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<aside>January 31, 2026</aside>
<a href="https://gto76.github.io" rel="author">Jure Šorn</a>
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<div><h1 id="comprehensivepythoncheatsheet">Comprehensive Python Cheatsheet</h1><p class="banner"><sup><a href="https://raw.githubusercontent.com/gto76/python-cheatsheet/main/README.md">Download text file</a>, <a href="https://github.com/gto76/python-cheatsheet">Fork me on GitHub</a>, <a href="https://github.com/gto76/python-cheatsheet/wiki/Frequently-Asked-Questions">Check out FAQ</a> or <a href="index.html?theme=dark3">Switch to dark theme</a>.
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</script><pre style="border-left: none;padding-left: 1.9px;"><code class="hljs bash" style="line-height: 1.327em;"><strong>ToC</strong> = {
<strong><span class="hljs-string"><span class="hljs-string">'1. Collections'</span></span></strong>: [<a href="#list">List</a>, <a href="#dictionary">Dictionary</a>, <a href="#set">Set</a>, <a href="#tuple">Tuple</a>, <a href="#range">Range</a>, <a href="#enumerate">Enumerate</a>, <a href="#iterator">Iterator</a>, <a href="#generator">Generator</a>],
<strong><span class="hljs-string"><span class="hljs-string">'2. Types'</span></span></strong>: [<a href="#type">Type</a>, <a href="#string">String</a>, <a href="#regex">Regular_Exp</a>, <a href="#format">Format</a>, <a href="#numbers">Numbers</a>, <a href="#combinatorics">Combinatorics</a>, <a href="#datetime">Datetime</a>],
<strong><span class="hljs-string"><span class="hljs-string">'3. Syntax'</span></span></strong>: [<a href="#function">Function</a>, <a href="#inline">Inline</a>, <a href="#imports">Import</a>, <a href="#decorator">Decorator</a>, <a href="#class">Class</a>, <a href="#ducktypes">Duck_Type</a>, <a href="#enum">Enum</a>, <a href="#exceptions">Except</a>],
<strong><span class="hljs-string"><span class="hljs-string">'4. System'</span></span></strong>: [<a href="#exit">Exit</a>, <a href="#print">Print</a>, <a href="#input">Input</a>, <a href="#commandlinearguments">Command_Line_Arguments</a>, <a href="#open">Open</a>, <a href="#paths">Path</a>, <a href="#oscommands">OS_Commands</a>],
<strong><span class="hljs-string"><span class="hljs-string">'5. Data'</span></span></strong>: [<a href="#json">JSON</a>, <a href="#pickle">Pickle</a>, <a href="#csv">CSV</a>, <a href="#sqlite">SQLite</a>, <a href="#bytes">Bytes</a>, <a href="#struct">Struct</a>, <a href="#array">Array</a>, <a href="#memoryview">Memory_View</a>, <a href="#deque">Deque</a>],
<strong><span class="hljs-string"><span class="hljs-string">'6. Advanced'</span></span></strong>: [<a href="#operator">Operator</a>, <a href="#matchstatement">Match_Stmt</a>, <a href="#logging">Logging</a>, <a href="#introspection">Introspection</a>, <a href="#threading">Threading</a>, <a href="#coroutines">Coroutines</a>],
<strong><span class="hljs-string"><span class="hljs-string">'7. Libraries'</span></span></strong>: [<a href="#progressbar">Progress_Bar</a>, <a href="#plot">Plot</a>, <a href="#table">Table</a>, <a href="#consoleapp">Console_App</a>, <a href="#guiapp">GUI</a>, <a href="#scraping">Scraping</a>, <a href="#webapp">Web</a>, <a href="#profiling">Profile</a>],
<strong><span class="hljs-string"><span class="hljs-string">'8. Multimedia'</span></span></strong>: [<a href="#numpy">NumPy</a>, <a href="#image">Image</a>, <a href="#animation">Animation</a>, <a href="#audio">Audio</a>, <a href="#synthesizer">Synthesizer</a>, <a href="#pygame">Pygame</a>, <a href="#pandas">Pandas</a>, <a href="#plotly">Plotly</a>]
}
</code></pre></div>
<div><h2 id="main"><a href="#main" name="main">#</a>Main</h2><pre><code class="python language-python hljs"><span class="hljs-keyword">if</span> __name__ == <span class="hljs-string">'__main__'</span>: <span class="hljs-comment"># Skips indented lines of code if file was imported.</span>
main() <span class="hljs-comment"># Executes user-defined `def main(): ...` function.</span>
</code></pre></div>
<div><h2 id="list"><a href="#list" name="list">#</a>List</h2><pre><code class="python language-python hljs"><list> = [<el_1>, <el_2>, ...] <span class="hljs-comment"># Creates new list object. E.g. `list_a = [1, 2, 3]`.</span>
</code></pre></div>
<pre><code class="python language-python hljs"><el> = <list>[index] <span class="hljs-comment"># First index is 0, last -1. Also `<list>[i] = <el>`.</span>
<list> = <list>[<slice>] <span class="hljs-comment"># Also <list>[from_inclusive : to_exclusive : ±step].</span>
</code></pre>
<pre><code class="python language-python hljs"><list>.append(<el>) <span class="hljs-comment"># Appends element to the end. Also `<list> += [<el>]`.</span>
<list>.extend(<collection>) <span class="hljs-comment"># Appends multiple elements. Also `<list> += <coll>`.</span>
</code></pre>
<pre><code class="python language-python hljs"><list>.sort(reverse=<span class="hljs-keyword">False</span>) <span class="hljs-comment"># Sorts the elements of the list in ascending order.</span>
<list>.reverse() <span class="hljs-comment"># Reverses the order of elements. Takes linear time.</span>
<list> = sorted(<collection>) <span class="hljs-comment"># Returns a new sorted list. Accepts `reverse=True`.</span>
<iter> = reversed(<list>) <span class="hljs-comment"># Returns reversed iterator. Also list(<iterator>).</span>
</code></pre>
<pre><code class="python language-python hljs"><el> = max(<collection>) <span class="hljs-comment"># Returns the largest element. Also min(<el_1>, ...).</span>
<num> = sum(<collection>) <span class="hljs-comment"># Returns a sum of elements. Also math.prod(<coll>).</span>
</code></pre>
<pre><code class="python language-python hljs">elementwise_sum = [sum(pair) <span class="hljs-keyword">for</span> pair <span class="hljs-keyword">in</span> zip(list_a, list_b)]
sorted_by_second = sorted(<collection>, key=<span class="hljs-keyword">lambda</span> el: el[<span class="hljs-number">1</span>])
sorted_by_both = sorted(<collection>, key=<span class="hljs-keyword">lambda</span> el: (el[<span class="hljs-number">1</span>], el[<span class="hljs-number">0</span>]))
flatter_list = list(itertools.chain.from_iterable(<list>))
</code></pre>
<ul>
<li><strong>For details about sort(), sorted(), min() and max() see <a href="#sortable">Sortable</a>.</strong></li>
<li><strong>Module <a href="#operator">operator</a> has function itemgetter() that can replace listed <a href="#lambda">lambdas</a>.</strong></li>
<li><strong>This text uses the term collection instead of <a href="#abstractbaseclasses">iterable</a>. For rationale see <a href="#collection">duck types</a>.</strong></li>
</ul>
<pre><code class="python language-python hljs"><int> = len(<list/dict/set/…>) <span class="hljs-comment"># Returns number of items. Doesn't accept iterators.</span>
<int> = <list>.count(<el>) <span class="hljs-comment"># Counts occurrences. Also `if <el> in <coll>: ...`.</span>
<int> = <list>.index(<el>) <span class="hljs-comment"># Returns index of first occ. or raises ValueError.</span>
<el> = <list>.pop() <span class="hljs-comment"># Removes item from the end (or at index if passed).</span>
<list>.insert(<int>, <el>) <span class="hljs-comment"># Inserts item at index and shifts remaining items.</span>
<list>.remove(<el>) <span class="hljs-comment"># Removes the first occurrence or raises ValueError.</span>
<list>.clear() <span class="hljs-comment"># Removes all items. Also provided by dict and set.</span>
</code></pre>
<div><h2 id="dictionary"><a href="#dictionary" name="dictionary">#</a>Dictionary</h2><pre><code class="python language-python hljs"><dict> = {key_1: val_1, key_2: val_2, ...} <span class="hljs-comment"># Use `<dict>[key]` to get or assign the value.</span>
</code></pre></div>
<pre><code class="python language-python hljs"><view> = <dict>.keys() <span class="hljs-comment"># A collection of keys that reflects changes.</span>
<view> = <dict>.values() <span class="hljs-comment"># A collection of values that reflects changes.</span>
<view> = <dict>.items() <span class="hljs-comment"># Coll. of key-value tuples that reflects chgs.</span>
</code></pre>
<pre><code class="python language-python hljs">value = <dict>.get(key, default=<span class="hljs-keyword">None</span>) <span class="hljs-comment"># Returns argument default if key is missing.</span>
value = <dict>.setdefault(key, default=<span class="hljs-keyword">None</span>) <span class="hljs-comment"># Returns and writes default if key is missing.</span>
<dict> = collections.defaultdict(<type>) <span class="hljs-comment"># Dict with automatic default value `<type>()`.</span>
<dict> = collections.defaultdict(<span class="hljs-keyword">lambda</span>: <span class="hljs-number">1</span>) <span class="hljs-comment"># Dictionary with automatic default value `1`.</span>
</code></pre>
<pre><code class="python language-python hljs"><dict> = dict(<collection>) <span class="hljs-comment"># Creates a dict from coll. of key-value pairs.</span>
<dict> = dict(zip(keys, values)) <span class="hljs-comment"># Creates key-value pairs from two collections.</span>
<dict> = dict.fromkeys(keys [, value]) <span class="hljs-comment"># Items get value None if only keys are passed.</span>
</code></pre>
<pre><code class="python language-python hljs"><dict>.update(<dict>) <span class="hljs-comment"># Adds items to dict. Passed dict has priority.</span>
value = <dict>.pop(key) <span class="hljs-comment"># Removes item or raises KeyError when missing.</span>
{k <span class="hljs-keyword">for</span> k, v <span class="hljs-keyword">in</span> <dict>.items() <span class="hljs-keyword">if</span> v == <span class="hljs-number">123</span>} <span class="hljs-comment"># Returns a set of keys whose value equals 123.</span>
{k: v <span class="hljs-keyword">for</span> k, v <span class="hljs-keyword">in</span> <dict>.items() <span class="hljs-keyword">if</span> k <span class="hljs-keyword">in</span> keys} <span class="hljs-comment"># Returns a dict of items with specified keys.</span>
</code></pre>
<div><h3 id="counter">Counter</h3><pre><code class="python language-python hljs"><span class="hljs-meta">>>> </span><span class="hljs-keyword">from</span> collections <span class="hljs-keyword">import</span> Counter
<span class="hljs-meta">>>> </span>counter = Counter([<span class="hljs-string">'blue'</span>, <span class="hljs-string">'blue'</span>, <span class="hljs-string">'blue'</span>, <span class="hljs-string">'red'</span>, <span class="hljs-string">'red'</span>])
<span class="hljs-meta">>>> </span>counter[<span class="hljs-string">'yellow'</span>] += <span class="hljs-number">1</span>
<span class="hljs-meta">>>> </span>print(counter.most_common())
[(<span class="hljs-string">'blue'</span>, <span class="hljs-number">3</span>), (<span class="hljs-string">'red'</span>, <span class="hljs-number">2</span>), (<span class="hljs-string">'yellow'</span>, <span class="hljs-number">1</span>)]
</code></pre></div>
<div><h2 id="set"><a href="#set" name="set">#</a>Set</h2><pre><code class="python language-python hljs"><set> = {<el_1>, <el_2>, ...} <span class="hljs-comment"># Coll. of unique items. Also set(), set(<coll>).</span>
</code></pre></div>
<pre><code class="python language-python hljs"><set>.add(<el>) <span class="hljs-comment"># Adds item to the set. Same as `<set> |= {<el>}`.</span>
<set>.update(<collection> [, ...]) <span class="hljs-comment"># Adds items to the set. Same as `<set> |= <set>`.</span>
</code></pre>
<pre><code class="python language-python hljs"><set> = <set>.union(<coll>) <span class="hljs-comment"># Returns a set of all items. Also <set> | <set>.</span>
<set> = <set>.intersection(<coll>) <span class="hljs-comment"># Returns all shared items. Also <set> & <set>.</span>
