Comprehensive Python Cheatsheet

^{Download text file, Buy PDF, Fork me on GitHub or Check out FAQ.}

Contents

    1. Collections:   List, Dictionary, Set, Tuple, Range, Enumerate, Iterator, Generator.
    2. Types:            Type, String, Regular_Exp, Format, Numbers, Combinatorics, Datetime.
    3. Syntax:           Args, Inline, Import, Decorator, Class, Duck_Types, Enum, Exception.
    4. System:          Exit, Print, Input, Command_Line_Arguments, Open, Path, OS_Commands.
    5. Data:               JSON, Pickle, CSV, SQLite, Bytes, Struct, Array, Memory_View, Deque.
    6. Advanced:     Threading, Operator, Match_Stmt, Logging, Introspection, Coroutines.
    7. Libraries:        Progress_Bar, Plot, Table, Console_App, GUI, Scraping, Web, Profile.
    8. Multimedia:    NumPy, Image, Animation, Audio, Synthesizer, Pygame, Pandas, Plotly.

Main

if __name__ == '__main__':      # Skips next line if file was imported.
    main()                      # Runs `def main(): ...` function.

List

<el>   = <list>[index]          # First index is 0. Last -1. Allows assignments.
<list> = <list>[<slice>]        # Or: <list>[from_inclusive : to_exclusive : ±step]

<list>.append(<el>)             # Or: <list> += [<el>]
<list>.extend(<collection>)     # Or: <list> += <collection>

<list>.sort()                   # Sorts in ascending order.
<list>.reverse()                # Reverses the list in-place.
<list> = sorted(<collection>)   # Returns a new sorted list.
<iter> = reversed(<list>)       # Returns reversed iterator.

sum_of_elements  = sum(<collection>)
elementwise_sum  = [sum(pair) for pair in zip(list_a, list_b)]
sorted_by_second = sorted(<collection>, key=lambda el: el[1])
sorted_by_both   = sorted(<collection>, key=lambda el: (el[1], el[0]))
flatter_list     = list(itertools.chain.from_iterable(<list>))
product_of_elems = functools.reduce(lambda out, el: out * el, <collection>)
list_of_chars    = list(<str>)

• For details about sorted(), min() and max() see sortable.
• Module operator provides functions itemgetter() and mul() that offer the same functionality as lambda expressions above.

<int> = len(<list>)             # Returns number of items. Also works on other collections.
<int> = <list>.count(<el>)      # Returns number of occurrences. Also `if <el> in <coll>: ...`.
<int> = <list>.index(<el>)      # Returns index of the first occurrence or raises ValueError.
<el>  = <list>.pop()            # Removes and returns item from the end or at index if passed.
<list>.insert(<int>, <el>)      # Inserts item at index and moves the rest to the right.
<list>.remove(<el>)             # Removes first occurrence of the item or raises ValueError.
<list>.clear()                  # Removes all items. Also works on dictionary and set.

Dictionary

<view> = <dict>.keys()                          # Coll. of keys that reflects changes.
<view> = <dict>.values()                        # Coll. of values that reflects changes.
<view> = <dict>.items()                         # Coll. of key-value tuples that reflects chgs.

value  = <dict>.get(key, default=None)          # Returns default if key is missing.
value  = <dict>.setdefault(key, default=None)   # Returns and writes default if key is missing.
<dict> = collections.defaultdict(<type>)        # Returns a dict with default value `<type>()`.
<dict> = collections.defaultdict(lambda: 1)     # Returns a dict with default value 1.

<dict> = dict(<collection>)                     # Creates a dict from coll. of key-value pairs.
<dict> = dict(zip(keys, values))                # Creates a dict from two collections.
<dict> = dict.fromkeys(keys [, value])          # Creates a dict from collection of keys.

<dict>.update(<dict>)                           # Adds items. Replaces ones with matching keys.
value = <dict>.pop(key)                         # Removes item or raises KeyError if missing.
{k for k, v in <dict>.items() if v == value}    # Returns set of keys that point to the value.
{k: v for k, v in <dict>.items() if k in keys}  # Filters the dictionary by keys.

Counter

>>> from collections import Counter
>>> counter = Counter(['blue', 'blue', 'blue', 'red', 'red'])
>>> counter['yellow'] += 1
>>> print(counter)
Counter({'blue': 3, 'red': 2, 'yellow': 1})
>>> counter.most_common()[0]
('blue', 3)

Set

<set> = set()                                   # `{}` returns a dictionary.

<set>.add(<el>)                                 # Or: <set> |= {<el>}
<set>.update(<collection> [, ...])              # Or: <set> |= <set>

<set>  = <set>.union(<coll.>)                   # Or: <set> | <set>
<set>  = <set>.intersection(<coll.>)            # Or: <set> & <set>
<set>  = <set>.difference(<coll.>)              # Or: <set> - <set>
<set>  = <set>.symmetric_difference(<coll.>)    # Or: <set> ^ <set>
<bool> = <set>.issubset(<coll.>)                # Or: <set> <= <set>
<bool> = <set>.issuperset(<coll.>)              # Or: <set> >= <set>

<el> = <set>.pop()                              # Raises KeyError if empty.
<set>.remove(<el>)                              # Raises KeyError if missing.
<set>.discard(<el>)                             # Doesn't raise an error.

