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<aside>June 27, 2022</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://transactions.sendowl.com/products/78175486/4422834F/view">Buy PDF</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><br><div><h2 id="toc"><a href="#toc" name="toc">#</a>Contents</h2><pre><code class="hljs bash" style="line-height: 1.3em;"><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="#arguments">Args</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_Types</a>, <a href="#enum">Enum</a>, <a href="#exceptions">Exception</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="#threading">Threading</a>, <a href="#operator">Operator</a>, <a href="#introspection">Introspection</a>, <a href="#metaprogramming">Metaprograming</a>, <a href="#eval">Eval</a>, <a href="#coroutines">Coroutine</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="#curses">Curses</a>, <a href="#logging">Logging</a>, <a href="#scraping">Scraping</a>, <a href="#web">Web</a>, <a href="#profiling">Profile</a>,
<a href="#numpy">NumPy</a>, <a href="#image">Image</a>, <a href="#audio">Audio</a>, <a href="#pygame">Games</a>, <a href="#pandas">Data</a>]
}
</code></pre></div></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"># Runs main() if file wasn't imported.</span>
main()
</code></pre></div>
<div><h2 id="list"><a href="#list" name="list">#</a>List</h2><pre><code class="python language-python hljs"><list> = <list>[<slice>] <span class="hljs-comment"># Or: <list>[from_inclusive : to_exclusive : ±step]</span>
</code></pre></div>
<pre><code class="python language-python hljs"><list>.append(<el>) <span class="hljs-comment"># Or: <list> += [<el>]</span>
<list>.extend(<collection>) <span class="hljs-comment"># Or: <list> += <collection></span>
</code></pre>
<pre><code class="python language-python hljs"><list>.sort() <span class="hljs-comment"># Sorts in ascending order.</span>
<list>.reverse() <span class="hljs-comment"># Reverses the list in-place.</span>
<list> = sorted(<collection>) <span class="hljs-comment"># Returns a new sorted list.</span>
<iter> = reversed(<list>) <span class="hljs-comment"># Returns reversed iterator.</span>
</code></pre>
<pre><code class="python language-python hljs">sum_of_elements = sum(<collection>)
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>))
product_of_elems = functools.reduce(<span class="hljs-keyword">lambda</span> out, el: out * el, <collection>)
list_of_chars = list(<str>)
</code></pre>
<ul>
<li><strong>For details about sorted(), min() and max() see <a href="#sortable">sortable</a>.</strong></li>
<li><strong>Module <a href="#operator">operator</a> provides functions itemgetter() and mul() that offer the same functionality as <a href="#lambda">lambda</a> expressions above.</strong></li>
</ul>
<pre><code class="python language-python hljs"><list>.insert(<int>, <el>) <span class="hljs-comment"># Inserts item at index and moves the rest to the right.</span>
<el> = <list>.pop([<int>]) <span class="hljs-comment"># Removes and returns item at index or from the end.</span>
<int> = <list>.count(<el>) <span class="hljs-comment"># Returns number of occurrences. Also works on strings.</span>
<int> = <list>.index(<el>) <span class="hljs-comment"># Returns index of the first occurrence or raises ValueError.</span>
<list>.remove(<el>) <span class="hljs-comment"># Removes first occurrence of the item or raises ValueError.</span>
<list>.clear() <span class="hljs-comment"># Removes all items. Also works on dictionary and set.</span>
</code></pre>
<div><h2 id="dictionary"><a href="#dictionary" name="dictionary">#</a>Dictionary</h2><pre><code class="python language-python hljs"><view> = <dict>.keys() <span class="hljs-comment"># Coll. of keys that reflects changes.</span>
<view> = <dict>.values() <span class="hljs-comment"># Coll. of values that reflects changes.</span>
<view> = <dict>.items() <span class="hljs-comment"># Coll. of key-value tuples that reflects chgs.</span>
</code></pre></div>
<pre><code class="python language-python hljs">value = <dict>.get(key, default=<span class="hljs-keyword">None</span>) <span class="hljs-comment"># Returns 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"># Creates a dict with default value of type.</span>
<dict> = collections.defaultdict(<span class="hljs-keyword">lambda</span>: <span class="hljs-number">1</span>) <span class="hljs-comment"># Creates a dict with 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 a dict from two collections.</span>
<dict> = dict.fromkeys(keys [, value]) <span class="hljs-comment"># Creates a dict from collection of keys.</span>
</code></pre>
<pre><code class="python language-python hljs"><dict>.update(<dict>) <span class="hljs-comment"># Adds items. Replaces ones with matching keys.</span>
value = <dict>.pop(key) <span class="hljs-comment"># Removes item or raises KeyError.</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 == value} <span class="hljs-comment"># Returns set of keys that point to the value.</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 dictionary, filtered by 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>colors = [<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 = Counter(colors)
<span class="hljs-meta">>>> </span>counter[<span class="hljs-string">'yellow'</span>] += <span class="hljs-number">1</span>
Counter({<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>})
<span class="hljs-meta">>>> </span>counter.most_common()[<span class="hljs-number">0</span>]
(<span class="hljs-string">'blue'</span>, <span class="hljs-number">3</span>)
</code></pre></div>