<set> = <set>.difference(<coll>) <span class="hljs-comment"># Returns set's unique items. Also <set> - <set>.</span>
<set> = <set>.symmetric_diff…(<coll>) <span class="hljs-comment"># Returns non-shared items. Also <set> ^ <set>.</span>
<bool> = <set>.issuperset(<coll>) <span class="hljs-comment"># Returns False if collection has unique items.</span>
<bool> = <set>.issubset(<coll>) <span class="hljs-comment"># Is collection a superset? Also <set> <= <set>.</span>
</code></pre>
<pre><code class="python language-python hljs"><el> = <set>.pop() <span class="hljs-comment"># Removes and returns an item or raises KeyError.</span>
<set>.remove(<el>) <span class="hljs-comment"># Removes the item or raises KeyError if missing.</span>
<set>.discard(<el>) <span class="hljs-comment"># Same as remove() but it doesn't raise an error.</span>
</code></pre>
<div><h3 id="frozenset">Frozen Set</h3><ul>
<li><strong>Frozenset is immutable and hashable version of the normal set.</strong></li>
<li><strong>That means it can be used as a key in a dictionary or as an item in a set.</strong></li>
</ul><pre><code class="python language-python hljs"><frozenset> = frozenset(<collection>)
</code></pre></div>
<div><h2 id="tuple"><a href="#tuple" name="tuple">#</a>Tuple</h2><p><strong>Tuple is an immutable and hashable list.</strong></p><pre><code class="python language-python hljs"><tuple> = () <span class="hljs-comment"># Returns an empty tuple. Also tuple(), tuple(<coll>).</span>
<tuple> = (<el>,) <span class="hljs-comment"># Returns a tuple with single element. Same as `<el>,`.</span>
<tuple> = (<el_1>, <el_2> [, ...]) <span class="hljs-comment"># Returns a tuple. Same as `<el_1>, <el_2> [, ...]`.</span>
</code></pre></div>
<div><h3 id="namedtuple">Named Tuple</h3><p><strong>Tuple's subclass with named elements.</strong></p><pre><code class="python language-python hljs"><span class="hljs-meta">>>> </span><span class="hljs-keyword">from</span> collections <span class="hljs-keyword">import</span> namedtuple
<span class="hljs-meta">>>> </span>Point = namedtuple(<span class="hljs-string">'Point'</span>, <span class="hljs-string">'x y'</span>)
<span class="hljs-meta">>>> </span>p = Point(<span class="hljs-number">1</span>, y=<span class="hljs-number">2</span>)
<span class="hljs-meta">>>> </span>print(p)
Point(x=<span class="hljs-number">1</span>, y=<span class="hljs-number">2</span>)
<span class="hljs-meta">>>> </span>p.x, p[<span class="hljs-number">1</span>]
(<span class="hljs-number">1</span>, <span class="hljs-number">2</span>)
</code></pre></div>
<div><h2 id="range"><a href="#range" name="range">#</a>Range</h2><p><strong>A sequence of evenly spaced integers.</strong></p><pre><code class="python language-python hljs"><range> = range(stop) <span class="hljs-comment"># I.e. range(to_exclusive). Integers from 0 to `stop-1`.</span>
<range> = range(start, stop) <span class="hljs-comment"># I.e. range(from_inc, to_exc). From start to `stop-1`.</span>
<range> = range(start, stop, ±step) <span class="hljs-comment"># I.e. range(from_inclusive, to_exclusive, ±step_size).</span>
</code></pre></div>
<pre><code class="python language-python hljs"><span class="hljs-meta">>>> </span>[i <span class="hljs-keyword">for</span> i <span class="hljs-keyword">in</span> range(<span class="hljs-number">3</span>)]
[<span class="hljs-number">0</span>, <span class="hljs-number">1</span>, <span class="hljs-number">2</span>]
</code></pre>
<div><h2 id="enumerate"><a href="#enumerate" name="enumerate">#</a>Enumerate</h2><pre><code class="python language-python hljs"><span class="hljs-keyword">for</span> i, el <span class="hljs-keyword">in</span> enumerate(<coll>, start=<span class="hljs-number">0</span>): <span class="hljs-comment"># Returns next element and its index on each pass.</span>
...
</code></pre></div>
<div><h2 id="iterator"><a href="#iterator" name="iterator">#</a>Iterator</h2><p><strong>Potentially endless stream of elements.</strong></p><pre><code class="python language-python hljs"><iter> = iter(<collection>) <span class="hljs-comment"># Iterator that returns passed elements one by one.</span>
<iter> = iter(<func>, to_exc) <span class="hljs-comment"># Calls `<func>()` until it receives 'to_exc' value.</span>
<iter> = (<expr> <span class="hljs-keyword">for</span> <name> <span class="hljs-keyword">in</span> <coll>) <span class="hljs-comment"># E.g. `(i+1 for i in range(3))`. Evaluates lazily.</span>
<el> = next(<iter> [, default]) <span class="hljs-comment"># Raises StopIteration or returns 'default' on end.</span>
<list> = list(<iter>) <span class="hljs-comment"># Returns a list of iterator's remaining elements.</span>
</code></pre></div>
<ul>
<li><strong>For loops call <code class="python hljs"><span class="hljs-string">'iter(<collection>)'</span></code> at the start and <code class="python hljs"><span class="hljs-string">'next(<iter>)'</span></code> on each pass.</strong></li>
<li><strong>Calling <code class="python hljs"><span class="hljs-string">'iter(<iter>)'</span></code> returns unmodified iterator. For details see <a href="#iterator-1">Iterator</a> duck type.</strong></li>
</ul>
<pre><code class="python language-python hljs"><span class="hljs-keyword">import</span> itertools <span class="hljs-keyword">as</span> it
</code></pre>
<pre><code class="python language-python hljs"><iter> = it.count(start=<span class="hljs-number">0</span>, step=<span class="hljs-number">1</span>) <span class="hljs-comment"># Returns updated 'start' endlessly. Accepts floats.</span>
<iter> = it.repeat(<obj> [, times]) <span class="hljs-comment"># Returns passed element endlessly or 'times' times.</span>
<iter> = it.cycle(<collection>) <span class="hljs-comment"># Repeats the sequence endlessly. Accepts iterators.</span>
</code></pre>
<pre><code class="python language-python hljs"><iter> = it.chain(<coll>, <coll>, ...) <span class="hljs-comment"># Returns each element of each collection in order.</span>
<iter> = it.chain.from_iterable(<coll>) <span class="hljs-comment"># Accepts collection (i.e. iterable) of collections.</span>
<iter> = it.islice(<coll>, stop) <span class="hljs-comment"># Also accepts 'start' and 'step'. Args can be None.</span>
<iter> = it.product(<coll>, ...) <span class="hljs-comment"># Same as `((a, b) for a in arg_1 for b in arg_2)`.</span>
</code></pre>
<div><h2 id="generator"><a href="#generator" name="generator">#</a>Generator</h2><ul>
<li><strong>Any function that contains a yield statement returns a generator.</strong></li>
<li><strong>Generators and iterators are interchangeable.</strong></li>
</ul><pre><code class="python language-python hljs"><span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">count</span><span class="hljs-params">(start, step)</span>:</span>
<span class="hljs-keyword">while</span> <span class="hljs-keyword">True</span>:
<span class="hljs-keyword">yield</span> start
start += step
</code></pre></div>
<pre><code class="python language-python hljs"><span class="hljs-meta">>>> </span>counter = count(<span class="hljs-number">10</span>, <span class="hljs-number">2</span>)
<span class="hljs-meta">>>> </span>next(counter), next(counter), next(counter)
(<span class="hljs-number">10</span>, <span class="hljs-number">12</span>, <span class="hljs-number">14</span>)
</code></pre>
<div><h2 id="type"><a href="#type" name="type">#</a>Type</h2><ul>
<li><strong>Everything in Python is an object.</strong></li>
<li><strong>Every object has a certain type.</strong></li>
<li><strong>Type and class are synonymous.</strong></li>
</ul><pre><code class="python language-python hljs"><type> = type(<obj>) <span class="hljs-comment"># Returns object's type. Same as `<obj>.__class__`.</span>
<bool> = isinstance(<obj>, <type>) <span class="hljs-comment"># Same result as `issubclass(type(<obj>), <type>)`.</span>
</code></pre></div>
<pre><code class="python language-python hljs"><span class="hljs-meta">>>> </span>type(<span class="hljs-string">'a'</span>), <span class="hljs-string">'a'</span>.__class__, str
(<<span class="hljs-class"><span class="hljs-title">class</span> '<span class="hljs-title">str</span>'>, <<span class="hljs-title">class</span> '<span class="hljs-title">str</span>'>, <<span class="hljs-title">class</span> '<span class="hljs-title">str</span>'>)
</span></code></pre>
<div><h4 id="sometypesdonothavebuiltinnamessotheymustbeimported">Some types do not have built-in names, so they must be imported:</h4><pre><code class="python language-python hljs"><span class="hljs-keyword">from</span> types <span class="hljs-keyword">import</span> FunctionType, MethodType, LambdaType, GeneratorType, ModuleType
</code></pre></div>
<div><h3 id="abstractbaseclasses">Abstract Base Classes</h3><p><strong>Each abstract base class specifies a set of virtual subclasses. These classes are then recognized by isinstance() and issubclass() as subclasses of the ABC, although they are really not. An ABC can also manually decide whether or not a specific class is its virtual subclass, usually based on which methods that class has implemented. For instance, Iterable ABC looks for method iter(), while Collection ABC looks for iter(), contains() and len().</strong></p><pre><code class="python language-python hljs"><span class="hljs-meta">>>> </span><span class="hljs-keyword">from</span> collections.abc <span class="hljs-keyword">import</span> Iterable, Collection, Sequence
<span class="hljs-meta">>>> </span>isinstance([<span class="hljs-number">1</span>, <span class="hljs-number">2</span>, <span class="hljs-number">3</span>], Iterable)
<span class="hljs-keyword">True</span>
</code></pre></div>
<pre><code class="text language-text">┏━━━━━━━━━━━━━━━━━━┯━━━━━━━━━━━━┯━━━━━━━━━━━━┯━━━━━━━━━━━━┓
┃ │ Iterable │ Collection │ Sequence ┃
┠──────────────────┼────────────┼────────────┼────────────┨
┃ list, range, str │ ✓ │ ✓ │ ✓ ┃
┃ dict, set │ ✓ │ ✓ │ ┃
┃ iter │ ✓ │ │ ┃
┗━━━━━━━━━━━━━━━━━━┷━━━━━━━━━━━━┷━━━━━━━━━━━━┷━━━━━━━━━━━━┛
</code></pre>
<pre><code class="python language-python hljs"><span class="hljs-meta">>>> </span><span class="hljs-keyword">from</span> numbers <span class="hljs-keyword">import</span> Number, Complex, Real, Rational, Integral
<span class="hljs-meta">>>> </span>isinstance(<span class="hljs-number">123</span>, Number)
<span class="hljs-keyword">True</span>
</code></pre>
<pre><code class="text language-text">┏━━━━━━━━━━━━━━━━━━━━┯━━━━━━━━━━━┯━━━━━━━━━━━┯━━━━━━━━━━┯━━━━━━━━━━┯━━━━━━━━━━┓
┃ │ Number │ Complex │ Real │ Rational │ Integral ┃
┠────────────────────┼───────────┼───────────┼──────────┼──────────┼──────────┨
┃ int │ ✓ │ ✓ │ ✓ │ ✓ │ ✓ ┃
┃ fractions.Fraction │ ✓ │ ✓ │ ✓ │ ✓ │ ┃
┃ float │ ✓ │ ✓ │ ✓ │ │ ┃
┃ complex │ ✓ │ ✓ │ │ │ ┃
┃ decimal.Decimal │ ✓ │ │ │ │ ┃
┗━━━━━━━━━━━━━━━━━━━━┷━━━━━━━━━━━┷━━━━━━━━━━━┷━━━━━━━━━━┷━━━━━━━━━━┷━━━━━━━━━━┛
</code></pre>
<div><h2 id="string"><a href="#string" name="string">#</a>String</h2><p><strong>Immutable sequence of characters.</strong></p><pre><code class="python language-python hljs"><str> = <span class="hljs-string">'abc'</span> <span class="hljs-comment"># Also "abc". Interprets \n, \t, \x00-\xff, etc.</span>
</code></pre></div>
<pre><code class="python language-python hljs"><str> = <str>.strip() <span class="hljs-comment"># Strips all whitespace characters from both ends.</span>
<str> = <str>.strip(<span class="hljs-string">'<chars>'</span>) <span class="hljs-comment"># Strips passed characters. Also lstrip/rstrip().</span>
</code></pre>
<pre><code class="python language-python hljs"><list> = <str>.split() <span class="hljs-comment"># Splits it on one or more whitespace characters.</span>