Frozen Set

• Is immutable and hashable.
• That means it can be used as a key in a dictionary or as an element in a set.

<frozenset> = frozenset(<collection>)

Tuple

Tuple is an immutable and hashable list.

<tuple> = ()                               # Empty tuple.
<tuple> = (<el>,)                          # Or: <el>,
<tuple> = (<el_1>, <el_2> [, ...])         # Or: <el_1>, <el_2> [, ...]

Named Tuple

Tuple's subclass with named elements.

>>> from collections import namedtuple
>>> Point = namedtuple('Point', 'x y')
>>> p = Point(1, y=2); p
Point(x=1, y=2)
>>> p[0]
1
>>> p.x
1
>>> getattr(p, 'y')
2

Range

Immutable and hashable sequence of integers.

<range> = range(stop)                      # range(to_exclusive)
<range> = range(start, stop)               # range(from_inclusive, to_exclusive)
<range> = range(start, stop, ±step)        # range(from_inclusive, to_exclusive, ±step_size)

>>> [i for i in range(3)]
[0, 1, 2]

Enumerate

for i, el in enumerate(<coll>, start=0):   # Returns next element and its index on each pass.
    ...

Iterator

<iter> = iter(<collection>)                # `iter(<iter>)` returns unmodified iterator.
<iter> = iter(<function>, to_exclusive)    # A sequence of return values until 'to_exclusive'.
<el>   = next(<iter> [, default])          # Raises StopIteration or returns 'default' on end.
<list> = list(<iter>)                      # Returns a list of iterator's remaining elements.

Itertools

import itertools as it

<iter> = it.count(start=0, step=1)         # Returns updated value endlessly. Accepts floats.
<iter> = it.repeat(<el> [, times])         # Returns element endlessly or 'times' times.
<iter> = it.cycle(<collection>)            # Repeats the sequence endlessly.

<iter> = it.chain(<coll>, <coll> [, ...])  # Empties collections in order (figuratively).
<iter> = it.chain.from_iterable(<coll>)    # Empties collections inside a collection in order.

<iter> = it.islice(<coll>, to_exclusive)   # Only returns first 'to_exclusive' elements.
<iter> = it.islice(<coll>, from_inc, …)    # `to_exclusive, +step_size`. Indices can be None.

Generator

• Any function that contains a yield statement returns a generator.
• Generators and iterators are interchangeable.

def count(start, step):
    while True:
        yield start
        start += step

>>> counter = count(10, 2)
>>> next(counter), next(counter), next(counter)
(10, 12, 14)

Type

• Everything is an object.
• Every object has a type.
• Type and class are synonymous.

<type> = type(<el>)                          # Or: <el>.__class__
<bool> = isinstance(<el>, <type>)            # Or: issubclass(type(<el>), <type>)

>>> type('a'), 'a'.__class__, str
(<class 'str'>, <class 'str'>, <class 'str'>)

Some types do not have built-in names, so they must be imported:

from types import FunctionType, MethodType, LambdaType, GeneratorType, ModuleType

Abstract Base Classes

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().

>>> from collections.abc import Iterable, Collection, Sequence
>>> isinstance([1, 2, 3], Iterable)
True

+------------------+------------+------------+------------+
|                  |  Iterable  | Collection |  Sequence  |
+------------------+------------+------------+------------+
| list, range, str |    yes     |    yes     |    yes     |
| dict, set        |    yes     |    yes     |            |
| iter             |    yes     |            |            |
+------------------+------------+------------+------------+

>>> from numbers import Number, Complex, Real, Rational, Integral
>>> isinstance(123, Number)
True

+--------------------+----------+----------+----------+----------+----------+
|                    |  Number  |  Complex |   Real   | Rational | Integral |
+--------------------+----------+----------+----------+----------+----------+
| int                |   yes    |   yes    |   yes    |   yes    |   yes    |
| fractions.Fraction |   yes    |   yes    |   yes    |   yes    |          |
| float              |   yes    |   yes    |   yes    |          |          |
| complex            |   yes    |   yes    |          |          |          |
| decimal.Decimal    |   yes    |          |          |          |          |
+--------------------+----------+----------+----------+----------+----------+

String

Immutable sequence of characters.

<str>  = <str>.strip()                       # Strips all whitespace characters from both ends.
<str>  = <str>.strip('<chars>')              # Strips passed characters. Also lstrip/rstrip().

<list> = <str>.split()                       # Splits on one or more whitespace characters.
<list> = <str>.split(sep=None, maxsplit=-1)  # Splits on 'sep' str at most 'maxsplit' times.
<list> = <str>.splitlines(keepends=False)    # On [\n\r\f\v\x1c-\x1e\x85\u2028\u2029] and \r\n.
<str>  = <str>.join(<coll_of_strings>)       # Joins elements using string as a separator.

<bool> = <sub_str> in <str>                  # Checks if string contains the substring.
<bool> = <str>.startswith(<sub_str>)         # Pass tuple of strings for multiple options.
<int>  = <str>.find(<sub_str>)               # Returns start index of the first match or -1.
<int>  = <str>.index(<sub_str>)              # Same, but raises ValueError if there's no match.

``python <str> = <str>.lower() # Changes the case. Also upper/capitalize/title(). <str> = <str>.replace(old, new [, count]) # Replaces 'old' with 'new' at most 'count' times. <str> = <str>.translate(<table>) # Usestr.m