<div><h2 id="set"><a href="#set" name="set">#</a>Set</h2><pre><code class="python language-python hljs"><set> = set() <span class="hljs-comment"># {} returns a dictionary.</span>
</code></pre></div>
<pre><code class="python language-python hljs"><set>.add(<el>) <span class="hljs-comment"># Or: <set> |= {<el>}</span>
<set>.update(<collection> [, ...]) <span class="hljs-comment"># Or: <set> |= <set></span>
</code></pre>
<pre><code class="python language-python hljs"><set> = <set>.union(<coll.>) <span class="hljs-comment"># Or: <set> | <set></span>
<set> = <set>.intersection(<coll.>) <span class="hljs-comment"># Or: <set> & <set></span>
<set> = <set>.difference(<coll.>) <span class="hljs-comment"># Or: <set> - <set></span>
<set> = <set>.symmetric_difference(<coll.>) <span class="hljs-comment"># Or: <set> ^ <set></span>
<bool> = <set>.issubset(<coll.>) <span class="hljs-comment"># Or: <set> <= <set></span>
<bool> = <set>.issuperset(<coll.>) <span class="hljs-comment"># Or: <set> >= <set></span>
</code></pre>
<pre><code class="python language-python hljs"><el> = <set>.pop() <span class="hljs-comment"># Raises KeyError if empty.</span>
<set>.remove(<el>) <span class="hljs-comment"># Raises KeyError if missing.</span>
<set>.discard(<el>) <span class="hljs-comment"># Doesn't raise an error.</span>
</code></pre>
<div><h3 id="frozenset">Frozen Set</h3><ul>
<li><strong>Is immutable and hashable.</strong></li>
<li><strong>That means it can be used as a key in a dictionary or as an element 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"># Or: tuple()</span>
<tuple> = (<el>,) <span class="hljs-comment"># Or: <el>,</span>
<tuple> = (<el_1>, <el_2> [, ...]) <span class="hljs-comment"># Or: <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>)
Point(x=<span class="hljs-number">1</span>, y=<span class="hljs-number">2</span>)
<span class="hljs-meta">>>> </span>p[<span class="hljs-number">0</span>]
<span class="hljs-number">1</span>
<span class="hljs-meta">>>> </span>p.x
<span class="hljs-number">1</span>
<span class="hljs-meta">>>> </span>getattr(p, <span class="hljs-string">'y'</span>)
<span class="hljs-number">2</span>
<span class="hljs-meta">>>> </span>p._fields <span class="hljs-comment"># Or: Point._fields</span>
(<span class="hljs-string">'x'</span>, <span class="hljs-string">'y'</span>)
</code></pre></div>
<div><h2 id="range"><a href="#range" name="range">#</a>Range</h2><pre><code class="python language-python hljs"><range> = range(to_exclusive)
<range> = range(from_inclusive, to_exclusive)
<range> = range(from_inclusive, to_exclusive, ±step_size)
</code></pre></div>
<pre><code class="python language-python hljs">from_inclusive = <range>.start
to_exclusive = <range>.stop
</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(<collection> [, i_start]):
...
</code></pre></div>
<div><h2 id="iterator"><a href="#iterator" name="iterator">#</a>Iterator</h2><pre><code class="python language-python hljs"><iter> = iter(<collection>) <span class="hljs-comment"># `iter(<iter>)` returns unmodified iterator.</span>
<iter> = iter(<function>, to_exclusive) <span class="hljs-comment"># A sequence of return values until 'to_exclusive'.</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>
<div><h3 id="itertools">Itertools</h3><pre><code class="python language-python hljs"><span class="hljs-keyword">from</span> itertools <span class="hljs-keyword">import</span> count, repeat, cycle, chain, islice
</code></pre></div>
<pre><code class="python language-python hljs"><iter> = count(start=<span class="hljs-number">0</span>, step=<span class="hljs-number">1</span>) <span class="hljs-comment"># Returns updated value endlessly. Accepts floats.</span>
<iter> = repeat(<el> [, times]) <span class="hljs-comment"># Returns element endlessly or 'times' times.</span>
<iter> = cycle(<collection>) <span class="hljs-comment"># Repeats the sequence endlessly.</span>
</code></pre>
<pre><code class="python language-python hljs"><iter> = chain(<coll_1>, <coll_2> [, ...]) <span class="hljs-comment"># Empties collections in order (figuratively).</span>
<iter> = chain.from_iterable(<collection>) <span class="hljs-comment"># Empties collections inside a collection in order.</span>
</code></pre>
<pre><code class="python language-python hljs"><iter> = islice(<coll>, to_exclusive) <span class="hljs-comment"># Only returns first 'to_exclusive' elements.</span>
<iter> = islice(<coll>, from_inclusive, …) <span class="hljs-comment"># `to_exclusive, step_size`.</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 is an object.</strong></li>
<li><strong>Every object has a type.</strong></li>
<li><strong>Type and class are synonymous.</strong></li>
</ul><pre><code class="python language-python hljs"><type> = type(<el>) <span class="hljs-comment"># Or: <el>.__class__</span>
<bool> = isinstance(<el>, <type>) <span class="hljs-comment"># Or: issubclass(type(<el>), <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. ABC can also manually decide whether or not a specific class is its virtual subclass, usually based on which methods the 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><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 all passed characters from both ends.</span>
</code></pre></div>
<pre><code class="python language-python hljs"><list> = <str>.split() <span class="hljs-comment"># Splits 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' str 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 elements using 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"># Checks if string contains a substring.</span>
<bool> = <str>.startswith(<sub_str>) <span class="hljs-comment"># Pass tuple of strings for multiple options.</span>