<list> = <str>.split(sep=<span class="hljs-keyword">None</span>, maxsplit=<span class="hljs-number">-1</span>) <span class="hljs-comment"># Splits on 'sep' string at most 'maxsplit' times.</span>
<list> = <str>.splitlines(keepends=<span class="hljs-keyword">False</span>) <span class="hljs-comment"># On [\n\r\f\v\x1c-\x1e\x85\u2028\u2029] and \r\n.</span>
<str> = <str>.join(<coll_of_strings>) <span class="hljs-comment"># Joins items by using the string as a separator.</span>
</code></pre>
<pre><code class="python language-python hljs"><bool> = <sub_str> <span class="hljs-keyword">in</span> <str> <span class="hljs-comment"># Returns True if string contains the substring.</span>
<bool> = <str>.startswith(<sub_str>) <span class="hljs-comment"># Pass tuple of strings to give multiple options.</span>
<int> = <str>.find(<sub_str>) <span class="hljs-comment"># Returns start index of the first match or `-1`.</span>
</code></pre>
<pre><code class="python language-python hljs"><str> = <str>.lower() <span class="hljs-comment"># Lowers the case. Also upper/capitalize/title().</span>
<str> = <str>.casefold() <span class="hljs-comment"># Lower() that converts ẞ/ß to ss, Σ/ς to σ, etc.</span>
<str> = <str>.replace(old, new [, count]) <span class="hljs-comment"># Replaces 'old' with 'new' at most 'count' times.</span>
<str> = <str>.translate(table) <span class="hljs-comment"># Use `str.maketrans(<dict>)` to generate table.</span>
</code></pre>
<pre><code class="python language-python hljs"><str> = chr(<int>) <span class="hljs-comment"># Converts passed integer into Unicode character.</span>
<int> = ord(<str>) <span class="hljs-comment"># Converts passed Unicode character into integer.</span>
</code></pre>
<ul>
<li><strong>Use <code class="python hljs"><span class="hljs-string">'unicodedata.normalize("NFC", <str>)'</span></code> on strings like <code class="python hljs"><span class="hljs-string">'Motörhead'</span></code> before comparing them to other strings, because <code class="python hljs"><span class="hljs-string">'ö'</span></code> can be stored as one or two characters.</strong></li>
<li><strong><code class="python hljs"><span class="hljs-string">'NFC'</span></code> converts such characters to a single character, while <code class="python hljs"><span class="hljs-string">'NFD'</span></code> converts them to two.</strong></li>
</ul>
<pre><code class="python language-python hljs"><bool> = <str>.isdecimal() <span class="hljs-comment"># Checks all chars for [0-9]. Also [०-९], [٠-٩].</span>
<bool> = <str>.isdigit() <span class="hljs-comment"># Checks for [²³¹…] and isdecimal(). Also [፩-፱].</span>
<bool> = <str>.isnumeric() <span class="hljs-comment"># Checks for [¼½¾…] and isdigit(). Also [零〇一…].</span>
<bool> = <str>.isalnum() <span class="hljs-comment"># Checks for [ABC…] and isnumeric(). Also [ªµº…].</span>
<bool> = <str>.isprintable() <span class="hljs-comment"># Checks for [ !"#…], basic emojis and isalnum().</span>
<bool> = <str>.isspace() <span class="hljs-comment"># Checks for [ \t\n\r\f\v\x1c\x1d\x1e\x1f\x85…].</span>
</code></pre>
<div><h2 id="regex"><a href="#regex" name="regex">#</a>Regex</h2><p><strong>Functions for regular expression matching.</strong></p><pre><code class="python language-python hljs"><span class="hljs-keyword">import</span> re
<str> = re.sub(<span class="hljs-string">r'<regex>'</span>, new, text) <span class="hljs-comment"># Substitutes occurrences with string 'new'.</span>
<list> = re.findall(<span class="hljs-string">r'<regex>'</span>, text) <span class="hljs-comment"># Returns all occurrences as string objects.</span>
<list> = re.split(<span class="hljs-string">r'<regex>'</span>, text) <span class="hljs-comment"># Add brackets around regex to keep matches.</span>
<Match> = re.search(<span class="hljs-string">r'<regex>'</span>, text) <span class="hljs-comment"># Returns first occ. of the pattern or None.</span>
<Match> = re.match(<span class="hljs-string">r'<regex>'</span>, text) <span class="hljs-comment"># Only searches at the start of the 'text'.</span>
<iter> = re.finditer(<span class="hljs-string">r'<regex>'</span>, text) <span class="hljs-comment"># Returns all occurrences as Match objects.</span>
</code></pre></div>
<ul>
<li><strong>Raw string literals do not interpret escape sequences, thus enabling us to use the regex-specific escape sequences that cause SyntaxWarning in normal string literals (since 3.12).</strong></li>
<li><strong>Argument <code class="python hljs"><span class="hljs-string">'new'</span></code> can also be a function that accepts a Match object and returns a string.</strong></li>
<li><strong>Argument <code class="python hljs"><span class="hljs-string">'flags=re.IGNORECASE'</span></code> can be used with all functions that are listed above.</strong></li>
<li><strong>Argument <code class="python hljs"><span class="hljs-string">'flags=re.MULTILINE'</span></code> makes <code class="python hljs"><span class="hljs-string">'^'</span></code> and <code class="python hljs"><span class="hljs-string">'$'</span></code> match the start/end of each line.</strong></li>
<li><strong>Argument <code class="python hljs"><span class="hljs-string">'flags=re.DOTALL'</span></code> makes <code class="python hljs"><span class="hljs-string">'.'</span></code> also accept the <code class="python hljs"><span class="hljs-string">'\n'</span></code> (besides all other chars).</strong></li>
<li><strong><code class="python hljs"><span class="hljs-string">'re.compile(r"<regex>")'</span></code> returns a Pattern object with methods sub(), findall(), etc.</strong></li>
</ul>
<div><h3 id="matchobject">Match Object</h3><pre><code class="python language-python hljs"><str> = <Match>.group() <span class="hljs-comment"># Returns the whole match. Also group(0).</span>
<str> = <Match>.group(<span class="hljs-number">1</span>) <span class="hljs-comment"># Returns part inside the first brackets.</span>
<tuple> = <Match>.groups() <span class="hljs-comment"># Returns all bracketed parts as strings.</span>
<int> = <Match>.start() <span class="hljs-comment"># Returns start index of the whole match.</span>
<int> = <Match>.end() <span class="hljs-comment"># Returns the match's end index plus one.</span>
</code></pre></div>
<div><h3 id="specialsequences">Special Sequences</h3><pre><code class="python language-python hljs"><span class="hljs-string">'\d'</span> == <span class="hljs-string">'[0-9]'</span> <span class="hljs-comment"># Also [०-९…]. Matches decimal character.</span>
<span class="hljs-string">'\w'</span> == <span class="hljs-string">'[a-zA-Z0-9_]'</span> <span class="hljs-comment"># Also [ª²³…]. Matches alphanumeric or _.</span>
<span class="hljs-string">'\s'</span> == <span class="hljs-string">'[ \t\n\r\f\v]'</span> <span class="hljs-comment"># Also [\x1c-\x1f…]. Matches whitespace.</span>
</code></pre></div>
<ul>
<li><strong>By default, decimal characters and alphanumerics from all alphabets are matched unless <code class="python hljs"><span class="hljs-string">'flags=re.ASCII'</span></code> is used. It restricts special sequence matches to the first 128 Unicode characters and also prevents <code class="python hljs"><span class="hljs-string">'\s'</span></code> from accepting <code class="python hljs"><span class="hljs-string">'\x1c'</span></code>, <code class="python hljs"><span class="hljs-string">'\x1d'</span></code>, <code class="python hljs"><span class="hljs-string">'\x1e'</span></code> and <code class="python hljs"><span class="hljs-string">'\x1f'</span></code> (non-printable characters that divide text into files, tables, rows and fields, respectively).</strong></li>
<li><strong>Use a capital letter, i.e. <code class="python hljs"><span class="hljs-string">'\D'</span></code>, <code class="python hljs"><span class="hljs-string">'\W'</span></code> or <code class="python hljs"><span class="hljs-string">'\S'</span></code>, for negation. All non-ASCII characters are matched if ASCII flag is used in conjunction with a capital letter.</strong></li>
</ul>
<div><h2 id="format"><a href="#format" name="format">#</a>Format</h2><pre><code class="python hljs"><str> = <span class="hljs-string">f'<span class="hljs-subst">{<obj>}</span>, <span class="hljs-subst">{<obj>}</span>'</span> <span class="hljs-comment"># Curly brackets can contain any expression.</span>
<str> = <span class="hljs-string">'{}, {}'</span>.format(<obj>, <obj>) <span class="hljs-comment"># Same as '{0}, {a}'.format(<obj>, a=<obj>).</span>
<str> = <span class="hljs-string">'%s, %s'</span> % (<obj>, <obj>) <span class="hljs-comment"># Redundant and inferior C-style formatting.</span>
</code></pre></div>
<div><h3 id="example">Example</h3><pre><code class="python language-python hljs"><span class="hljs-meta">>>> </span>Person = collections.namedtuple(<span class="hljs-string">'Person'</span>, <span class="hljs-string">'name height'</span>)
<span class="hljs-meta">>>> </span>person = Person(<span class="hljs-string">'Jean-Luc'</span>, <span class="hljs-number">187</span>)
<span class="hljs-meta">>>> </span><span class="hljs-string">f'<span class="hljs-subst">{person.name}</span> is <span class="hljs-subst">{person.height / <span class="hljs-number">100</span>}</span> meters tall.'</span>
<span class="hljs-string">'Jean-Luc is 1.87 meters tall.'</span>
</code></pre></div>
<div><h3 id="generaloptions">General Options</h3><pre><code class="python language-python hljs">{<obj>:<<span class="hljs-number">10</span>} <span class="hljs-comment"># '<obj> '.</span>
{<obj>:^<span class="hljs-number">10</span>} <span class="hljs-comment"># ' <obj> '.</span>
{<obj>:><span class="hljs-number">10</span>} <span class="hljs-comment"># ' <obj>'.</span>
{<obj>:.<<span class="hljs-number">10</span>} <span class="hljs-comment"># '<obj>.....'.</span>
{<obj>:<span class="hljs-number">0</span>} <span class="hljs-comment"># '<obj>'.</span>
</code></pre></div>
<ul>
<li><strong>Objects are converted to strings with format() function, e.g. <code class="python hljs"><span class="hljs-string">'format(<obj>, "<10")'</span></code>.</strong></li>
<li><strong>Options can be generated dynamically via nested braces: <code class="python hljs"><span class="hljs-string">f'<span class="hljs-subst">{<obj>:{<str/int>}</span>[…]}'</span></code>.</strong></li>
<li><strong>Adding <code class="python hljs"><span class="hljs-string">'='</span></code> to the expression prepends it to its result, e.g. <code class="python hljs"><span class="hljs-string">f'<span class="hljs-subst">{<span class="hljs-number">1</span>+<span class="hljs-number">1</span>=}</span>'</span></code> returns <code class="python hljs"><span class="hljs-string">'1+1=2'</span></code>.</strong></li>
<li><strong>Adding <code class="python hljs"><span class="hljs-string">'!r'</span></code> to the expression first calls result's <a href="#class">repr()</a> method and only then format().</strong></li>
</ul>
<div><h3 id="strings">Strings</h3><pre><code class="python language-python hljs">{<span class="hljs-string">'abcde'</span>:<span class="hljs-number">10</span>} <span class="hljs-comment"># 'abcde '.</span>
{<span class="hljs-string">'abcde'</span>:<span class="hljs-number">10.3</span>} <span class="hljs-comment"># 'abc '.</span>
{<span class="hljs-string">'abcde'</span>:<span class="hljs-number">.3</span>} <span class="hljs-comment"># 'abc'.</span>
{<span class="hljs-string">'abcde'</span>!r:<span class="hljs-number">10</span>} <span class="hljs-comment"># "'abcde' ".</span>
</code></pre></div>
<div><h3 id="numbers-1">Numbers</h3><pre><code class="python language-python hljs">{<span class="hljs-number">123456</span>:<span class="hljs-number">10</span>} <span class="hljs-comment"># ' 123456'.</span>
{<span class="hljs-number">123456</span>:<span class="hljs-number">10</span>,} <span class="hljs-comment"># ' 123,456'.</span>
