Comprehensive Python Cheatsheet =============================== [Download text file](https://raw.githubusercontent.com/gto76/python-cheatsheet/main/README.md), [Buy PDF](https://transactions.sendowl.com/products/78175486/4422834F/view), [Fork me on GitHub](https://github.com/gto76/python-cheatsheet) or [Check out FAQ](https://github.com/gto76/python-cheatsheet/wiki/Frequently-Asked-Questions). ![Monty Python](web/image_888.jpeg) Contents -------- **   ** **1. Collections:** ** ** **[`List`](#list)**__,__ **[`Dictionary`](#dictionary)**__,__ **[`Set`](#set)**__,__ **[`Tuple`](#tuple)**__,__ **[`Range`](#range)**__,__ **[`Enumerate`](#enumerate)**__,__ **[`Iterator`](#iterator)**__,__ **[`Generator`](#generator)**__.__ **   ** **2. Types:** **          ** **[`Type`](#type)**__,__ **[`String`](#string)**__,__ **[`Regular_Exp`](#regex)**__,__ **[`Format`](#format)**__,__ **[`Numbers`](#numbers-1)**__,__ **[`Combinatorics`](#combinatorics)**__,__ **[`Datetime`](#datetime)**__.__ **   ** **3. Syntax:** **         ** **[`Args`](#arguments)**__,__ **[`Inline`](#inline)**__,__ **[`Import`](#imports)**__,__ **[`Decorator`](#decorator)**__,__ **[`Class`](#class)**__,__ **[`Duck_Types`](#duck-types)**__,__ **[`Enum`](#enum)**__,__ **[`Exception`](#exceptions)**__.__ **   ** **4. System:** **        ** **[`Exit`](#exit)**__,__ **[`Print`](#print)**__,__ **[`Input`](#input)**__,__ **[`Command_Line_Arguments`](#command-line-arguments)**__,__ **[`Open`](#open)**__,__ **[`Path`](#paths)**__,__ **[`OS_Commands`](#os-commands)**__.__ **   ** **5. Data:** **             ** **[`JSON`](#json)**__,__ **[`Pickle`](#pickle)**__,__ **[`CSV`](#csv)**__,__ **[`SQLite`](#sqlite)**__,__ **[`Bytes`](#bytes)**__,__ **[`Struct`](#struct)**__,__ **[`Array`](#array)**__,__ **[`Memory_View`](#memory-view)**__,__ **[`Deque`](#deque)**__.__ **   ** **6. Advanced:** **   ** **[`Threading`](#threading)**__,__ **[`Operator`](#operator)**__,__ **[`Introspection`](#introspection)**__,__ **[`Metaprograming`](#metaprogramming)**__,__ **[`Eval`](#eval)**__,__ **[`Coroutines`](#coroutines)**__.__ **   ** **7. Libraries:** **      ** **[`Progress_Bar`](#progress-bar)**__,__ **[`Plot`](#plot)**__,__ **[`Table`](#table)**__,__ **[`Curses`](#curses)**__,__ **[`Logging`](#logging)**__,__ **[`Scraping`](#scraping)**__,__ **[`Web`](#web)**__,__ **[`Profile`](#profiling)**__,__ **                                 ** **[`NumPy`](#numpy)**__,__ **[`Image`](#image)**__,__ **[`Audio`](#audio)**__,__ **[`Games`](#pygame)**__,__ **[`Data`](#pandas)**__.__ Main ---- ```python if __name__ == '__main__': # Runs main() if file wasn't imported. main() ``` List ---- ```python = [] # Or: [from_inclusive : to_exclusive : ±step] ``` ```python .append() # Or: += [] .extend() # Or: += ``` ```python .sort() # Sorts in ascending order. .reverse() # Reverses the list in-place. = sorted() # Returns a new sorted list. = reversed() # Returns reversed iterator. ``` ```python sum_of_elements = sum() elementwise_sum = [sum(pair) for pair in zip(list_a, list_b)] sorted_by_second = sorted(, key=lambda el: el[1]) sorted_by_both = sorted(, key=lambda el: (el[1], el[0])) flatter_list = list(itertools.chain.from_iterable()) product_of_elems = functools.reduce(lambda out, el: out * el, ) list_of_chars = list() ``` * **For details about sorted(), min() and max() see [sortable](#sortable).** * **Module [operator](#operator) provides functions itemgetter() and mul() that offer the same functionality as [lambda](#lambda) expressions above.** ```python .insert(, ) # Inserts item at index and moves the rest to the right. = .pop([]) # Removes and returns item at index or from the end. = .count() # Returns number of occurrences. Also works on strings. = .index() # Returns index of the first occurrence or raises ValueError. .remove() # Removes first occurrence of the item or raises ValueError. .clear() # Removes all items. Also works on dictionary and set. ``` Dictionary ---------- ```python = .keys() # Coll. of keys that reflects changes. = .values() # Coll. of values that reflects changes. = .items() # Coll. of key-value tuples that reflects chgs. ``` ```python value = .get(key, default=None) # Returns default if key is missing. value = .setdefault(key, default=None) # Returns and writes default if key is missing. = collections.defaultdict() # Returns a dict with default value of type. = collections.defaultdict(lambda: 1) # Returns a dict with default value 1. ``` ```python = dict() # Creates a dict from coll. of key-value pairs. = dict(zip(keys, values)) # Creates