<bool> = <str>.endswith(<sub_str>) <span class="hljs-comment"># Pass tuple of strings for multiple options.</span>
<int> = <str>.find(<sub_str>) <span class="hljs-comment"># Returns start index of the first match or -1.</span>
<int> = <str>.index(<sub_str>) <span class="hljs-comment"># Same, but raises ValueError if missing.</span>
</code></pre>
<pre><code class="python language-python hljs"><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 int to Unicode character.</span>
<int> = ord(<str>) <span class="hljs-comment"># Converts Unicode character to int.</span>
</code></pre>
<ul>
<li><strong>Also: <code class="python hljs"><span class="hljs-string">'lstrip()'</span></code>, <code class="python hljs"><span class="hljs-string">'rstrip()'</span></code> and <code class="python hljs"><span class="hljs-string">'rsplit()'</span></code>.</strong></li>
<li><strong>Also: <code class="python hljs"><span class="hljs-string">'lower()'</span></code>, <code class="python hljs"><span class="hljs-string">'upper()'</span></code>, <code class="python hljs"><span class="hljs-string">'capitalize()'</span></code> and <code class="python hljs"><span class="hljs-string">'title()'</span></code>.</strong></li>
</ul>
<div><h3 id="propertymethods">Property Methods</h3><pre><code class="text language-text">┏━━━━━━━━━━━━━━━┯━━━━━━━━━━┯━━━━━━━━━━┯━━━━━━━━━━┯━━━━━━━━━━┯━━━━━━━━━━┓
┃ │ [ !#$%…] │ [a-zA-Z] │ [¼½¾] │ [²³¹] │ [0-9] ┃
┠───────────────┼──────────┼──────────┼──────────┼──────────┼──────────┨
┃ isprintable() │ ✓ │ ✓ │ ✓ │ ✓ │ ✓ ┃
┃ isalnum() │ │ ✓ │ ✓ │ ✓ │ ✓ ┃
┃ isnumeric() │ │ │ ✓ │ ✓ │ ✓ ┃
┃ isdigit() │ │ │ │ ✓ │ ✓ ┃
┃ isdecimal() │ │ │ │ │ ✓ ┃
┗━━━━━━━━━━━━━━━┷━━━━━━━━━━┷━━━━━━━━━━┷━━━━━━━━━━┷━━━━━━━━━━┷━━━━━━━━━━┛
</code></pre></div>
<ul>
<li><strong>Also: <code class="python hljs"><span class="hljs-string">'isspace()'</span></code> checks for <code class="python hljs"><span class="hljs-string">'[ \t\n\r\f\v\x1c-\x1f\x85…]'</span></code>.</strong></li>
</ul>
<div><h2 id="regex"><a href="#regex" name="regex">#</a>Regex</h2><pre><code class="python language-python hljs"><span class="hljs-keyword">import</span> re
<str> = re.sub(<regex>, new, text, count=<span class="hljs-number">0</span>) <span class="hljs-comment"># Substitutes all occurrences with 'new'.</span>
<list> = re.findall(<regex>, text) <span class="hljs-comment"># Returns all occurrences as strings.</span>
<list> = re.split(<regex>, text, maxsplit=<span class="hljs-number">0</span>) <span class="hljs-comment"># Use brackets in regex to include the matches.</span>
<Match> = re.search(<regex>, text) <span class="hljs-comment"># Searches for first occurrence of the pattern.</span>
<Match> = re.match(<regex>, text) <span class="hljs-comment"># Searches only at the beginning of the text.</span>
<iter> = re.finditer(<regex>, text) <span class="hljs-comment"># Returns all occurrences as Match objects.</span>
</code></pre></div>
<ul>
<li><strong>Argument 'new' can be a function that accepts a Match object and returns a string.</strong></li>
<li><strong>Search() and match() return None if they can't find a match.</strong></li>
<li><strong>Argument <code class="python hljs"><span class="hljs-string">'flags=re.IGNORECASE'</span></code> can be used with all functions.</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 dot also accept the <code class="python hljs"><span class="hljs-string">'\n'</span></code>.</strong></li>
<li><strong>Use <code class="python hljs"><span class="hljs-string">r'\1'</span></code> or <code class="python hljs"><span class="hljs-string">'\\1'</span></code> for backreference (<code class="python hljs"><span class="hljs-string">'\1'</span></code> returns a character with octal code 1).</strong></li>
<li><strong>Add <code class="python hljs"><span class="hljs-string">'?'</span></code> after <code class="python hljs"><span class="hljs-string">'*'</span></code> and <code class="python hljs"><span class="hljs-string">'+'</span></code> to make them non-greedy.</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 in the first bracket.</span>
<tuple> = <Match>.groups() <span class="hljs-comment"># Returns all bracketed parts.</span>
<int> = <Match>.start() <span class="hljs-comment"># Returns start index of the match.</span>
<int> = <Match>.end() <span class="hljs-comment"># Returns exclusive end index of the match.</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"># Matches decimal characters.</span>
<span class="hljs-string">'\w'</span> == <span class="hljs-string">'[a-zA-Z0-9_]'</span> <span class="hljs-comment"># Matches alphanumerics and underscore.</span>
<span class="hljs-string">'\s'</span> == <span class="hljs-string">'[ \t\n\r\f\v]'</span> <span class="hljs-comment"># Matches whitespaces.</span>
</code></pre></div>
<ul>
<li><strong>By default, decimal characters, alphanumerics and whitespaces from all alphabets are matched unless <code class="python hljs"><span class="hljs-string">'flags=re.ASCII'</span></code> argument is used.</strong></li>
<li><strong>As shown above, it restricts all special sequence matches to the first 128 characters and prevents <code class="python hljs"><span class="hljs-string">'\s'</span></code> from accepting <code class="python hljs"><span class="hljs-string">'[\x1c-\x1f]'</span></code> (the so-called separator characters).</strong></li>
<li><strong>Use a capital letter for negation (all non-ASCII characters will be matched when used in combination with ASCII flag).</strong></li>
</ul>
<div><h2 id="format"><a href="#format" name="format">#</a>Format</h2><pre><code class="python language-python hljs"><str> = <span class="hljs-string">f'<span class="hljs-subst">{<el_1>}</span>, <span class="hljs-subst">{<el_2>}</span>'</span> <span class="hljs-comment"># Curly brackets can also contain expressions.</span>