{<span class="hljs-number">123456</span>:<span class="hljs-number">10</span>_} <span class="hljs-comment"># ' 123_456'.</span>
{<span class="hljs-number">123456</span>:+<span class="hljs-number">10</span>} <span class="hljs-comment"># ' +123456'.</span>
{<span class="hljs-number">123456</span>:=+<span class="hljs-number">10</span>} <span class="hljs-comment"># '+ 123456'.</span>
{<span class="hljs-number">123456</span>: } <span class="hljs-comment"># ' 123456'.</span>
{<span class="hljs-number">-123456</span>: } <span class="hljs-comment"># '-123456'.</span>
</code></pre></div>
<div><h3 id="floats">Floats</h3><pre><code class="python language-python hljs">{<span class="hljs-number">1.23456</span>:<span class="hljs-number">10.3</span>} <span class="hljs-comment"># ' 1.23'.</span>
{<span class="hljs-number">1.23456</span>:<span class="hljs-number">10.3</span>f} <span class="hljs-comment"># ' 1.235'.</span>
{<span class="hljs-number">1.23456</span>:<span class="hljs-number">10.3</span>e} <span class="hljs-comment"># ' 1.235e+00'.</span>
{<span class="hljs-number">1.23456</span>:<span class="hljs-number">10.3</span>%} <span class="hljs-comment"># ' 123.456%'.</span>
</code></pre></div>
<div><h4 id="comparisonofpresentationtypes">Comparison of presentation types:</h4><pre><code class="text language-text">┏━━━━━━━━━━━━━━┯━━━━━━━━━━━━━━━━┯━━━━━━━━━━━━━━━━┯━━━━━━━━━━━━━━━━┯━━━━━━━━━━━━━━━━┓
┃ │ {<float>} │ {<float>:f} │ {<float>:e} │ {<float>:%} ┃
┠──────────────┼────────────────┼────────────────┼────────────────┼────────────────┨
┃ 0.000056789 │ '5.6789e-05' │ '0.000057' │ '5.678900e-05' │ '0.005679%' ┃
┃ 0.00056789 │ '0.00056789' │ '0.000568' │ '5.678900e-04' │ '0.056789%' ┃
┃ 0.0056789 │ '0.0056789' │ '0.005679' │ '5.678900e-03' │ '0.567890%' ┃
┃ 0.056789 │ '0.056789' │ '0.056789' │ '5.678900e-02' │ '5.678900%' ┃
┃ 0.56789 │ '0.56789' │ '0.567890' │ '5.678900e-01' │ '56.789000%' ┃
┃ 5.6789 │ '5.6789' │ '5.678900' │ '5.678900e+00' │ '567.890000%' ┃
┃ 56.789 │ '56.789' │ '56.789000' │ '5.678900e+01' │ '5678.900000%' ┃
┗━━━━━━━━━━━━━━┷━━━━━━━━━━━━━━━━┷━━━━━━━━━━━━━━━━┷━━━━━━━━━━━━━━━━┷━━━━━━━━━━━━━━━━┛
┏━━━━━━━━━━━━━━┯━━━━━━━━━━━━━━━━┯━━━━━━━━━━━━━━━━┯━━━━━━━━━━━━━━━━┯━━━━━━━━━━━━━━━━┓
┃ │ {<float>:.2} │ {<float>:.2f} │ {<float>:.2e} │ {<float>:.2%} ┃
┠──────────────┼────────────────┼────────────────┼────────────────┼────────────────┨
┃ 0.000056789 │ '5.7e-05' │ '0.00' │ '5.68e-05' │ '0.01%' ┃
┃ 0.00056789 │ '0.00057' │ '0.00' │ '5.68e-04' │ '0.06%' ┃
┃ 0.0056789 │ '0.0057' │ '0.01' │ '5.68e-03' │ '0.57%' ┃
┃ 0.056789 │ '0.057' │ '0.06' │ '5.68e-02' │ '5.68%' ┃
┃ 0.56789 │ '0.57' │ '0.57' │ '5.68e-01' │ '56.79%' ┃
┃ 5.6789 │ '5.7' │ '5.68' │ '5.68e+00' │ '567.89%' ┃
┃ 56.789 │ '5.7e+01' │ '56.79' │ '5.68e+01' │ '5678.90%' ┃
┗━━━━━━━━━━━━━━┷━━━━━━━━━━━━━━━━┷━━━━━━━━━━━━━━━━┷━━━━━━━━━━━━━━━━┷━━━━━━━━━━━━━━━━┛
</code></pre></div>
<ul>
<li><strong><code class="python hljs"><span class="hljs-string">'{<num>:g}'</span></code> is <code class="python hljs"><span class="hljs-string">'{<float>:.6}'</span></code> that strips <code class="python hljs"><span class="hljs-string">'.0'</span></code> and has exponent starting at <code class="python hljs"><span class="hljs-string">'1e+06'</span></code>.</strong></li>
<li><strong>When both rounding up and rounding down are possible, the one that returns result with even last digit is chosen. Hence <code class="python hljs"><span class="hljs-string">'{6.5:.0f}'</span></code> becomes a <code class="python hljs"><span class="hljs-string">'6'</span></code>, while <code class="python hljs"><span class="hljs-string">'{7.5:.0f}'</span></code> an <code class="python hljs"><span class="hljs-string">'8'</span></code>.</strong></li>
<li><strong>The last rule only effects numbers that can be represented exactly by a float (<code class="python hljs"><span class="hljs-number">.5</span></code>, <code class="python hljs"><span class="hljs-number">.25</span></code>, …).</strong></li>
</ul>
<div><h3 id="ints">Ints</h3><pre><code class="python language-python hljs">{<span class="hljs-number">90</span>:c} <span class="hljs-comment"># Converts 90 to Unicode character 'Z'.</span>
{<span class="hljs-number">90</span>:b} <span class="hljs-comment"># Converts 90 to binary number '1011010'.</span>
{<span class="hljs-number">90</span>:X} <span class="hljs-comment"># Converts 90 to hexadecimal number '5A'.</span>
</code></pre></div>
<div><h2 id="numbers"><a href="#numbers" name="numbers">#</a>Numbers</h2><pre><code class="python language-python hljs"><int> = int(<float/str/bool>) <span class="hljs-comment"># A whole number. Truncates floats.</span>
<float> = float(<int/str/bool>) <span class="hljs-comment"># 64-bit decimal. Also <fl>e±<int>.</span>
<complex> = complex(real=<span class="hljs-number">0</span>, imag=<span class="hljs-number">0</span>) <span class="hljs-comment"># Complex number. Also <fl> ± <fl>j.</span>
<Fraction> = fractions.Fraction(numer, denom) <span class="hljs-comment"># `<Fraction> = <Fraction> / <int>`.</span>
<Decimal> = decimal.Decimal(<str/int/tuple>) <span class="hljs-comment"># `Decimal((1, (2,), 3)) == -2000`.</span>
</code></pre></div>
<ul>
<li><strong><code class="python hljs"><span class="hljs-string">'int(<str>)'</span></code> and <code class="python hljs"><span class="hljs-string">'float(<str>)'</span></code> raise ValueError exception if string is malformed.</strong></li>
<li><strong>Decimal objects store numbers exactly, unlike most floats where <code class="python hljs"><span class="hljs-string">'1.1 + 2.2 != 3.3'</span></code>.</strong></li>
<li><strong>Floats can be compared with: <code class="python hljs"><span class="hljs-string">'math.isclose(<float>, <float>, rel_tol=1e-9)'</span></code>.</strong></li>
<li><strong>Precision of decimal operations is set with: <code class="python hljs"><span class="hljs-string">'decimal.getcontext().prec = <int>'</span></code>.</strong></li>
<li><strong>Bools can be used anywhere ints can, since bool is a subclass of int: <code class="python hljs"><span class="hljs-string">'True + 1 == 2'</span></code>.</strong></li>
</ul>
<div><h3 id="builtinfunctions">Built-in Functions</h3><pre><code class="python language-python hljs"><num> = pow(<num>, <num>) <span class="hljs-comment"># E.g. `pow(3, 4) == 3 ** 4 == 81`.</span>
<num> = abs(<num>) <span class="hljs-comment"># E.g. `abs(-50) == abs(50) == 50`.</span>
<num> = round(<num> [, ±ndigits]) <span class="hljs-comment"># E.g. `round(123.45, -1) == 120`.</span>
<num> = min(<coll_of_nums>) <span class="hljs-comment"># Also `max(<num>, <num> [, ...])`.</span>
<num> = sum(<coll_of_nums>) <span class="hljs-comment"># Also `math.prod(<coll_of_nums>)`.</span>
</code></pre></div>
<div><h3 id="math">Math</h3><pre><code class="python language-python hljs"><span class="hljs-keyword">from</span> math <span class="hljs-keyword">import</span> floor, ceil, trunc <span class="hljs-comment"># Funcs that convert float into int.</span>
<span class="hljs-keyword">from</span> math <span class="hljs-keyword">import</span> pi, inf, nan, isnan <span class="hljs-comment"># `inf*0` and `nan+1` return `nan`.</span>
<span class="hljs-keyword">from</span> math <span class="hljs-keyword">import</span> sqrt, factorial <span class="hljs-comment"># `sqrt(-1)` will raise ValueError.</span>
<span class="hljs-keyword">from</span> math <span class="hljs-keyword">import</span> sin, cos, tan <span class="hljs-comment"># Also: degrees, radians, asin, etc.</span>
<span class="hljs-keyword">from</span> math <span class="hljs-keyword">import</span> log, log10, log2 <span class="hljs-comment"># Log() can accept 'base' argument.</span>
</code></pre></div>
<div><h3 id="statistics">Statistics</h3><pre><code class="python language-python hljs"><span class="hljs-keyword">from</span> statistics <span class="hljs-keyword">import</span> mean, median, mode <span class="hljs-comment"># Mode returns most common element.</span>
<span class="hljs-keyword">from</span> statistics <span class="hljs-keyword">import</span> variance, stdev <span class="hljs-comment"># Also `cuts = quantiles(data, n)`.</span>
</code></pre></div>
<div><h3 id="random">Random</h3><pre><code class="python language-python hljs"><span class="hljs-keyword">from</span> random <span class="hljs-keyword">import</span> random, randint, uniform <span class="hljs-comment"># Also: gauss, choice, shuffle, etc.</span>
</code></pre></div>
<pre><code class="python language-python hljs"><float> = random() <span class="hljs-comment"># Selects random float from [0, 1).</span>
<num> = randint/uniform(a, b) <span class="hljs-comment"># Selects an int/float from [a, b].</span>
<float> = gauss(mean, stdev) <span class="hljs-comment"># Also triangular(low, high, mode).</span>
<el> = choice(<sequence>) <span class="hljs-comment"># Doesn't modify. Also sample(p, n).</span>
shuffle(<list>) <span class="hljs-comment"># Works with all mutable sequences.</span>
</code></pre>
<div><h3 id="hexadecimalnumbers">Hexadecimal Numbers</h3><pre><code class="python language-python hljs"><int> = <span class="hljs-number">0x</span><hex> <span class="hljs-comment"># E.g. `0xFf == 255`. Also 0b<bin>.</span>
<int> = int(<span class="hljs-string">'±<hex>'</span>, <span class="hljs-number">16</span>) <span class="hljs-comment"># Also int('±0x<hex>/±0b<bin>', 0).</span>
<str> = hex(<int>) <span class="hljs-comment"># Returns '[-]0x<hex>'. Also bin().</span>
</code></pre></div>
<div><h3 id="bitwiseoperators">Bitwise Operators</h3><pre><code class="python language-python hljs"><int> = <int> & <int> <span class="hljs-comment"># E.g. `0b1100 & 0b1010 == 0b1000`.</span>
<int> = <int> | <int> <span class="hljs-comment"># E.g. `0b1100 | 0b1010 == 0b1110`.</span>
<int> = <int> ^ <int> <span class="hljs-comment"># E.g. `0b1100 ^ 0b1010 == 0b0110`.</span>
<int> = <int> << n_bits <span class="hljs-comment"># E.g. `0b1111 << 4 == 0b11110000`.</span>
<int> = ~<int> <span class="hljs-comment"># E.g. `~0b1 == -(0b1+1) == -0b10`.</span>
</code></pre></div>
<div><h2 id="combinatorics"><a href="#combinatorics" name="combinatorics">#</a>Combinatorics</h2><pre><code class="python language-python hljs"><span class="hljs-keyword">import</span> itertools <span class="hljs-keyword">as</span> it
</code></pre></div>
<pre><code class="python language-python hljs"><span class="hljs-meta">>>> </span>list(it.product(<span class="hljs-string">'abc'</span>, repeat=<span class="hljs-number">2</span>)) <span class="hljs-comment"># a b c</span>
[(<span class="hljs-string">'a'</span>, <span class="hljs-string">'a'</span>), (<span class="hljs-string">'a'</span>, <span class="hljs-string">'b'</span>), (<span class="hljs-string">'a'</span>, <span class="hljs-string">'c'</span>), <span class="hljs-comment"># a x x x</span>
(<span class="hljs-string">'b'</span>, <span class="hljs-string">'a'</span>), (<span class="hljs-string">'b'</span>, <span class="hljs-string">'b'</span>), (<span class="hljs-string">'b'</span>, <span class="hljs-string">'c'</span>), <span class="hljs-comment"># b x x x</span>
(<span class="hljs-string">'c'</span>, <span class="hljs-string">'a'</span>), (<span class="hljs-string">'c'</span>, <span class="hljs-string">'b'</span>), (<span class="hljs-string">'c'</span>, <span class="hljs-string">'c'</span>)] <span class="hljs-comment"># c x x x</span>
</code></pre>
<pre><code class="python language-python hljs"><span class="hljs-meta">>>> </span>list(it.permutations(<span class="hljs-string">'abc'</span>, <span class="hljs-number">2</span>)) <span class="hljs-comment"># a b c</span>
[(<span class="hljs-string">'a'</span>, <span class="hljs-string">'b'</span>), (<span class="hljs-string">'a'</span>, <span class="hljs-string">'c'</span>), <span class="hljs-comment"># a . x x</span>
