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<title>Comprehensive Python Cheatsheet</title>
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<header>
<aside>March 14, 2018</aside>
<a href="../" rel="author">Jure Šorn</a>
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<a href="javascript:" id="return-to-top"><i class="icon-chevron-up"></i></a>
<div><h1 id="comprehensivepythoncheatsheet">Comprehensive Python Cheatsheet</h1><p class="banner"><sup><a href="https://raw.githubusercontent.com/gto76/python-cheatsheet/master/README.md">Download text file</a>, <a href="web/python-cheatsheet-6bfe293.pdf">PDF</a>, <a href="https://github.com/gto76/python-cheatsheet">Fork me on GitHub</a> or <a href="https://github.com/gto76/python-cheatsheet/wiki/Frequently-Asked-Questions">Check out FAQ</a>.
</sup></p><p class="banner"><img src="web/image_888.jpeg" alt="Monty Python"></p><br><div><h2 id="toc"><a href="#toc" name="toc">#</a>Contents</h2><pre><code class="hljs bash"><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="#closure">Closure</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">Exceptions</a>],
<strong><span class="hljs-string"><span class="hljs-string">'4. System'</span></span></strong>: [<a href="#print">Print</a>, <a href="#input">Input</a>, <a href="#commandlinearguments">Command_Line_Arguments</a>, <a href="#open">Open</a>, <a href="#path">Path</a>, <a href="#commandexecution">Command_Execution</a>],
<strong><span class="hljs-string"><span class="hljs-string">'5. Data'</span></span></strong>: [<a href="#csv">CSV</a>, <a href="#json">JSON</a>, <a href="#pickle">Pickle</a>, <a href="#sqlite">SQLite</a>, <a href="#bytes">Bytes</a>, <a href="#struct">Struct</a>, <a href="#array">Array</a>, <a href="#memoryview">MemoryView</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="#metaprograming">Metaprograming</a>, <a href="#eval">Eval</a>, <a href="#coroutine">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="#profile">Profile</a>,
<a href="#numpy">NumPy</a>, <a href="#image">Image</a>, <a href="#audio">Audio</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>[from_inclusive : to_exclusive : ±step_size]
</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()
<list>.reverse()
<list> = sorted(<collection>)
<iter> = reversed(<list>)
</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, x: out * x, <collection>)
list_of_chars = list(<str>)
</code></pre>
<pre><code class="python language-python hljs"><int> = <list>.count(<el>) <span class="hljs-comment"># Returns number of occurrences. Also works on strings.</span>
index = <list>.index(<el>) <span class="hljs-comment"># Returns index of first occurrence or raises ValueError.</span>
<list>.insert(index, <el>) <span class="hljs-comment"># Inserts item at index and moves the rest to the right.</span>
<el> = <list>.pop([index]) <span class="hljs-comment"># Removes and returns item at index or from the end.</span>
<list>.remove(<el>) <span class="hljs-comment"># Removes first occurrence of 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.</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 does not exist.</span>
value = <dict>.setdefault(key, default=<span class="hljs-keyword">None</span>) <span class="hljs-comment"># Same, but also adds default to dict.</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>.update(<dict>)
<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">value = <dict>.pop(key) <span class="hljs-comment"># Removes item or raises KeyError.</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"># Filters dictionary 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">'red'</span>, <span class="hljs-string">'blue'</span>, <span class="hljs-string">'yellow'</span>, <span class="hljs-string">'blue'</span>, <span class="hljs-string">'red'</span>, <span class="hljs-string">'blue'</span>]
<span class="hljs-meta">>>> </span>counter = Counter(colors)
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()
</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> = ()
<tuple> = (<el>, )
<tuple> = (<el_1>, <el_2>, ...)