<str> = <span class="hljs-string">'{}, {}'</span>.format(<el_1>, <el_2>) <span class="hljs-comment"># Or: '{0}, {a}'.format(<el_1>, a=<el_2>)</span>
<str> = <span class="hljs-string">'%s, %s'</span> % (<el_1>, <el_2>) <span class="hljs-comment"># Redundant and inferior C style formatting.</span>
</code></pre></div>
<div><h3 id="attributes">Attributes</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.height}</span>'</span>
<span class="hljs-string">'187'</span>
<span class="hljs-meta">>>> </span><span class="hljs-string">'{p.height}'</span>.format(p=person)
<span class="hljs-string">'187'</span>
</code></pre></div>
<div><h3 id="generaloptions">General Options</h3><pre><code class="python language-python hljs">{<el>:<<span class="hljs-number">10</span>} <span class="hljs-comment"># '<el> '</span>
{<el>:^<span class="hljs-number">10</span>} <span class="hljs-comment"># ' <el> '</span>
{<el>:><span class="hljs-number">10</span>} <span class="hljs-comment"># ' <el>'</span>
{<el>:.<<span class="hljs-number">10</span>} <span class="hljs-comment"># '<el>......'</span>
{<el>:<span class="hljs-number">0</span>} <span class="hljs-comment"># '<el>'</span>
</code></pre></div>
<ul>
<li><strong>Options can be generated dynamically: <code class="python hljs"><span class="hljs-string">f'<span class="hljs-subst">{<el>:{<str/int>}</span>[…]}'</span></code>.</strong></li>
<li><strong>Adding <code class="python hljs"><span class="hljs-string">'!r'</span></code> before the colon converts object to string by calling its <a href="#class">repr()</a> method.</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>When both rounding up and rounding down are possible, the one that returns result with even last digit is chosen. That makes <code class="python hljs"><span class="hljs-string">'{6.5:.0f}'</span></code> a <code class="python hljs"><span class="hljs-string">'6'</span></code> and <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>This 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"># 'Z'</span>
{<span class="hljs-number">90</span>:b} <span class="hljs-comment"># '1011010'</span>
{<span class="hljs-number">90</span>:X} <span class="hljs-comment"># '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"># Or: math.floor(<float>)</span>
<float> = float(<int/str/bool>) <span class="hljs-comment"># Or: <real>e±<int></span>
<complex> = complex(real=<span class="hljs-number">0</span>, imag=<span class="hljs-number">0</span>) <span class="hljs-comment"># Or: <real> ± <real>j</span>
<Fraction> = fractions.Fraction(<span class="hljs-number">0</span>, <span class="hljs-number">1</span>) <span class="hljs-comment"># Or: Fraction(numerator=0, denominator=1)</span>
<Decimal> = decimal.Decimal(<str/int>) <span class="hljs-comment"># Or: Decimal((sign, digits, exponent))</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 on malformed strings.</strong></li>
<li><strong>Decimal numbers are stored 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>)'</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>
</ul>
<div><h3 id="basicfunctions">Basic Functions</h3><pre><code class="python language-python hljs"><num> = pow(<num>, <num>) <span class="hljs-comment"># Or: <num> ** <num></span>
<num> = abs(<num>) <span class="hljs-comment"># <float> = abs(<complex>)</span>
<num> = round(<num> [, ±ndigits]) <span class="hljs-comment"># `round(126, -1) == 130`</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> e, pi, inf, nan, isinf, isnan
<span class="hljs-keyword">from</span> math <span class="hljs-keyword">import</span> sin, cos, tan, asin, acos, atan, degrees, radians
<span class="hljs-keyword">from</span> math <span class="hljs-keyword">import</span> log, log10, log2
</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, variance, stdev, quantiles, groupby
</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, choice, shuffle, gauss, seed
<float> = random() <span class="hljs-comment"># A float inside [0, 1).</span>
<int> = randint(from_inc, to_inc) <span class="hljs-comment"># An int inside [from_inc, to_inc].</span>
<el> = choice(<sequence>) <span class="hljs-comment"># Keeps the sequence intact.</span>
</code></pre></div>
<div><h3 id="binhex">Bin, Hex</h3><pre><code class="python language-python hljs"><int> = ±<span class="hljs-number">0</span>b<bin> <span class="hljs-comment"># Or: ±0x<hex></span>
<int> = int(<span class="hljs-string">'±<bin>'</span>, <span class="hljs-number">2</span>) <span class="hljs-comment"># Or: int('±<hex>', 16)</span>
<int> = int(<span class="hljs-string">'±0b<bin>'</span>, <span class="hljs-number">0</span>) <span class="hljs-comment"># Or: int('±0x<hex>', 0)</span>
<str> = bin(<int>) <span class="hljs-comment"># Returns '[-]0b<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"># And (0b1100 & 0b1010 == 0b1000).</span>
<int> = <int> | <int> <span class="hljs-comment"># Or (0b1100 | 0b1010 == 0b1110).</span>
<int> = <int> ^ <int> <span class="hljs-comment"># Xor (0b1100 ^ 0b1010 == 0b0110).</span>
<int> = <int> << n_bits <span class="hljs-comment"># Left shift. Use >> for right.</span>
<int> = ~<int> <span class="hljs-comment"># Not. Also -<int> - 1.</span>
</code></pre></div>
<div><h2 id="combinatorics"><a href="#combinatorics" name="combinatorics">#</a>Combinatorics</h2><ul>
<li><strong>Every function returns an iterator.</strong></li>
<li><strong>If you want to print the iterator, you need to pass it to the list() function first!</strong></li>