(<span class="hljs-string">'b'</span>, <span class="hljs-string">'a'</span>), (<span class="hljs-string">'b'</span>, <span class="hljs-string">'c'</span>), <span class="hljs-comment"># b x . x</span>
(<span class="hljs-string">'c'</span>, <span class="hljs-string">'a'</span>), (<span class="hljs-string">'c'</span>, <span class="hljs-string">'b'</span>)] <span class="hljs-comment"># c x x .</span>
</code></pre>
<pre><code class="python language-python hljs"><span class="hljs-meta">>>> </span>list(it.combinations(<span class="hljs-string">'abc'</span>, <span class="hljs-number">2</span>)) <span class="hljs-comment"># a b c</span>
[(<span class="hljs-string">'a'</span>, <span class="hljs-string">'b'</span>), (<span class="hljs-string">'a'</span>, <span class="hljs-string">'c'</span>), <span class="hljs-comment"># a . x x</span>
(<span class="hljs-string">'b'</span>, <span class="hljs-string">'c'</span>) <span class="hljs-comment"># b . . x</span>
] <span class="hljs-comment"># c . . .</span>
</code></pre>
<div><h2 id="datetime"><a href="#datetime" name="datetime">#</a>Datetime</h2><p><strong>Provides 'date', 'time', 'datetime' and 'timedelta' classes. All are immutable and hashable.</strong></p><pre><code class="python language-python hljs"><span class="hljs-comment"># $ pip3 install python-dateutil</span>
<span class="hljs-keyword">from</span> datetime <span class="hljs-keyword">import</span> date, time, datetime, timedelta, timezone
<span class="hljs-keyword">import</span> zoneinfo, dateutil.tz
</code></pre></div>
<pre><code class="python language-python apache hljs"><D> = date(year, month, day) <span class="hljs-comment"># Only accepts valid dates between AD 1 and 9999.</span>
<T> = time(hour=<span class="hljs-number">0</span>, minute=<span class="hljs-number">0</span>, second=<span class="hljs-number">0</span>) <span class="hljs-comment"># Accepts `microsecond=0, tzinfo=None, fold=0`.</span>
<DT> = datetime(year, month, day, hour=<span class="hljs-number">0</span>) <span class="hljs-comment"># Accepts `minute=0, second=0, microsecond=0, …`.</span>
<TD> = timedelta(weeks=<span class="hljs-number">0</span>, days=<span class="hljs-number">0</span>, hours=<span class="hljs-number">0</span>) <span class="hljs-comment"># Accepts `minutes=0, seconds=0, microseconds=0`.</span>
</code></pre>
<ul>
<li><strong>Times and datetimes that have defined timezone are called aware and ones that don't, naive. If time or datetime object is naive, it is presumed to be in the system's timezone!</strong></li>
<li><strong><code class="python hljs"><span class="hljs-string">'fold=1'</span></code> means the second pass in case of time jumping back (usually for one hour).</strong></li>
<li><strong>Timedelta normalizes arguments to ±days, seconds (< 86 400) and microseconds (< 1M). Its str() method returns <code class="python hljs"><span class="hljs-string">'[±D, ]H:MM:SS[.…]'</span></code> and total_seconds() a float of seconds.</strong></li>
<li><strong>Use <code class="python hljs"><span class="hljs-string">'<D/DT>.weekday()'</span></code> to get the day of the week as an int (with Monday being 0).</strong></li>
</ul>
<div><h3 id="now">Now</h3><pre><code class="python language-python hljs"><D/DTn> = D/DT.today() <span class="hljs-comment"># Current local date or naive DT. Also DT.now().</span>
<DTa> = DT.now(<tzinfo>) <span class="hljs-comment"># Aware DT from current time in passed timezone.</span>
</code></pre></div>
<ul>
<li><strong>To extract time use <code class="python hljs"><span class="hljs-string">'<DTn>.time()'</span></code>, <code class="python hljs"><span class="hljs-string">'<DTa>.time()'</span></code> or <code class="python hljs"><span class="hljs-string">'<DTa>.timetz()'</span></code>.</strong></li>
</ul>
<div><h3 id="timezones">Timezones</h3><pre><code class="python language-python apache hljs"><tzinfo> = timezone.utc <span class="hljs-comment"># Coordinated universal time. London without DST.</span>
<tzinfo> = timezone(<timedelta>) <span class="hljs-comment"># Timezone with fixed offset from universal time.</span>
<tzinfo> = dateutil.tz.tzlocal() <span class="hljs-comment"># Local timezone with dynamic offset from the UTC.</span>
<tzinfo> = zoneinfo.ZoneInfo(<span class="hljs-string">'<iana_key>'</span>) <span class="hljs-comment"># 'Continent/City_Name' zone with dynamic offset.</span>
<DTa> = <DT>.astimezone([<tzinfo>]) <span class="hljs-comment"># Converts DT to the passed or local fixed zone.</span>
<Ta/DTa> = <T/DT>.replace(tzinfo=<tzinfo>) <span class="hljs-comment"># Changes the timezone object without conversion.</span>
</code></pre></div>
<ul>
<li><strong>Timezones returned by tzlocal(), ZoneInfo() and implicit local timezone of naive objects have offsets that vary through time due to DST and historical changes of the base offset.</strong></li>
<li><strong>To get ZoneInfo() to work on Windows run <code class="python hljs"><span class="hljs-string">'> pip3 install tzdata'</span></code>.</strong></li>
</ul>
<div><h3 id="encode">Encode</h3><pre><code class="python language-python apache hljs"><D/T/DT> = D/T/DT.fromisoformat(<str>) <span class="hljs-comment"># Object from the ISO string. Raises ValueError.</span>
<DT> = DT.strptime(<str>, <span class="hljs-string">'<format>'</span>) <span class="hljs-comment"># Naive or aware datetime from the custom string.</span>
<D/DTn> = D/DT.fromordinal(<int>) <span class="hljs-comment"># Date or DT from days since the Gregorian NYE 1.</span>
<DTn> = DT.fromtimestamp(<float>) <span class="hljs-comment"># A local naive DT from seconds since the epoch.</span>
<DTa> = DT.fromtimestamp(<float>, <tz>) <span class="hljs-comment"># An aware datetime from seconds since the epoch.</span>
</code></pre></div>
<ul>
<li><strong>ISO strings come in following forms: <code class="python hljs"><span class="hljs-string">'YYYY-MM-DD'</span></code>, <code class="python hljs"><span class="hljs-string">'HH:MM:SS.mmmuuu[±HH:MM]'</span></code>, or both separated by an arbitrary character. All parts following the hours are optional.</strong></li>
<li><strong>Python uses the Unix epoch: <code class="python hljs"><span class="hljs-string">'1970-01-01 00:00 UTC'</span></code>, <code class="python hljs"><span class="hljs-string">'1970-01-01 01:00 CET'</span></code>, …</strong></li>
</ul>
<div><h3 id="decode">Decode</h3><pre><code class="python language-python hljs"><str> = <D/T/DT>.isoformat(sep=<span class="hljs-string">'T'</span>) <span class="hljs-comment"># Also `timespec='auto/hours/minutes/seconds/…'`.</span>
<str> = <D/T/DT>.strftime(<span class="hljs-string">'<format>'</span>) <span class="hljs-comment"># Returns custom string representation of object.</span>
<int> = <D/DT>.toordinal() <span class="hljs-comment"># Days since NYE 1, ignoring DT's time and zone.</span>
<float> = <DTn>.timestamp() <span class="hljs-comment"># Seconds since the epoch from a local naive DT.</span>
<float> = <DTa>.timestamp() <span class="hljs-comment"># Seconds since the epoch from an aware datetime.</span>
</code></pre></div>
<div><h3 id="format-1">Format</h3><pre><code class="python language-python hljs"><span class="hljs-meta">>>> </span>dt = datetime.strptime(<span class="hljs-string">'2025-08-14 23:39:00.00 +0200'</span>, <span class="hljs-string">'%Y-%m-%d %H:%M:%S.%f %z'</span>)
<span class="hljs-meta">>>> </span>dt.strftime(<span class="hljs-string">"%dth of %B '%y (%a), %I:%M %p %Z"</span>)
<span class="hljs-string">"14th of August '25 (Thu), 11:39 PM UTC+02:00"</span>
</code></pre></div>
<ul>
<li><strong><code class="python hljs"><span class="hljs-string">'%z'</span></code> accepts <code class="python hljs"><span class="hljs-string">'±HH[:]MM'</span></code> and returns <code class="python hljs"><span class="hljs-string">'±HHMM'</span></code> or empty string if object is naive.</strong></li>
<li><strong><code class="python hljs"><span class="hljs-string">'%Z'</span></code> accepts <code class="python hljs"><span class="hljs-string">'UTC/GMT'</span></code> and local timezone's code and returns timezone's name, <code class="python hljs"><span class="hljs-string">'UTC[±HH:MM]'</span></code> if timezone is nameless, or an empty string if object is naive.</strong></li>
</ul>
<div><h3 id="arithmetics">Arithmetics</h3><pre><code class="python language-python apache hljs"><bool> = <D/T/DTn> > <D/T/DTn> <span class="hljs-comment"># Ignores time jumps (fold attribute). Also `==`.</span>
<bool> = <DTa> > <DTa> <span class="hljs-comment"># Ignores time jumps if they share tzinfo object.</span>
<TD> = <D/DTn> - <D/DTn> <span class="hljs-comment"># Ignores jumps. Convert to UTC for actual delta.</span>
<TD> = <DTa> - <DTa> <span class="hljs-comment"># Ignores jumps if they share the tzinfo object.</span>
<D/DT> = <D/DT> ± <TD> <span class="hljs-comment"># Returned datetime can fall into a missing hour.</span>
<TD> = <TD> * <float> <span class="hljs-comment"># Also `<TD> = <TD> ± <TD>`, `<TD> = abs(<TD>)`.</span>
<float> = <TD> / <TD> <span class="hljs-comment"># Calling divmod(<TD>, <TD>) returns int and TD.</span>
</code></pre></div>
<div><h2 id="function"><a href="#function" name="function">#</a>Function</h2><p><strong>Independent block of code that returns a value when called.</strong></p><pre><code class="python language-python hljs"><span class="hljs-function"><span class="hljs-keyword">def</span> <<span class="hljs-title">func_name</span>><span class="hljs-params">(<nondefault_args>)</span>:</span> ... <span class="hljs-comment"># E.g. `func(x, y):`.</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> <<span class="hljs-title">func_name</span>><span class="hljs-params">(<default_args>)</span>:</span> ... <span class="hljs-comment"># E.g. `func(x=0, y=0):`.</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> <<span class="hljs-title">func_name</span>><span class="hljs-params">(<nondefault_args>, <default_args>)</span>:</span> ... <span class="hljs-comment"># E.g. `func(x, y=0):`.</span>
</code></pre></div>
<ul>
<li><strong>Function returns None if it doesn't encounter the <code class="python hljs"><span class="hljs-string">'return <object/expr>'</span></code> statement.</strong></li>
<li><strong>Run <code class="python hljs"><span class="hljs-string">'global <var_name>'</span></code> inside the function before assigning to the global variable.</strong></li>
<li><strong>Value of a default argument is evaluated when function is first encountered in the scope.</strong></li>
<li><strong>Any mutation of a default argument value will persist between function invocations!</strong></li>
</ul>
<div><h3 id="functioncall">Function Call</h3><pre><code class="python language-python hljs"><obj> = <function>(<positional_args>) <span class="hljs-comment"># E.g. `func(0, 0)`.</span>
<obj> = <function>(<keyword_args>) <span class="hljs-comment"># E.g. `func(x=0, y=0)`.</span>
<obj> = <function>(<positional_args>, <keyword_args>) <span class="hljs-comment"># E.g. `func(0, y=0)`.</span>
</code></pre></div>
<div><h2 id="splatoperator"><a href="#splatoperator" name="splatoperator">#</a>Splat Operator</h2><p><strong>Splat expands a collection into positional arguments, while splatty-splat expands a dictionary into keyword arguments.</strong></p><pre><code class="python language-python hljs">args, kwargs = (<span class="hljs-number">1</span>, <span class="hljs-number">2</span>), {<span class="hljs-string">'z'</span>: <span class="hljs-number">3</span>}
func(*args, **kwargs)
</code></pre></div>
<div><h4 id="isthesameas">Is the same as:</h4><pre><code class="python language-python hljs">func(<span class="hljs-number">1</span>, <span class="hljs-number">2</span>, z=<span class="hljs-number">3</span>)