a dict from two collections. = dict.fromkeys(keys [, value]) # Creates a dict from collection of keys. ``` ```python .update() # Adds items. Replaces ones with matching keys. value = .pop(key) # Removes item or raises KeyError. {k for k, v in .items() if v == value} # Returns set of keys that point to the value. {k: v for k, v in .items() if k in keys} # Returns a dictionary, filtered by keys. ``` ### Counter ```python >>> from collections import Counter >>> colors = ['blue', 'blue', 'blue', 'red', 'red'] >>> counter = Counter(colors) >>> counter['yellow'] += 1 Counter({'blue': 3, 'red': 2, 'yellow': 1}) >>> counter.most_common()[0] ('blue', 3) ``` Set --- ```python = set() # `{}` returns a dictionary. ``` ```python .add() # Or: |= {} .update( [, ...]) # Or: |= ``` ```python = .union() # Or: | = .intersection() # Or: & = .difference() # Or: - = .symmetric_difference() # Or: ^ = .issubset() # Or: <= = .issuperset() # Or: >= ``` ```python = .pop() # Raises KeyError if empty. .remove() # Raises KeyError if missing. .discard() # 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.** ```python = frozenset() ``` Tuple ----- **Tuple is an immutable and hashable list.** ```python = () # Empty tuple. = (,) # Or: , = (, [, ...]) # Or: , [, ...] ``` ### Named Tuple **Tuple's subclass with named elements.** ```python >>> from collections import namedtuple >>> Point = namedtuple('Point', 'x y') >>> p = Point(1, y=2) Point(x=1, y=2) >>> p[0] 1 >>> p.x 1 >>> getattr(p, 'y') 2 ``` Range ----- **Immutable and hashable sequence of integers.** ```python = range(stop) # range(to_exclusive) = range(start, stop) # range(from_inclusive, to_exclusive) = range(start, stop, ±step) # range(from_inclusive, to_exclusive, ±step_size) ``` ```python >>> [i for i in range(3)] [0, 1, 2] ``` Enumerate --------- ```python for i, el in enumerate( [, i_start]): ... ``` Iterator -------- ```python = iter() # `iter()` returns unmodified iterator. = iter(, to_exclusive) # A sequence of return values until 'to_exclusive'. = next( [, default]) # Raises StopIteration or returns 'default' on end. = list() # Returns a list of iterator's remaining elements. ``` ### Itertools ```python import itertools as it ``` ```python = it.count(start=0, step=1) # Returns updated value endlessly. Accepts floats. = it.repeat( [, times]) # Returns element endlessly or 'times' times. = it.cycle() # Repeats the sequence endlessly. ``` ```python = it.chain(, [, ...]) # Empties collections in order (figuratively). = it.chain.from_iterable() # Empties collections inside a collection in order. ``` ```python = it.islice(, to_exclusive) # Only returns first 'to_exclusive' elements. = it.islice(, 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.** ```python def count(start, step): while True: yield start start += step ``` ```python >>> 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.** ```python = type() # Or: .__class__ = isinstance(, ) # Or: issubclass(type(), ) ``` ```python >>> type('a'), 'a'.__class__, str (, , ) ``` #### Some types do not have built-in names, so they must be imported: ```python 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().** ```python >>> from collections.abc import Iterable, Collection, Sequence >>> isinstance([1, 2, 3], Iterable) True ``` ```text +------------------+------------+------------+------------+ | | Iterable | Collection | Sequence | +------------------+------------+------------+------------+ | list, range, str | yes | yes | yes | | dict, set | yes | yes | | | iter | yes | | | +------------------+------------+------------+------------+ ``` ```python >>> from numbers import Number, Complex, Real, Rational, Integral >>> isinstance(123, Number) True ``` ```text +--------------------+----------+----------+----------+----------+----------+ | | 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 ------ ```python = .strip() # Strips all whitespace characters from both ends. = .strip('') # Strips all passed characters from both ends. ``` ```python = .split() # Splits on one or more whitespace characters. = .split(sep=None, maxsplit=-1) # Splits on 'sep' str at most 'maxsplit' times. = .splitlines(keepends=False) # On [\n\r\f\v\x1c-\x1e\x85\u2028\u2029] and \r\n. = .join() # Joins elements using string as a separator. ``` ```python = in # Checks if string contains a substring. = .startswith() # Pass tuple of strings for multiple options. = .endswith() # Pass tuple of strings for multiple options. = .find() # Returns start index of the first match or -1. = .index() # Same, but raises ValueError if missing. ``` ```python = .replace(old, new [, count]) # Replaces 'old' with 'new' at most 'count' times. = .translate() # Use `str.maketrans()` to generate table. ``` ```python = chr() # Converts int to Unicode character. = ord() # Converts Unicode character to int. ``` * **Also: `'lstrip()'`, `'rstrip()'` and `'rsplit()'`.