</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"># Sequence of return values until 'to_exclusive'.</span>
<el> = next(<iter> [, default]) <span class="hljs-comment"># Raises StopIteration or returns 'default' on end.</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 incremented value endlessly.</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 indefinitely.</span>
</code></pre>
<pre><code class="python language-python hljs"><iter> = chain(<coll.>, <coll.> [, ...]) <span class="hljs-comment"># Empties collections in order.</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(<collection>, to_exclusive)
<iter> = islice(<collection>, from_inclusive, to_exclusive)
<iter> = islice(<collection>, from_inclusive, to_exclusive, +step_size)
</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 builtin 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
</code></pre></div>
<div><h3 id="abc">ABC</h3><p><strong>An abstract base class introduces virtual subclasses, that don’t inherit from it but are still recognized by isinstance() and issubclass().</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> Sequence, Collection, Iterable
<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">+------------------+----------+------------+----------+
| | Sequence | Collection | Iterable |
+------------------+----------+------------+----------+
| list, range, str | yes | yes | yes |
| dict, set | | yes | yes |
| iter | | | yes |
+------------------+----------+------------+----------+
</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> Integral, Rational, Real, Complex, Number
<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">+--------------------+----------+----------+--------+---------+--------+
| | Integral | Rational | Real | Complex | Number |
+--------------------+----------+----------+--------+---------+--------+
| int | yes | yes | yes | yes | yes |
| fractions.Fraction | | yes | yes | yes | yes |
| float | | | yes | yes | yes |
| complex | | | | yes | yes |
| decimal.Decimal | | | | | yes |
+--------------------+----------+----------+--------+---------+--------+
</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"># Splits on line breaks. Keeps them if 'keepends'.</span>
<str> = <str>.join(<coll_of_strings>) <span class="hljs-comment"># Joins elements using string as 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 first match or -1.</span>
<int> = <str>.index(<sub_str>) <span class="hljs-comment"># Same but raises ValueError.</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>
<bool> = <str>.isnumeric() <span class="hljs-comment"># True if str contains only numeric characters.</span>
<list> = textwrap.wrap(<str>, width) <span class="hljs-comment"># Nicely breaks string into lines.</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>.</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="char">Char</h3><pre><code class="python language-python hljs"><str> = chr(<int>) <span class="hljs-comment"># Converts int to unicode char.</span>
<int> = ord(<str>) <span class="hljs-comment"># Converts unicode char to int.</span>
</code></pre></div>
<pre><code class="python language-python hljs"><span class="hljs-meta">>>> </span>ord(<span class="hljs-string">'0'</span>), ord(<span class="hljs-string">'9'</span>)
(<span class="hljs-number">48</span>, <span class="hljs-number">57</span>)
<span class="hljs-meta">>>> </span>ord(<span class="hljs-string">'A'</span>), ord(<span class="hljs-string">'Z'</span>)
(<span class="hljs-number">65</span>, <span class="hljs-number">90</span>)
<span class="hljs-meta">>>> </span>ord(<span class="hljs-string">'a'</span>), ord(<span class="hljs-string">'z'</span>)
(<span class="hljs-number">97</span>, <span class="hljs-number">122</span>)
</code></pre>
<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.</span>
<list> = re.findall(<regex>, text) <span class="hljs-comment"># Returns all occurrences.</span>
<list> = re.split(<regex>, text, maxsplit=<span class="hljs-number">0</span>) <span class="hljs-comment"># Use brackets in regex to keep the matches.</span>
<Match> = re.search(<regex>, text) <span class="hljs-comment"># Searches for first occurrence of 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>Search() and match() return None if there are no matches.</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 newline.</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.</strong></li>
<li><strong>Add <code class="python hljs"><span class="hljs-string">'?'</span></code> after an operator to make it 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"># Whole match. Also group(0).</span>
<str> = <Match>.group(<span class="hljs-number">1</span>) <span class="hljs-comment"># Part in first bracket.</span>
<tuple> = <Match>.groups() <span class="hljs-comment"># All bracketed parts.</span>
<int> = <Match>.start() <span class="hljs-comment"># Start index of a match.</span>
<int> = <Match>.end() <span class="hljs-comment"># Exclusive end index of a match.</span>
</code></pre></div>
<div><h3 id="specialsequences">Special Sequences</h3><ul>