</code></pre></div>
<div><h3 id="insidefunctiondefinition">Inside Function Definition</h3><p><strong>Splat combines zero or more positional arguments into a tuple, while splatty-splat combines zero or more keyword arguments into a dictionary.</strong></p><pre><code class="python language-python hljs"><span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">add</span><span class="hljs-params">(*a)</span>:</span>
<span class="hljs-keyword">return</span> sum(a)
</code></pre></div>
<pre><code class="python language-python hljs"><span class="hljs-meta">>>> </span>add(<span class="hljs-number">1</span>, <span class="hljs-number">2</span>, <span class="hljs-number">3</span>)
<span class="hljs-number">6</span>
</code></pre>
<div><h4 id="allowedcompositionsofargumentsandthewaystheycanbecalled">Allowed compositions of arguments and the ways they can be called:</h4><pre><code class="text language-text">┏━━━━━━━━━━━━━━━━━━━━━━━━━━━┯━━━━━━━━━━━━━━━━┯━━━━━━━━━━━━━━┯━━━━━━━━━━━━━━┓
┃ │ func(x=<span class="hljs-number">1</span>, y=<span class="hljs-number">2</span>) │ func(<span class="hljs-number">1</span>, y=<span class="hljs-number">2</span>) │ func(<span class="hljs-number">1</span>, <span class="hljs-number">2</span>) ┃
┠───────────────────────────┼────────────────┼──────────────┼──────────────┨
┃ <span class="hljs-title">func</span>(x, *args, **kwargs): │ ✓ │ ✓ │ ✓ ┃
┃ <span class="hljs-title">func</span>(*args, y, **kwargs): │ ✓ │ ✓ │ ┃
┃ <span class="hljs-title">func</span>(*, x, **kwargs): │ ✓ │ │ ┃
┗━━━━━━━━━━━━━━━━━━━━━━━━━━━┷━━━━━━━━━━━━━━━━┷━━━━━━━━━━━━━━┷━━━━━━━━━━━━━━┛
</code></pre></div>
<div><h3 id="otheruses">Other Uses</h3><pre><code class="python language-python hljs"><list> = [*<collection> [, ...]] <span class="hljs-comment"># Same as `list(<coll>) [+ ...]`.</span>
<tuple> = (*<collection>, [...]) <span class="hljs-comment"># Same as `tuple(<coll>) [+ ...]`.</span>
<set> = {*<collection> [, ...]} <span class="hljs-comment"># Same as `set(<coll>) [| ...]`.</span>
<dict> = {**<dict> [, ...]} <span class="hljs-comment"># Last dict has priority. Also |.</span>
</code></pre></div>
<pre><code class="python language-python hljs">head, *body, tail = <collection> <span class="hljs-comment"># Head or tail can be omitted.</span>
</code></pre>
<div><h2 id="inline"><a href="#inline" name="inline">#</a>Inline</h2><div><h3 id="lambda">Lambda</h3><pre><code class="python language-python hljs"><func> = <span class="hljs-keyword">lambda</span>: <return_value> <span class="hljs-comment"># A single statement function.</span>
<func> = <span class="hljs-keyword">lambda</span> <arg_1>, <arg_2>: <return_value> <span class="hljs-comment"># Also allows default arguments.</span>
</code></pre></div></div>
<div><h3 id="comprehensions">Comprehensions</h3><pre><code class="python language-python hljs"><list> = [i+<span class="hljs-number">1</span> <span class="hljs-keyword">for</span> i <span class="hljs-keyword">in</span> range(<span class="hljs-number">5</span>)] <span class="hljs-comment"># Returns `[1, 2, 3, 4, 5]`.</span>
<iter> = (i <span class="hljs-keyword">for</span> i <span class="hljs-keyword">in</span> range(<span class="hljs-number">10</span>) <span class="hljs-keyword">if</span> i > <span class="hljs-number">5</span>) <span class="hljs-comment"># Returns `iter([6, 7, 8, 9])`.</span>
<set> = {i+<span class="hljs-number">5</span> <span class="hljs-keyword">for</span> i <span class="hljs-keyword">in</span> range(<span class="hljs-number">5</span>)} <span class="hljs-comment"># Returns `{5, 6, 7, 8, 9}`.</span>
<dict> = {i: i**<span class="hljs-number">2</span> <span class="hljs-keyword">for</span> i <span class="hljs-keyword">in</span> range(<span class="hljs-number">1</span>, <span class="hljs-number">4</span>)} <span class="hljs-comment"># Returns `{1: 1, 2: 4, 3: 9}`.</span>
</code></pre></div>
<pre><code class="python language-python hljs"><span class="hljs-meta">>>> </span>[l+r <span class="hljs-keyword">for</span> l <span class="hljs-keyword">in</span> <span class="hljs-string">'abc'</span> <span class="hljs-keyword">for</span> r <span class="hljs-keyword">in</span> <span class="hljs-string">'abc'</span>] <span class="hljs-comment"># Inner loop is on right side.</span>
[<span class="hljs-string">'aa'</span>, <span class="hljs-string">'ab'</span>, <span class="hljs-string">'ac'</span>, ..., <span class="hljs-string">'cc'</span>]
</code></pre>
<div><h3 id="mapfilterreduce">Map, Filter, Reduce</h3><pre><code class="python language-python hljs"><span class="hljs-keyword">from</span> functools <span class="hljs-keyword">import</span> reduce
</code></pre></div>
<pre><code class="python language-python hljs"><iter> = map(<span class="hljs-keyword">lambda</span> x: x + <span class="hljs-number">1</span>, range(<span class="hljs-number">5</span>)) <span class="hljs-comment"># Returns `iter([1, 2, 3, 4, 5])`.</span>
<iter> = filter(<span class="hljs-keyword">lambda</span> x: x > <span class="hljs-number">5</span>, range(<span class="hljs-number">10</span>)) <span class="hljs-comment"># Returns `iter([6, 7, 8, 9])`.</span>
<obj> = reduce(<span class="hljs-keyword">lambda</span> out, x: out + x, range(<span class="hljs-number">5</span>)) <span class="hljs-comment"># Returns 10. Accepts 'initial'.</span>
</code></pre>
<div><h3 id="anyall">Any, All</h3><pre><code class="python language-python hljs"><bool> = any(<collection>) <span class="hljs-comment"># Is bool(<el>) True for any el?</span>
<bool> = all(<collection>) <span class="hljs-comment"># Is it True for all (or empty)?</span>
</code></pre></div>
<div><h3 id="conditionalexpression">Conditional Expression</h3><pre><code class="python language-python hljs"><obj> = <exp> <span class="hljs-keyword">if</span> <condition> <span class="hljs-keyword">else</span> <exp> <span class="hljs-comment"># Evaluates only one expression.</span>
</code></pre></div>
<pre><code class="python language-python hljs"><span class="hljs-meta">>>> </span>[i <span class="hljs-keyword">if</span> i <span class="hljs-keyword">else</span> <span class="hljs-string">'zero'</span> <span class="hljs-keyword">for</span> i <span class="hljs-keyword">in</span> (<span class="hljs-number">0</span>, <span class="hljs-number">1</span>, <span class="hljs-number">2</span>, <span class="hljs-number">3</span>)] <span class="hljs-comment"># `any(['', [], None])` is False.</span>
[<span class="hljs-string">'zero'</span>, <span class="hljs-number">1</span>, <span class="hljs-number">2</span>, <span class="hljs-number">3</span>]
</code></pre>
<div><h3 id="andor">And, Or</h3><pre><code class="python language-python hljs"><obj> = <exp> <span class="hljs-keyword">and</span> <exp> [<span class="hljs-keyword">and</span> ...] <span class="hljs-comment"># Returns first false or last obj.</span>
<obj> = <exp> <span class="hljs-keyword">or</span> <exp> [<span class="hljs-keyword">or</span> ...] <span class="hljs-comment"># Returns first true or last obj.</span>
</code></pre></div>
<div><h3 id="walrusoperator">Walrus Operator</h3><pre><code class="python language-python hljs"><span class="hljs-meta">>>> </span>[i <span class="hljs-keyword">for</span> ch <span class="hljs-keyword">in</span> <span class="hljs-string">'0123'</span> <span class="hljs-keyword">if</span> (i := int(ch)) > <span class="hljs-number">0</span>] <span class="hljs-comment"># Assigns to var in mid-sentence.</span>
[<span class="hljs-number">1</span>, <span class="hljs-number">2</span>, <span class="hljs-number">3</span>]
</code></pre></div>
<div><h3 id="namedtupleenumdataclass">Named Tuple, Enum, Dataclass</h3><pre><code class="python language-python hljs"><span class="hljs-keyword">from</span> collections <span class="hljs-keyword">import</span> namedtuple
Point = namedtuple(<span class="hljs-string">'Point'</span>, <span class="hljs-string">'x y'</span>) <span class="hljs-comment"># Creates tuple's subclass.</span>
point = Point(<span class="hljs-number">0</span>, <span class="hljs-number">0</span>) <span class="hljs-comment"># Returns its instance.</span>
<span class="hljs-keyword">from</span> enum <span class="hljs-keyword">import</span> Enum
Direction = Enum(<span class="hljs-string">'Direction'</span>, <span class="hljs-string">'N E S W'</span>) <span class="hljs-comment"># Creates an enumeration.</span>
direction = Direction.N <span class="hljs-comment"># Returns its member.</span>
<span class="hljs-keyword">from</span> dataclasses <span class="hljs-keyword">import</span> make_dataclass
Player = make_dataclass(<span class="hljs-string">'Player'</span>, [<span class="hljs-string">'loc'</span>, <span class="hljs-string">'dir'</span>]) <span class="hljs-comment"># Creates a normal class.</span>
player = Player(point, direction) <span class="hljs-comment"># Returns its instance.</span>
</code></pre></div>
<div><h2 id="imports"><a href="#imports" name="imports">#</a>Imports</h2><p><strong>Mechanism that makes code in one file available to another file.</strong></p><pre><code class="python language-python hljs"><span class="hljs-keyword">import</span> <module> <span class="hljs-comment"># Imports a built-in module or the '<module>.py'.</span>
<span class="hljs-keyword">import</span> <package> <span class="hljs-comment"># A built-in package or '<package>/__init__.py'.</span>
<span class="hljs-keyword">import</span> <package>.<module> <span class="hljs-comment"># A package's module or '<package>/<module>.py'.</span>
<span class="hljs-keyword">from</span> <pkg/mod>[.…] <span class="hljs-keyword">import</span> <obj> <span class="hljs-comment"># Imports a module, function, variable or class.</span>
</code></pre></div>
<ul>
<li><strong>Package is a collection of modules, but it can also define its own functions, variables, etc. On a filesystem this corresponds to a directory of Python files with an optional init script.</strong></li>
<li><strong><code class="python hljs"><span class="hljs-string">'import <package>'</span></code> only exposes modules that are imported inside <code class="python hljs"><span class="hljs-string">'__init__.py'</span></code>.</strong></li>
<li><strong>Directory of the file that is passed to python command serves as the root of local imports.</strong></li>
<li><strong>Use relative imports, i.e. <code class="python hljs"><span class="hljs-string">'from .[…][<pkg/mod>[.…]] import <obj>'</span></code>, if project has scattered entry points. Another option is to install the whole project by moving its code into 'src' dir, adding <a href="https://packaging.python.org/en/latest/guides/writing-pyproject-toml/#basic-information">'pyproject.toml'</a> to its root, and running <code class="python hljs"><span class="hljs-string">'$ pip3 install -e .'</span></code>.</strong></li>
</ul>
<div><h2 id="closure"><a href="#closure" name="closure">#</a>Closure</h2><p><strong>We have/get a closure in Python when a nested function references a value of its enclosing function and then the enclosing function returns its nested function (any value that is referenced from within multiple nested functions gets shared).</strong></p><pre><code class="python language-python hljs"><span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">get_multiplier</span><span class="hljs-params">(a)</span>:</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">out</span><span class="hljs-params">(b)</span>:</span>
<span class="hljs-keyword">return</span> a * b
<span class="hljs-keyword">return</span> out
</code></pre></div>
<pre><code class="python language-python hljs"><span class="hljs-meta">>>> </span>multiply_by_3 = get_multiplier(<span class="hljs-number">3</span>)
<span class="hljs-meta">>>> </span>multiply_by_3(<span class="hljs-number">10</span>)