</ul><pre><code class="python language-python hljs"><span class="hljs-keyword">from</span> itertools <span class="hljs-keyword">import</span> product, combinations, combinations_with_replacement, permutations
</code></pre></div>
<pre><code class="python language-python hljs"><span class="hljs-meta">>>> </span>product([<span class="hljs-number">0</span>, <span class="hljs-number">1</span>], repeat=<span class="hljs-number">3</span>)
[(<span class="hljs-number">0</span>, <span class="hljs-number">0</span>, <span class="hljs-number">0</span>), (<span class="hljs-number">0</span>, <span class="hljs-number">0</span>, <span class="hljs-number">1</span>), (<span class="hljs-number">0</span>, <span class="hljs-number">1</span>, <span class="hljs-number">0</span>), (<span class="hljs-number">0</span>, <span class="hljs-number">1</span>, <span class="hljs-number">1</span>), ..., (<span class="hljs-number">1</span>, <span class="hljs-number">1</span>, <span class="hljs-number">1</span>)]
</code></pre>
<pre><code class="python language-python hljs"><span class="hljs-meta">>>> </span>product(<span class="hljs-string">'abc'</span>, <span class="hljs-string">'abc'</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>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>
</code></pre>
<pre><code class="python language-python hljs"><span class="hljs-meta">>>> </span>combinations_with_replacement(<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">'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">'b'</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">'c'</span>)] <span class="hljs-comment"># c . . x</span>
</code></pre>
<pre><code class="python language-python hljs"><span class="hljs-meta">>>> </span>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>
<div><h2 id="datetime"><a href="#datetime" name="datetime">#</a>Datetime</h2><ul>
<li><strong>Module 'datetime' provides 'date' <code class="apache hljs"><span class="hljs-section"><D></span></code>, 'time' <code class="apache hljs"><span class="hljs-section"><T></span></code>, 'datetime' <code class="apache hljs"><span class="hljs-section"><DT></span></code> and 'timedelta' <code class="apache hljs"><span class="hljs-section"><TD></span></code> classes. All are immutable and hashable.</strong></li>
<li><strong>Time and datetime objects can be 'aware' <code class="apache hljs"><span class="hljs-section"><a></span></code>, meaning they have defined timezone, or 'naive' <code class="apache hljs"><span class="hljs-section"><n></span></code>, meaning they don't.</strong></li>
<li><strong>If object is naive, it is presumed to be in the system's timezone.</strong></li>
</ul><pre><code class="python language-python hljs"><span class="hljs-keyword">from</span> datetime <span class="hljs-keyword">import</span> date, time, datetime, timedelta
<span class="hljs-keyword">from</span> dateutil.tz <span class="hljs-keyword">import</span> UTC, tzlocal, gettz, datetime_exists, resolve_imaginary
</code></pre></div>
<div><h3 id="constructors">Constructors</h3><pre><code class="python language-python apache hljs"><D> = date(year, month, day)
<T> = time(hour=<span class="hljs-number">0</span>, minute=<span class="hljs-number">0</span>, second=<span class="hljs-number">0</span>, microsecond=<span class="hljs-number">0</span>, tzinfo=<span class="hljs-keyword">None</span>, fold=<span class="hljs-number">0</span>)
<DT> = datetime(year, month, day, hour=<span class="hljs-number">0</span>, minute=<span class="hljs-number">0</span>, second=<span class="hljs-number">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>, minutes=<span class="hljs-number">0</span>, seconds=<span class="hljs-number">0</span>, ...)
</code></pre></div>
<ul>
<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>
<li><strong><code class="python hljs"><span class="hljs-string">'fold=1'</span></code> means the second pass in case of time jumping back for one hour.</strong></li>
<li><strong>Timedelta normalizes arguments to ±days, seconds (< 86 400) and microseconds (< 1M).</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 datetime.</span>
<DTn> = DT.utcnow() <span class="hljs-comment"># Naive datetime from current UTC time.</span>
<DTa> = DT.now(<tzinfo>) <span class="hljs-comment"># Aware datetime from current tz time.</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="timezone">Timezone</h3><pre><code class="python language-python apache hljs"><tzinfo> = UTC <span class="hljs-comment"># UTC timezone. London without DST.</span>
<tzinfo> = tzlocal() <span class="hljs-comment"># Local timezone. Also gettz().</span>
<tzinfo> = gettz(<span class="hljs-string">'<Continent>/<City>'</span>) <span class="hljs-comment"># 'Continent/City_Name' timezone or None.</span>
<DTa> = <DT>.astimezone(<tzinfo>) <span class="hljs-comment"># Datetime, converted to the passed timezone.</span>
<Ta/DTa> = <T/DT>.replace(tzinfo=<tzinfo>) <span class="hljs-comment"># Unconverted object with a new timezone.</span>
</code></pre></div>
<div><h3 id="encode">Encode</h3><pre><code class="python language-python apache hljs"><D/T/DT> = D/T/DT.fromisoformat(<span class="hljs-string">'<iso>'</span>) <span class="hljs-comment"># Object from ISO string. Raises ValueError.</span>
<DT> = DT.strptime(<str>, <span class="hljs-string">'<format>'</span>) <span class="hljs-comment"># Datetime from str, according to format.</span>
<D/DTn> = D/DT.fromordinal(<int>) <span class="hljs-comment"># D/DTn from days since the Gregorian NYE 1.</span>
<DTn> = DT.fromtimestamp(<real>) <span class="hljs-comment"># Local time DTn from seconds since the Epoch.</span>