<li><strong>By default digits, whitespaces and alphanumerics 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>Use capital letters for negation.</strong></li>
</ul><pre><code class="python language-python hljs"><span class="hljs-string">'\d'</span> == <span class="hljs-string">'[0-9]'</span> <span class="hljs-comment"># Digit</span>
<span class="hljs-string">'\s'</span> == <span class="hljs-string">'[ \t\n\r\f\v]'</span> <span class="hljs-comment"># Whitespace</span>
<span class="hljs-string">'\w'</span> == <span class="hljs-string">'[a-zA-Z0-9_]'</span> <span class="hljs-comment"># Alphanumeric</span>
</code></pre></div>
<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>
<str> = <span class="hljs-string">'{}, {}'</span>.format(<el_1>, <el_2>)
</code></pre></div>
<div><h3 id="attributes">Attributes</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> namedtuple
<span class="hljs-meta">>>> </span>Person = 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>
</code></pre></div>
<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">0</span>} <span class="hljs-comment"># '<el>'</span>
</code></pre>
<div><h3 id="strings">Strings</h3><p><strong><code class="python hljs"><span class="hljs-string">'!r'</span></code> calls object's repr() method, instead of str(), to get a string.</strong></p><pre><code class="python language-python hljs">{<span class="hljs-string">'abcde'</span>!r:<<span class="hljs-number">10</span>} <span class="hljs-comment"># "'abcde' "</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>:<span class="hljs-number">10.3</span>} <span class="hljs-comment"># 'abc '</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"># ' 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="comparisonoffloatpresentationtypes">Comparison of float 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%' |
| 567.89 | '567.89' | '567.890000' | '5.678900e+02' | '56789.000000%' |
+----------------+----------------+---------------+----------------+-----------------+
</code></pre></div>
<pre><code class="text language-text">+----------------+----------------+---------------+----------------+-----------------+
| | {<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%' |
| 567.89 | '5.7e+02' | '567.89' | '5.68e+02' | '56789.00%' |
+----------------+----------------+---------------+----------------+-----------------+
</code></pre>
<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>:X} <span class="hljs-comment"># '5A'</span>
{<span class="hljs-number">90</span>:b} <span class="hljs-comment"># '1011010'</span>
</code></pre></div>
<div><h2 id="numbers"><a href="#numbers" name="numbers">#</a>Numbers</h2><div><h3 id="types">Types</h3><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>)
<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(numerator=<span class="hljs-number">0</span>, denominator=<span class="hljs-number">1</span>)
<Decimal> = decimal.Decimal(<int/float/str>)
</code></pre></div></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 can be represented exactly, unlike floats where <code class="python hljs"><span class="hljs-string">'1.1 + 2.2 != 3.3'</span></code>.</strong></li>
<li><strong>Their precision can be adjusted 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>)
<int> = round(<num>)
<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
<span class="hljs-keyword">from</span> math <span class="hljs-keyword">import</span> cos, acos, sin, asin, tan, 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, pvariance, pstdev
</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
<float> = random()
<int> = randint(from_inclusive, to_inclusive)
<el> = choice(<list>)
shuffle(<list>)
</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>
<span class="hljs-string">'0b<bin>'</span> = bin(<int>) <span class="hljs-comment"># Or: '0x<hex>' = hex(<int>)</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</span>
<int> = <int> | <int> <span class="hljs-comment"># Or</span>
<int> = <int> ^ <int> <span class="hljs-comment"># Xor (0 if both bits equal)</span>
<int> = <int> << n_bits <span class="hljs-comment"># Shift left</span>
<int> = <int> >> n_bits <span class="hljs-comment"># Shift right</span>
<int> = ~<int> <span class="hljs-comment"># Compliment (flips bits)</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!</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">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">1</span>, <span class="hljs-number">1</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>)]
</code></pre>
<pre><code class="python language-python hljs"><span class="hljs-meta">>>> </span>product(<span class="hljs-string">'ab'</span>, <span class="hljs-string">'12'</span>)
[(<span class="hljs-string">'a'</span>, <span class="hljs-string">'1'</span>), (<span class="hljs-string">'a'</span>, <span class="hljs-string">'2'</span>),
(<span class="hljs-string">'b'</span>, <span class="hljs-string">'1'</span>), (<span class="hljs-string">'b'</span>, <span class="hljs-string">'2'</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-string">'a'</span>, <span class="hljs-string">'b'</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>)]