<span class="hljs-number">30</span>
</code></pre>
<div><h3 id="partial">Partial</h3><pre><code class="python language-python hljs"><span class="hljs-keyword">from</span> functools <span class="hljs-keyword">import</span> partial
<function> = partial(<function> [, <arg_1> [, ...]])
</code></pre></div>
<pre><code class="python language-python hljs"><span class="hljs-meta">>>> </span><span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">multiply</span><span class="hljs-params">(a, b)</span>:</span>
<span class="hljs-meta">... </span> <span class="hljs-keyword">return</span> a * b
<span class="hljs-meta">>>> </span>multiply_by_3 = partial(multiply, <span class="hljs-number">3</span>)
<span class="hljs-meta">>>> </span>multiply_by_3(<span class="hljs-number">10</span>)
<span class="hljs-number">30</span>
</code></pre>
<ul>
<li><strong>Partial is also useful in cases when a function needs to be passed as an argument because it enables us to set its arguments beforehand (<code class="python hljs"><span class="hljs-string">'collections.defaultdict(<func>)'</span></code>, <code class="python hljs"><span class="hljs-string">'iter(<func>, to_exc)'</span></code> and <code class="python hljs"><span class="hljs-string">'dataclasses.field(default_factory=<func>)'</span></code>).</strong></li>
</ul>
<div><h3 id="nonlocal">Non-Local</h3><p><strong>If variable is being assigned to anywhere in the scope (i.e., body of a function), it is treated as a local variable unless it is declared <code class="python hljs"><span class="hljs-string">'global'</span></code> or <code class="python hljs"><span class="hljs-string">'nonlocal'</span></code> before its first usage.</strong></p><pre><code class="python language-python hljs"><span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">get_counter</span><span class="hljs-params">()</span>:</span>
i = <span class="hljs-number">0</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">out</span><span class="hljs-params">()</span>:</span>
<span class="hljs-keyword">nonlocal</span> i
i += <span class="hljs-number">1</span>
<span class="hljs-keyword">return</span> i
<span class="hljs-keyword">return</span> out
</code></pre></div>
<pre><code class="python language-python hljs"><span class="hljs-meta">>>> </span>counter = get_counter()
<span class="hljs-meta">>>> </span>counter(), counter(), counter()
(<span class="hljs-number">1</span>, <span class="hljs-number">2</span>, <span class="hljs-number">3</span>)
</code></pre>
<div class="pagebreak"></div><div><h2 id="decorator"><a href="#decorator" name="decorator">#</a>Decorator</h2><p><strong>A decorator takes a function, adds some functionality and returns it. It can be any <a href="#callable">callable</a>, but is usually implemented as a function that returns a <a href="#closure">closure</a>.</strong></p><pre><code class="python language-python hljs"><span class="hljs-meta">@decorator_name</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">function_that_gets_passed_to_decorator</span><span class="hljs-params">()</span>:</span>
...
</code></pre></div>
<div><h3 id="debuggerexample">Debugger Example</h3><p><strong>Decorator that prints function's name every time <em>that</em> function is called.</strong></p><pre><code class="python language-python hljs"><span class="hljs-keyword">from</span> functools <span class="hljs-keyword">import</span> wraps
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">debug</span><span class="hljs-params">(func)</span>:</span>
<span class="hljs-meta"> @wraps(func)</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">out</span><span class="hljs-params">(*args, **kwargs)</span>:</span>
print(func.__name__)
<span class="hljs-keyword">return</span> func(*args, **kwargs)
<span class="hljs-keyword">return</span> out
<span class="hljs-meta">@debug</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">add</span><span class="hljs-params">(x, y)</span>:</span>
<span class="hljs-keyword">return</span> x + y
</code></pre></div>
<ul>
<li><strong>Wraps is a helper decorator that copies the metadata of the passed function (func) to the function it is decorating (out). Without it, <code class="python hljs"><span class="hljs-string">'add.__name__'</span></code> would return <code class="python hljs"><span class="hljs-string">'out'</span></code>.</strong></li>
</ul>
<div><h3 id="cache">Cache</h3><p><strong>Decorator that caches function's return values. All function's arguments must be hashable.</strong></p><pre><code class="python language-python hljs"><span class="hljs-keyword">from</span> functools <span class="hljs-keyword">import</span> cache
<span class="hljs-meta">@cache</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">fibonacci</span><span class="hljs-params">(n)</span>:</span>
<span class="hljs-keyword">return</span> n <span class="hljs-keyword">if</span> n < <span class="hljs-number">2</span> <span class="hljs-keyword">else</span> fibonacci(n<span class="hljs-number">-2</span>) + fibonacci(n<span class="hljs-number">-1</span>)
</code></pre></div>
<ul>
<li><strong>Potential problem with cache is that it can grow indefinitely. To clear stored values run <code class="python hljs"><span class="hljs-string">'<func>.cache_clear()'</span></code>, or use <code class="python hljs"><span class="hljs-string">'@lru_cache(maxsize=<int>)'</span></code> decorator instead.</strong></li>
<li><strong>CPython interpreter limits recursion depth to 3000 by default. To increase the limit run <code class="python hljs"><span class="hljs-string">'sys.setrecursionlimit(<int>)'</span></code>.</strong></li>
</ul>
<div><h3 id="parametrizeddecorator">Parametrized Decorator</h3><p><strong>A decorator that accepts arguments and returns a normal decorator that accepts a function.</strong></p><pre><code class="python language-python hljs"><span class="hljs-keyword">from</span> functools <span class="hljs-keyword">import</span> wraps
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">debug</span><span class="hljs-params">(print_result=<span class="hljs-keyword">False</span>)</span>:</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">decorator</span><span class="hljs-params">(func)</span>:</span>
<span class="hljs-meta"> @wraps(func)</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">out</span><span class="hljs-params">(*args, **kwargs)</span>:</span>
result = func(*args, **kwargs)
print(func.__name__, result <span class="hljs-keyword">if</span> print_result <span class="hljs-keyword">else</span> <span class="hljs-string">''</span>)
<span class="hljs-keyword">return</span> result
<span class="hljs-keyword">return</span> out
<span class="hljs-keyword">return</span> decorator
<span class="hljs-meta">@debug(print_result=True)</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">add</span><span class="hljs-params">(x, y)</span>:</span>
<span class="hljs-keyword">return</span> x + y
</code></pre></div>
<ul>
<li><strong>Using only <code class="python hljs"><span class="hljs-string">'@debug'</span></code> to decorate the add() function would not work here, because debug would then receive the add() function as a 'print_result' argument. Decorators can however manually check if the argument they received is a function and act accordingly.</strong></li>
</ul>
<div><h2 id="class"><a href="#class" name="class">#</a>Class</h2><p><strong>A template for creating user-defined objects.</strong></p><pre><code class="python language-python hljs"><span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">MyClass</span>:</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__init__</span><span class="hljs-params">(self, a)</span>:</span>
self.a = a
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__str__</span><span class="hljs-params">(self)</span>:</span>
<span class="hljs-keyword">return</span> str(self.a)
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__repr__</span><span class="hljs-params">(self)</span>:</span>
class_name = self.__class__.__name__
<span class="hljs-keyword">return</span> <span class="hljs-string">f'<span class="hljs-subst">{class_name}</span>(<span class="hljs-subst">{self.a!r}</span>)'</span>
<span class="hljs-meta"> @classmethod</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">get_class_name</span><span class="hljs-params">(cls)</span>:</span>
<span class="hljs-keyword">return</span> cls.__name__
</code></pre></div>
<pre><code class="python language-python hljs"><span class="hljs-meta">>>> </span>obj = MyClass(<span class="hljs-number">1</span>)
<span class="hljs-meta">>>> </span>obj.a, str(obj), repr(obj)
(<span class="hljs-number">1</span>, <span class="hljs-string">'1'</span>, <span class="hljs-string">'MyClass(1)'</span>)
</code></pre>
<ul>
<li><strong>Methods whose names start and end with two underscores are called special methods. They are executed when object is passed to a built-in function or used as an operand, for example, <code class="python hljs"><span class="hljs-string">'print(a)'</span></code> calls <code class="python hljs"><span class="hljs-string">'a.__str__()'</span></code> and <code class="python hljs"><span class="hljs-string">'a + b'</span></code> calls <code class="python hljs"><span class="hljs-string">'a.__add__(b)'</span></code>.</strong></li>
<li><strong>Methods decorated with <code class="python hljs"><span class="hljs-string">'@staticmethod'</span></code> receive neither 'self' nor 'cls' argument.</strong></li>
<li><strong>Return value of str() special method should be readable and of repr() unambiguous. If only repr() is defined, it will also be used for str().</strong></li>
</ul>
<div><h4 id="expressionsthatcallstrspecialmethod">Expressions that call str() special method:</h4><pre><code class="python language-python hljs"><span class="hljs-string">f'<span class="hljs-subst">{obj}</span>'</span>
print(obj)
logging.warning(obj)
<csv_writer>.writerow([obj])
</code></pre></div>
<div><h4 id="expressionsthatcallreprspecialmethod">Expressions that call repr() special method:</h4><pre><code class="python language-python hljs"><span class="hljs-string">f'<span class="hljs-subst">{obj!r}</span>'</span>
print/str/repr([obj])
print/str/repr({obj: obj})
print/str/repr(MyDataClass(obj))
</code></pre></div>
<div><h3 id="subclass">Subclass</h3><ul>
<li><strong>Inheritance is a mechanism that enables a class to extend some other class (i.e., a subclass to extend its parent), and by doing so inherit all of its methods and attributes.</strong></li>
<li><strong>Subclass can then add its own methods and attributes or override inherited ones by reusing their names.</strong></li>
</ul><pre><code class="python language-python hljs"><span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">Person</span>:</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__init__</span><span class="hljs-params">(self, name)</span>:</span>
self.name = name
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__repr__</span><span class="hljs-params">(self)</span>:</span>
<span class="hljs-keyword">return</span> <span class="hljs-string">f'Person(<span class="hljs-subst">{self.name!r}</span>)'</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__lt__</span><span class="hljs-params">(self, other)</span>:</span>
<span class="hljs-keyword">return</span> self.name < other.name
<span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">Employee</span><span class="hljs-params">(Person)</span>:</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__init__</span><span class="hljs-params">(self, name, staff_num)</span>:</span>
super().__init__(name)
self.staff_num = staff_num
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__repr__</span><span class="hljs-params">(self)</span>:</span>