<DTa> = DT.fromtimestamp(<real>, <tz.>) <span class="hljs-comment"># 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 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"># Custom string representation.</span>
<int> = <D/DT>.toordinal() <span class="hljs-comment"># Days since Gregorian NYE 1, ignoring time and tz.</span>
<float> = <DTn>.timestamp() <span class="hljs-comment"># Seconds since the Epoch, from DTn in local tz.</span>
<float> = <DTa>.timestamp() <span class="hljs-comment"># Seconds since the Epoch, from 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">'2015-05-14 23:39:00.00 +2000'</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">"%A, %dth of %B '%y, %I:%M%p %Z"</span>)
<span class="hljs-string">"Thursday, 14th of May '15, 11:39PM UTC+02:00"</span>
</code></pre></div>
<ul>
<li><strong><code class="python hljs"><span class="hljs-string">'%Z'</span></code> only accepts <code class="python hljs"><span class="hljs-string">'UTC/GMT'</span></code> and local timezone's code. <code class="python hljs"><span class="hljs-string">'%z'</span></code> also accepts <code class="python hljs"><span class="hljs-string">'±HH:MM'</span></code>.</strong></li>
<li><strong>For abbreviated weekday and month use <code class="python hljs"><span class="hljs-string">'%a'</span></code> and <code class="python hljs"><span class="hljs-string">'%b'</span></code>.</strong></li>
</ul>
<div><h3 id="arithmetics">Arithmetics</h3><pre><code class="python language-python apache hljs"><D/DT> = <D/DT> ± <TD> <span class="hljs-comment"># Returned datetime can fall into missing hour.</span>
<TD> = <D/DTn> - <D/DTn> <span class="hljs-comment"># Returns the difference, ignoring time jumps.</span>
<TD> = <DTa> - <DTa> <span class="hljs-comment"># Ignores time jumps if they share tzinfo object.</span>
<TD> = <TD> * <real> <span class="hljs-comment"># Also: <TD> = abs(<TD>) and <TD> = <TD> ±% <TD>.</span>
<float> = <TD> / <TD> <span class="hljs-comment"># How many weeks/years there are in TD. Also //.</span>
</code></pre></div>
<div><h2 id="arguments"><a href="#arguments" name="arguments">#</a>Arguments</h2><div><h3 id="insidefunctioncall">Inside Function Call</h3><pre><code class="python language-python hljs">func(<positional_args>) <span class="hljs-comment"># func(0, 0)</span>
func(<keyword_args>) <span class="hljs-comment"># func(x=0, y=0)</span>
func(<positional_args>, <keyword_args>) <span class="hljs-comment"># func(0, y=0)</span>
</code></pre></div></div>
<div><h3 id="insidefunctiondefinition">Inside Function Definition</h3><pre><code class="python language-python hljs"><span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">func</span><span class="hljs-params">(<nondefault_args>)</span>:</span> ... <span class="hljs-comment"># def func(x, y): ...</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">func</span><span class="hljs-params">(<default_args>)</span>:</span> ... <span class="hljs-comment"># def func(x=0, y=0): ...</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">func</span><span class="hljs-params">(<nondefault_args>, <default_args>)</span>:</span> ... <span class="hljs-comment"># def func(x, y=0): ...</span>
</code></pre></div>
<ul>
<li><strong>Default values are evaluated when function is first encountered in the scope.</strong></li>
<li><strong>Any mutations of mutable default values will persist between invocations.</strong></li>
</ul>
<div><h2 id="splatoperator"><a href="#splatoperator" name="splatoperator">#</a>Splat Operator</h2><div><h3 id="insidefunctioncall-1">Inside Function Call</h3><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 = (<span class="hljs-number">1</span>, <span class="hljs-number">2</span>)
kwargs = {<span class="hljs-string">'x'</span>: <span class="hljs-number">3</span>, <span class="hljs-string">'y'</span>: <span class="hljs-number">4</span>, <span class="hljs-string">'z'</span>: <span class="hljs-number">5</span>}
func(*args, **kwargs)
</code></pre></div></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>, x=<span class="hljs-number">3</span>, y=<span class="hljs-number">4</span>, z=<span class="hljs-number">5</span>)
</code></pre></div>
<div><h3 id="insidefunctiondefinition-1">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="legalargumentcombinations">Legal argument combinations:</h4><pre><code class="python language-python hljs"><span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">f</span><span class="hljs-params">(*, x, y, z)</span>:</span> ... <span class="hljs-comment"># f(x=1, y=2, z=3)</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">f</span><span class="hljs-params">(x, *, y, z)</span>:</span> ... <span class="hljs-comment"># f(x=1, y=2, z=3) | f(1, y=2, z=3)</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">f</span><span class="hljs-params">(x, y, *, z)</span>:</span> ... <span class="hljs-comment"># f(x=1, y=2, z=3) | f(1, y=2, z=3) | f(1, 2, z=3)</span>
</code></pre></div>
<pre><code class="python language-python hljs"><span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">f</span><span class="hljs-params">(*args)</span>:</span> ... <span class="hljs-comment"># f(1, 2, 3)</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">f</span><span class="hljs-params">(x, *args)</span>:</span> ... <span class="hljs-comment"># f(1, 2, 3)</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">f</span><span class="hljs-params">(*args, z)</span>:</span> ... <span class="hljs-comment"># f(1, 2, z=3)</span>
</code></pre>