</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-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-string">'b'</span>, <span class="hljs-string">'b'</span>), (<span class="hljs-string">'b'</span>, <span class="hljs-string">'c'</span>),
(<span class="hljs-string">'c'</span>, <span class="hljs-string">'c'</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-string">'a'</span>, <span class="hljs-string">'b'</span>), (<span class="hljs-string">'a'</span>, <span class="hljs-string">'c'</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-string">'c'</span>, <span class="hljs-string">'a'</span>), (<span class="hljs-string">'c'</span>, <span class="hljs-string">'b'</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 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 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
</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(days=<span class="hljs-number">0</span>, seconds=<span class="hljs-number">0</span>, microseconds=<span class="hljs-number">0</span>, milliseconds=<span class="hljs-number">0</span>,
minutes=<span class="hljs-number">0</span>, hours=<span class="hljs-number">0</span>, weeks=<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 (Mon == 0).</strong></li>
<li><strong><code class="python hljs"><span class="hljs-string">'fold=1'</span></code> means second pass in case of time jumping back for one hour.</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 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">'<Cont.>/<City>'</span>) <span class="hljs-comment"># 'Continent/City_Name' timezone or None.</span>
</code></pre></div>
<pre><code class="python language-python apache hljs"><DTa> = <DT>.astimezone(<tzinfo>) <span class="hljs-comment"># Datetime, converted to passed timezone.</span>
<Ta/DTa> = <T/DT>.replace(tzinfo=<tzinfo>) <span class="hljs-comment"># Unconverted object with new timezone.</span>
</code></pre>
<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 Christ, at midnight.</span>
<DTn> = DT.fromtimestamp(<real>) <span class="hljs-comment"># Local time DTn from seconds since Epoch.</span>
<DTa> = DT.fromtimestamp(<real>, <tz.>) <span class="hljs-comment"># Aware datetime from seconds since 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.ffffff[±<offset>]'</span></code>, or both separated by <code class="python hljs"><span class="hljs-string">'T'</span></code>. Offset is formatted as: <code class="python hljs"><span class="hljs-string">'HH:MM'</span></code>.</strong></li>
<li><strong>On Unix systems Epoch is <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() <span class="hljs-comment"># ISO string representation.</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 Christ, ignoring time and tz.</span>
<float> = <DTn>.timestamp() <span class="hljs-comment"># Seconds since Epoch from DTn in local time.</span>
<float> = <DTa>.timestamp() <span class="hljs-comment"># Seconds since Epoch from DTa.</span>
</code></pre></div>
<div><h3 id="format-1">Format</h3><pre><code class="python language-python hljs"><span class="hljs-meta">>>> </span><span class="hljs-keyword">from</span> datetime <span class="hljs-keyword">import</span> datetime
<span class="hljs-meta">>>> </span>dt = datetime.strptime(<span class="hljs-string">'2015-05-14 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">"%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>When parsing, <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"><TD> = <D/DT> - <D/DT>
<D/DT> = <D/DT> ± <TD>
<TD> = <TD> ± <TD>
<TD> = <TD> */ <real>
</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"><function>(<positional_args>) <span class="hljs-comment"># f(0, 0)</span>
<function>(<keyword_args>) <span class="hljs-comment"># f(x=0, y=0)</span>
<function>(<positional_args>, <keyword_args>) <span class="hljs-comment"># f(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">f</span><span class="hljs-params">(<nondefault_args>)</span>:</span> <span class="hljs-comment"># def f(x, y):</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">f</span><span class="hljs-params">(<default_args>)</span>:</span> <span class="hljs-comment"># def f(x=0, y=0):</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">f</span><span class="hljs-params">(<nondefault_args>, <default_args>)</span>:</span> <span class="hljs-comment"># def f(x, y=0):</span>
</code></pre></div>
<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) | 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, 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>
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">f</span><span class="hljs-params">(x, *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>
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">f</span><span class="hljs-params">(x, *args, z, **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)</span>
</code></pre>
<div><h3 id="otheruses">Other Uses</h3><pre><code class="python language-python hljs"><list> = [*<collection> [, ...]]
<set> = {*<collection> [, ...]}
<tuple> = (*<collection>, [...])