<span class="hljs-keyword">return</span> <span class="hljs-string">f'Employee(<span class="hljs-subst">{self.name!r}</span>, <span class="hljs-subst">{self.staff_num}</span>)'</span>
</code></pre></div>
<pre><code class="python language-python hljs"><span class="hljs-meta">>>> </span>people = [Person(<span class="hljs-string">'Bob'</span>), Employee(<span class="hljs-string">'Ann'</span>, <span class="hljs-number">0</span>)]
<span class="hljs-meta">>>> </span>sorted(people)
[Employee(<span class="hljs-string">'Ann'</span>, <span class="hljs-number">0</span>), Person(<span class="hljs-string">'Bob'</span>)]
</code></pre>
<div><h3 id="typeannotations">Type Annotations</h3><ul>
<li><strong>They add type hints to variables, arguments and functions (<code class="python hljs"><span class="hljs-string">'def f() -> <type>:'</span></code>).</strong></li>
<li><strong>Hints are used by type checkers like <a href="https://pypi.org/project/mypy/">mypy</a>, data validation libraries such as <a href="https://pypi.org/project/pydantic/">Pydantic</a> and lately also by <a href="https://pypi.org/project/Cython/">Cython</a> compiler. However, they are not enforced by CPython interpreter.</strong></li>
</ul><pre><code class="python language-python hljs"><span class="hljs-keyword">from</span> collections <span class="hljs-keyword">import</span> abc
<name>: <type> [| ...] [= <obj>]
<name>: list/set/abc.Iterable/abc.Sequence[<type>] [= <obj>]
<name>: tuple/dict[<type>, ...] [= <obj>]
</code></pre></div>
<div><h3 id="dataclass">Dataclass</h3><p><strong>Decorator that uses class variables to generate init(), repr() and eq() special methods.</strong></p><pre><code class="python language-python hljs"><span class="hljs-keyword">from</span> dataclasses <span class="hljs-keyword">import</span> dataclass, field, make_dataclass
<span class="hljs-meta">@dataclass(order=False, frozen=False)</span>
<span class="hljs-class"><span class="hljs-keyword">class</span> <<span class="hljs-title">class_name</span>>:</span>
<attr_name>: <type>
<attr_name>: <type> = <default_value>
<attr_name>: list/dict/set = field(default_factory=list/dict/set)
</code></pre></div>
<ul>
<li><strong>Objects can be made <a href="#sortable">sortable</a> with <code class="python hljs"><span class="hljs-string">'order=True'</span></code> and immutable with <code class="python hljs"><span class="hljs-string">'frozen=True'</span></code>.</strong></li>
<li><strong>For object to be <a href="#hashable">hashable</a>, all attributes must be hashable and <code class="python hljs"><span class="hljs-string">'frozen'</span></code> must be <code class="python hljs"><span class="hljs-string">'True'</span></code>.</strong></li>
<li><strong>Function field() is needed because <code class="python hljs"><span class="hljs-string">'<attr_name>: list = []'</span></code> would make a list that is shared among all instances. Its 'default_factory' argument accepts any <a href="#callable">callable</a> object.</strong></li>
<li><strong>For attributes of arbitrary type use <code class="python hljs"><span class="hljs-string">'typing.Any'</span></code>.</strong></li>
</ul>
<div><h4 id="inline-1">Inline:</h4><pre><code class="python language-python hljs">P = make_dataclass(<span class="hljs-string">'P'</span>, [<span class="hljs-string">'x'</span>, <span class="hljs-string">'y'</span>])
P = make_dataclass(<span class="hljs-string">'P'</span>, [(<span class="hljs-string">'x'</span>, float), (<span class="hljs-string">'y'</span>, float)])
P = make_dataclass(<span class="hljs-string">'P'</span>, [(<span class="hljs-string">'x'</span>, float, <span class="hljs-number">0</span>), (<span class="hljs-string">'y'</span>, float, <span class="hljs-number">0</span>)])
</code></pre></div>
<div><h3 id="property">Property</h3><p><strong>Pythonic way of implementing getters and setters.</strong></p><pre><code class="python language-python hljs"><span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">Person</span>:</span>
<span class="hljs-meta"> @property</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">name</span><span class="hljs-params">(self)</span>:</span>
<span class="hljs-keyword">return</span> <span class="hljs-string">' '</span>.join(self._name)
<span class="hljs-meta"> @name.setter</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">name</span><span class="hljs-params">(self, value)</span>:</span>
self._name = value.split()
</code></pre></div>
<pre><code class="python language-python hljs"><span class="hljs-meta">>>> </span>person = Person()
<span class="hljs-meta">>>> </span>person.name = <span class="hljs-string">'\t Guido van Rossum \n'</span>
<span class="hljs-meta">>>> </span>person.name
<span class="hljs-string">'Guido van Rossum'</span>
</code></pre>
<div><h3 id="slots">Slots</h3><p><strong>Mechanism that restricts objects to attributes listed in 'slots'.</strong></p><pre><code class="python language-python hljs"><span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">MyClassWithSlots</span>:</span>
__slots__ = [<span class="hljs-string">'a'</span>]
</code></pre></div>
<div><h3 id="copy">Copy</h3><pre><code class="python language-python hljs"><span class="hljs-keyword">from</span> copy <span class="hljs-keyword">import</span> copy, deepcopy
<object> = copy/deepcopy(<object>)
</code></pre></div>
<div><h2 id="ducktypes"><a href="#ducktypes" name="ducktypes">#</a>Duck Types</h2><p><strong>A duck type is an implicit type that prescribes a set of special methods. Any object that has those methods defined is considered a member of that duck type.</strong></p><div><h3 id="comparable">Comparable</h3><ul>
<li><strong>If eq() method is not overridden, it returns <code class="python hljs"><span class="hljs-string">'id(self) == id(other)'</span></code>, which is the same as <code class="python hljs"><span class="hljs-string">'self is other'</span></code>. That means all user-defined objects compare not equal by default (because id() returns object's memory address that is guaranteed to be unique).</strong></li>
<li><strong>Only the left side object has eq() method called, unless it returns NotImplemented, in which case the right object is consulted. Result is False if both return NotImplemented.</strong></li>
<li><strong>Method ne() (called by <code class="python hljs"><span class="hljs-string">'!='</span></code>) automatically works on any object that has eq() defined.</strong></li>
</ul><pre><code class="python language-python hljs"><span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">MyComparable</span>:</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__init__</span><span class="hljs-params">(self, a)</span>:</span>
self.a = a
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__eq__</span><span class="hljs-params">(self, other)</span>:</span>
<span class="hljs-keyword">if</span> isinstance(other, type(self)):
<span class="hljs-keyword">return</span> self.a == other.a
<span class="hljs-keyword">return</span> <span class="hljs-built_in">NotImplemented</span>
</code></pre></div></div>
<div><h3 id="hashable">Hashable</h3><ul>
<li><strong>Hashable object needs both hash() and eq() methods and its hash value must not change.</strong></li>
<li><strong>Hashable objects that compare equal must have the same hash value, meaning default hash() that returns <code class="python hljs"><span class="hljs-string">'id(self)'</span></code> will not do. That is why Python automatically makes classes unhashable if you only implement the eq() method.</strong></li>
</ul><pre><code class="python language-python hljs"><span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">MyHashable</span>:</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__init__</span><span class="hljs-params">(self, a)</span>:</span>
self._a = a
<span class="hljs-meta"> @property</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">a</span><span class="hljs-params">(self)</span>:</span>
<span class="hljs-keyword">return</span> self._a
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__eq__</span><span class="hljs-params">(self, other)</span>:</span>
<span class="hljs-keyword">if</span> isinstance(other, type(self)):
<span class="hljs-keyword">return</span> self.a == other.a
<span class="hljs-keyword">return</span> <span class="hljs-built_in">NotImplemented</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__hash__</span><span class="hljs-params">(self)</span>:</span>
<span class="hljs-keyword">return</span> hash(self.a)
</code></pre></div>
<div><h3 id="sortable">Sortable</h3><ul>
<li><strong>With 'total_ordering' decorator, you only need to provide eq() and one of lt(), gt(), le() or ge() special methods (used by <, >, <=, >=) and the rest will be automatically generated.</strong></li>
<li><strong>Functions sorted() and min() only require lt() method, while max() only requires gt(). However, it is best to define them all so that confusion doesn't arise in other contexts.</strong></li>
<li><strong>When two lists, strings or dataclasses are compared, their values get compared in order until a pair of unequal values is found. The comparison of this two values is then returned. The shorter sequence is considered smaller in case of all values being equal.</strong></li>
<li><strong>To sort a coll. of strings in proper alphabetical order pass <code class="python hljs"><span class="hljs-string">'key=locale.strxfrm'</span></code> to sorted() after running <code class="python hljs"><span class="hljs-string">'locale.setlocale(locale.LC_COLLATE, "en_US.UTF-8")'</span></code>.</strong></li>
</ul><pre><code class="python language-python hljs"><span class="hljs-keyword">from</span> functools <span class="hljs-keyword">import</span> total_ordering
<span class="hljs-meta">@total_ordering</span>
<span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">MySortable</span>:</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__init__</span><span class="hljs-params">(self, a)</span>:</span>
self.a = a
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__eq__</span><span class="hljs-params">(self, other)</span>:</span>
<span class="hljs-keyword">if</span> isinstance(other, type(self)):
<span class="hljs-keyword">return</span> self.a == other.a
<span class="hljs-keyword">return</span> <span class="hljs-built_in">NotImplemented</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__lt__</span><span class="hljs-params">(self, other)</span>:</span>
<span class="hljs-keyword">if</span> isinstance(other, type(self)):
<span class="hljs-keyword">return</span> self.a < other.a
<span class="hljs-keyword">return</span> <span class="hljs-built_in">NotImplemented</span>
</code></pre></div>
<div><h3 id="iterator-1">Iterator</h3><ul>
<li><strong>Any object that has methods next() and iter() is an iterator.</strong></li>
<li><strong>Next() should return next item or raise StopIteration exception.</strong></li>
<li><strong>Iter() should return unmodified iterator, i.e. the 'self' argument.</strong></li>
<li><strong>Any object that has iter() method can be used in a for loop.</strong></li>
</ul><pre><code class="python language-python hljs"><span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">Counter</span>:</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__init__</span><span class="hljs-params">(self)</span>:</span>
self.i = <span class="hljs-number">0</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__next__</span><span class="hljs-params">(self)</span>:</span>
self.i += <span class="hljs-number">1</span>
<span class="hljs-keyword">return</span> self.i
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__iter__</span><span class="hljs-params">(self)</span>:</span>
<span class="hljs-keyword">return</span> self
</code></pre></div>