<pre><code class="python language-python hljs"><span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">f</span><span class="hljs-params">(**kwargs)</span>:</span> ... <span class="hljs-comment"># f(x=1, y=2, z=3)</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">f</span><span class="hljs-params">(x, **kwargs)</span>:</span> ... <span class="hljs-comment"># f(x=1, y=2, z=3) | f(1, y=2, z=3)</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">f</span><span class="hljs-params">(*, x, **kwargs)</span>:</span> ... <span class="hljs-comment"># f(x=1, y=2, z=3)</span>
</code></pre>
<pre><code class="python language-python hljs"><span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">f</span><span class="hljs-params">(*args, **kwargs)</span>:</span> ... <span class="hljs-comment"># f(x=1, y=2, z=3) | f(1, y=2, z=3) | f(1, 2, z=3) | f(1, 2, 3)</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">f</span><span class="hljs-params">(x, *args, **kwargs)</span>:</span> ... <span class="hljs-comment"># f(x=1, y=2, z=3) | f(1, y=2, z=3) | f(1, 2, z=3) | f(1, 2, 3)</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">f</span><span class="hljs-params">(*args, y, **kwargs)</span>:</span> ... <span class="hljs-comment"># f(x=1, y=2, z=3) | f(1, y=2, z=3)</span>
</code></pre>
<div><h3 id="otheruses">Other Uses</h3><pre><code class="python language-python hljs"><list> = [*<coll.> [, ...]] <span class="hljs-comment"># Or: list(<collection>) [+ ...]</span>
<tuple> = (*<coll.>, [...]) <span class="hljs-comment"># Or: tuple(<collection>) [+ ...]</span>
<set> = {*<coll.> [, ...]} <span class="hljs-comment"># Or: set(<collection>) [| ...]</span>
<dict> = {**<dict> [, ...]} <span class="hljs-comment"># Or: dict(**<dict> [, ...])</span>
</code></pre></div>
<pre><code class="python language-python hljs">head, *body, tail = <coll.> <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 accepts 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">10</span>)] <span class="hljs-comment"># Or: [1, 2, ..., 10]</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"># Or: 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">10</span>)} <span class="hljs-comment"># Or: {5, 6, ..., 14}</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">10</span>)} <span class="hljs-comment"># Or: {0: 0, 1: 2, ..., 9: 18}</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-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"><iter> = map(<span class="hljs-keyword">lambda</span> x: x + <span class="hljs-number">1</span>, range(<span class="hljs-number">10</span>)) <span class="hljs-comment"># Or: iter([1, 2, ..., 10])</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"># Or: iter([6, 7, 8, 9])</span>
<obj> = reduce(<span class="hljs-keyword">lambda</span> out, x: out + x, range(<span class="hljs-number">10</span>)) <span class="hljs-comment"># Or: 45</span>
</code></pre></div>
<ul>
<li><strong>Reduce must be imported from the functools module.</strong></li>
</ul>
<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 element.</span>
<bool> = all(<collection>) <span class="hljs-comment"># Is True for all elements 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"># Only one expression gets evaluated.</span>
</code></pre></div>
<pre><code class="python language-python hljs"><span class="hljs-meta">>>> </span>[a <span class="hljs-keyword">if</span> a <span class="hljs-keyword">else</span> <span class="hljs-string">'zero'</span> <span class="hljs-keyword">for</span> a <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-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="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 a 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>
</code></pre></div>
<pre><code class="python language-python hljs"><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 enum.</span>
direction = Direction.n <span class="hljs-comment"># Returns its member.</span>
</code></pre>
<pre><code class="python language-python hljs"><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 class.</span>
player = Player(point, direction) <span class="hljs-comment"># Returns its instance.</span>
</code></pre>
<div><h2 id="imports"><a href="#imports" name="imports">#</a>Imports</h2><pre><code class="python language-python hljs"><span class="hljs-keyword">import</span> <module> <span class="hljs-comment"># Imports a built-in or '<module>.py'.</span>
<span class="hljs-keyword">import</span> <package> <span class="hljs-comment"># Imports a built-in or '<package>/__init__.py'.</span>
<span class="hljs-keyword">import</span> <package>.<module> <span class="hljs-comment"># Imports a built-in or '<package>/<module>.py'.</span>
</code></pre></div>
<ul>
<li><strong>Package is a collection of modules, but it can also define its own objects.</strong></li>
<li><strong>On a filesystem this corresponds to a directory of Python files with an optional init script.</strong></li>
<li><strong>Running <code class="python hljs"><span class="hljs-string">'import <package>'</span></code> does not automatically provide access to the package's modules unless they are explicitly imported in its init script.</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:</strong></p><ul>
<li><strong>A nested function references a value of its enclosing function and then</strong></li>
<li><strong>the enclosing function returns the nested function.</strong></li>
</ul><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>
<ul>
<li><strong>If multiple nested functions within enclosing function reference the same value, that value gets shared.</strong></li>
<li><strong>To dynamically access function's first free variable use <code class="python hljs"><span class="hljs-string">'<function>.__closure__[0].cell_contents'</span></code>.</strong></li>
</ul>
<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>, <arg_2>, ...])