<dict> = {**<dict> [, ...]}
</code></pre></div>
<pre><code class="python language-python hljs">head, *body, tail = <collection>
</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"><function> = <span class="hljs-keyword">lambda</span>: <return_value>
<function> = <span class="hljs-keyword">lambda</span> <argument_1>, <argument_2>: <return_value>
</code></pre></div></div>
<div><h3 id="comprehension">Comprehension</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"># [1, 2, ..., 10]</span>
<set> = {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"># {6, 7, 8, 9}</span>
<iter> = (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"># (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"># {0: 0, 1: 2, ..., 9: 18}</span>
</code></pre></div>
<pre><code class="python language-python hljs">out = [i+j <span class="hljs-keyword">for</span> i <span class="hljs-keyword">in</span> range(<span class="hljs-number">10</span>) <span class="hljs-keyword">for</span> j <span class="hljs-keyword">in</span> range(<span class="hljs-number">10</span>)]
</code></pre>
<div><h4 id="isthesameas-1">Is the same as:</h4><pre><code class="python language-python hljs">out = []
<span class="hljs-keyword">for</span> i <span class="hljs-keyword">in</span> range(<span class="hljs-number">10</span>):
<span class="hljs-keyword">for</span> j <span class="hljs-keyword">in</span> range(<span class="hljs-number">10</span>):
out.append(i+j)
</code></pre></div>
<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
<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"># (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"># (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"># 45</span>
</code></pre></div>
<div><h3 id="anyall">Any, All</h3><pre><code class="python language-python hljs"><bool> = any(<collection>) <span class="hljs-comment"># False if empty.</span>
<bool> = all(el[<span class="hljs-number">1</span>] <span class="hljs-keyword">for</span> el <span class="hljs-keyword">in</span> <collection>) <span class="hljs-comment"># True if empty.</span>
</code></pre></div>
<div><h3 id="ifelse">If - Else</h3><pre><code class="python language-python hljs"><expression_if_true> <span class="hljs-keyword">if</span> <condition> <span class="hljs-keyword">else</span> <expression_if_false>
</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">0</span>, <span class="hljs-number">3</span>)]
[<span class="hljs-string">'zero'</span>, <span class="hljs-number">1</span>, <span class="hljs-string">'zero'</span>, <span class="hljs-number">3</span>]
</code></pre>
<div><h3 id="namedtupleenumdataclass">Namedtuple, 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>)
point = Point(<span class="hljs-number">0</span>, <span class="hljs-number">0</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>)
direction = Direction.n
</code></pre>
<pre><code class="python language-python hljs"><span class="hljs-keyword">from</span> dataclasses <span class="hljs-keyword">import</span> make_dataclass
Creature = make_dataclass(<span class="hljs-string">'Creature'</span>, [<span class="hljs-string">'location'</span>, <span class="hljs-string">'direction'</span>])
creature = Creature(Point(<span class="hljs-number">0</span>, <span class="hljs-number">0</span>), Direction.n)
</code></pre>
<div><h2 id="closure"><a href="#closure" name="closure">#</a>Closure</h2><p><strong>We have 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 a 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">Nonlocal</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><h2 id="decorator"><a href="#decorator" name="decorator">#</a>Decorator</h2><p><strong>A decorator takes a function, adds some functionality and returns it.</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 it gets 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 metadata of function add() to function 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>Recursion depth is limited 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=False)</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>
<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 be also used for str().</strong></li>
</ul>
<div><h4 id="strusecases">Str() use cases:</h4><pre><code class="python language-python hljs">print(<el>)
print(<span class="hljs-string">f'<span class="hljs-subst">{<el>}</span>'</span>)
<span class="hljs-keyword">raise</span> Exception(<el>)
loguru.logger.debug(<el>)
csv.writer(<file>).writerow([<el>])
</code></pre></div>
<div><h4 id="reprusecases">Repr() use cases:</h4><pre><code class="python language-python hljs">print([<el>])
print(<span class="hljs-string">f'<span class="hljs-subst">{<el>!r}</span>'</span>)
<span class="hljs-meta">>>> </span><el>
loguru.logger.exception()
Z = dataclasses.make_dataclass(<span class="hljs-string">'Z'</span>, [<span class="hljs-string">'a'</span>]); print(Z(<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=None)</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:</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><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-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-meta"> @a.setter</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">a</span><span class="hljs-params">(self, value)</span>:</span>
self._a = value
</code></pre></div>
<pre><code class="python language-python hljs"><span class="hljs-meta">>>> </span>el = MyClass()
<span class="hljs-meta">>>> </span>el.a = <span class="hljs-number">123</span>
<span class="hljs-meta">>>> </span>el.a
<span class="hljs-number">123</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> or immutable and hashable with <code class="python hljs"><span class="hljs-string">'frozen=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.</strong></li>
<li><strong>Default_factory can be any <a href="#callable">callable</a>.</strong></li>
</ul>
<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>