</code></pre></div>
<pre><code class="python language-python hljs"><span class="hljs-meta">>>> </span><span class="hljs-keyword">import</span> operator <span class="hljs-keyword">as</span> op
<span class="hljs-meta">>>> </span>multiply_by_3 = partial(op.mul, <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 function needs to be passed as an argument because it enables us to set its arguments beforehand.</strong></li>
<li><strong>A few examples being: <code class="python hljs"><span class="hljs-string">'defaultdict(<function>)'</span></code>, <code class="python hljs"><span class="hljs-string">'iter(<function>, to_exclusive)'</span></code> and dataclass's <code class="python hljs"><span class="hljs-string">'field(default_factory=<function>)'</span></code>.</strong></li>
</ul>
<div><h3 id="nonlocal">Non-Local</h3><p><strong>If variable is being assigned to anywhere in the scope, it is regarded as a local variable, unless it is declared as a 'global' or a 'nonlocal'.</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><ul>
<li><strong>A decorator takes a function, adds some functionality and returns it.</strong></li>
<li><strong>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></li>
</ul><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 the 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 wrapping (out).</strong></li>
<li><strong>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="lrucache">LRU 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> lru_cache
<span class="hljs-meta">@lru_cache(maxsize=None)</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">fib</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> fib(n-<span class="hljs-number">2</span>) + fib(n-<span class="hljs-number">1</span>)
</code></pre></div>
<ul>
<li><strong>Default size of the cache is 128 values. Passing <code class="python hljs"><span class="hljs-string">'maxsize=None'</span></code> makes it unbounded.</strong></li>
<li><strong>CPython interpreter limits recursion depth to 1000 by default. To increase it use <code class="python hljs"><span class="hljs-string">'sys.setrecursionlimit(<depth>)'</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><pre><code class="python language-python hljs"><span class="hljs-class"><span class="hljs-keyword">class</span> <<span class="hljs-title">name</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">__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-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-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>
<ul>
<li><strong>Return value of repr() should be unambiguous and of str() readable.</strong></li>
<li><strong>If only repr() is defined, it will also be used for str().</strong></li>
<li><strong>Methods decorated with <code class="python hljs"><span class="hljs-string">'@staticmethod'</span></code> do not receive 'self' nor 'cls' as their first arg.</strong></li>
</ul>
<div><h4 id="strusecases">Str() use cases:</h4><pre><code class="python language-python hljs">print(<el>)
<span class="hljs-string">f'<span class="hljs-subst">{<el>}</span>'</span>
logging.warning(<el>)
csv.writer(<file>).writerow([<el>])
<span class="hljs-keyword">raise</span> Exception(<el>)
</code></pre></div>
<div><h4 id="reprusecases">Repr() use cases:</h4><pre><code class="python language-python hljs">print/str/repr([<el>])
<span class="hljs-string">f'<span class="hljs-subst">{<el>!r}</span>'</span>
Z = dataclasses.make_dataclass(<span class="hljs-string">'Z'</span>, [<span class="hljs-string">'a'</span>]); print/str/repr(Z(<el>))
<span class="hljs-meta">>>> </span><el>
</code></pre></div>
<div><h3 id="constructoroverloading">Constructor Overloading</h3><pre><code class="python language-python hljs"><span class="hljs-class"><span class="hljs-keyword">class</span> <<span class="hljs-title">name</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 class="hljs-keyword">None</span>)</span>:</span>
self.a = a
</code></pre></div>
<div><h3 id="inheritance">Inheritance</h3><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, age)</span>:</span>
self.name = name
self.age = age
<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, age, staff_num)</span>:</span>
super().__init__(name, age)
self.staff_num = staff_num
</code></pre></div>
<div><h3 id="multipleinheritance">Multiple Inheritance</h3><pre><code class="python language-python hljs"><span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">A</span>:</span> <span class="hljs-keyword">pass</span>
<span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">B</span>:</span> <span class="hljs-keyword">pass</span>
<span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">C</span><span class="hljs-params">(A, B)</span>:</span> <span class="hljs-keyword">pass</span>
</code></pre></div>
<p><strong>MRO determines the order in which parent classes are traversed when searching for a method or an attribute:</strong></p>
<pre><code class="python language-python hljs"><span class="hljs-meta">>>> </span>C.mro()
[<<span class="hljs-class"><span class="hljs-title">class</span> '<span class="hljs-title">C</span>'>, <<span class="hljs-title">class</span> '<span class="hljs-title">A</span>'>, <<span class="hljs-title">class</span> '<span class="hljs-title">B</span>'>, <<span class="hljs-title">class</span> '<span class="hljs-title">object</span>'>]
</span></code></pre>
<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="dataclass">Dataclass</h3><p><strong>Decorator that automatically generates 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
<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_1>: <type>
<attr_name_2>: <type> = <default_value>
<attr_name_3>: list/dict/set = field(default_factory=list/dict/set)
</code></pre></div>
<ul>
<li><strong>Objects can be made sortable 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 hashable, all attributes must be hashable and 'frozen' must be True.</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 can be any <a href="#callable">callable</a>.</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"><span class="hljs-keyword">from</span> dataclasses <span class="hljs-keyword">import</span> make_dataclass
<class> = make_dataclass(<span class="hljs-string">'<class_name>'</span>, <coll_of_attribute_names>)
<class> = make_dataclass(<span class="hljs-string">'<class_name>'</span>, <coll_of_tuples>)
<tuple> = (<span class="hljs-string">'<attr_name>'</span>, <type> [, <default_value>])</code></pre></div>
<div><h4 id="restoftypeannotationscpythoninterpreterignoresthemall">Rest of type annotations (CPython interpreter ignores them all):</h4><pre><code class="python language-python hljs"><span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">func</span><span class="hljs-params">(<arg_name>: <type> [= <obj>])</span> -> <type>:</span> ...
<var_name>: typing.List/Set/Iterable/Sequence/Optional[<type>]
<var_name>: typing.Dict/Tuple/Union[<type>, ...]
</code></pre></div>
<div><h3 id="slots">Slots</h3><p><strong>Mechanism that restricts objects to attributes listed in 'slots' and significantly reduces their memory footprint.</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>]
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__init__</span><span class="hljs-params">(self)</span>:</span>
self.a = <span class="hljs-number">1</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(<object>)
<object> = 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>.</strong></li>
<li><strong>That means all objects compare not equal by default.</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. False is returned if both return NotImplemented.</strong></li>
<li><strong>Ne() 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 should never 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.</strong></li>
<li><strong>That is why Python automatically makes classes unhashable if you only implement eq().</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 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>
</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.</strong></li>
<li><strong>Iter() should return 'self'.</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>
<pre><code class="python language-python hljs"><span class="hljs-meta">>>> </span>counter = Counter()
<span class="hljs-meta">>>> </span>next(counter), next(counter), next(counter)
(<span class="hljs-number">1</span>, <span class="hljs-number">2</span>, <span class="hljs-number">3</span>)
</code></pre>
<div><h4 id="pythonhasmanydifferentiteratorobjects">Python has many different iterator objects:</h4><ul>
<li><strong>Sequence iterators returned by the <a href="#iterator">iter()</a> function, such as list_iterator and set_iterator.</strong></li>