** * **Also: `'lower()'`, `'upper()'`, `'capitalize()'` and `'title()'`.** ### Property Methods ```text +---------------+----------+----------+----------+----------+----------+ | | [ !#$%…] | [a-zA-Z] | [¼½¾] | [²³¹] | [0-9] | +---------------+----------+----------+----------+----------+----------+ | isprintable() | yes | yes | yes | yes | yes | | isalnum() | | yes | yes | yes | yes | | isnumeric() | | | yes | yes | yes | | isdigit() | | | | yes | yes | | isdecimal() | | | | | yes | +---------------+----------+----------+----------+----------+----------+ ``` * **`'isspace()'` checks for whitespaces: `'[ \t\n\r\f\v\x1c-\x1f\x85\xa0\u1680…]'`.** Regex ----- ```python import re = re.sub(, new, text, count=0) # Substitutes all occurrences with 'new'. = re.findall(, text) # Returns all occurrences as strings. = re.split(, text, maxsplit=0) # Use brackets in regex to include the matches. = re.search(, text) # Searches for first occurrence of the pattern. = re.match(, text) # Searches only at the beginning of the text. = re.finditer(, text) # Returns all occurrences as Match objects. ``` * **Argument 'new' can be a function that accepts a Match object and returns a string.** * **Search() and match() return None if they can't find a match.** * **Argument `'flags=re.IGNORECASE'` can be used with all functions.** * **Argument `'flags=re.MULTILINE'` makes `'^'` and `'$'` match the start/end of each line.** * **Argument `'flags=re.DOTALL'` makes `'.'` also accept the `'\n'`.** * **Use `r'\1'` or `'\\1'` for backreference (`'\1'` returns a character with octal code 1).** * **Add `'?'` after `'*'` and `'+'` to make them non-greedy.** ### Match Object ```python = .group() # Returns the whole match. Also group(0). = .group(1) # Returns part in the first bracket. = .groups() # Returns all bracketed parts. = .start() # Returns start index of the match. = .end() # Returns exclusive end index of the match. ``` ### Special Sequences ```python '\d' == '[0-9]' # Matches decimal characters. '\w' == '[a-zA-Z0-9_]' # Matches alphanumerics and underscore. '\s' == '[ \t\n\r\f\v]' # Matches whitespaces. ``` * **By default, decimal characters, alphanumerics and whitespaces from all alphabets are matched unless `'flags=re.ASCII'` argument is used.** * **As shown above, it restricts all special sequence matches to the first 128 characters and prevents `'\s'` from accepting `'[\x1c-\x1f]'` (the so-called separator characters).** * **Use a capital letter for negation (all non-ASCII characters will be matched when used in combination with ASCII flag).** Format ------ ```python = f'{}, {}' # Curly brackets can also contain expressions. = '{}, {}'.format(, ) # Or: '{0}, {a}'.format(, a=) = '%s, %s' % (, ) # Redundant and inferior C style formatting. ``` ### Attributes ```python >>> Person = collections.namedtuple('Person', 'name height') >>> person = Person('Jean-Luc', 187) >>> f'{person.height}' '187' >>> '{p.height}'.format(p=person) '187' ``` ### General Options ```python {:<10} # ' ' {:^10} # ' ' {:>10} # ' ' {:.<10} # '......' {:0} # '' ``` * **Options can be generated dynamically: `f'{:{}[…]}'`.** * **Adding `'!r'` before the colon converts object to string by calling its [repr()](#class) method.** ### Strings ```python {'abcde':10} # 'abcde ' {'abcde':10.3} # 'abc ' {'abcde':.3} # 'abc' {'abcde'!r:10} # "'abcde' " ``` ### Numbers ```python {123456:10} # ' 123456' {123456:10,} # ' 123,456' {123456:10_} # ' 123_456' {123456:+10} # ' +123456' {123456:=+10} # '+ 123456' {123456: } # ' 123456' {-123456: } # '-123456' ``` ### Floats ```python {1.23456:10.3} # ' 1.23' {1.23456:10.3f} # ' 1.235' {1.23456:10.3e} # ' 1.235e+00' {1.23456:10.3%} # ' 123.456%' ``` #### Comparison of presentation types: ```text +--------------+----------------+----------------+----------------+----------------+ | | {} | {:f} | {:e} | {:%} | +--------------+----------------+----------------+----------------+----------------+ | 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%' | +--------------+----------------+----------------+----------------+----------------+ ``` ```text +--------------+----------------+----------------+----------------+----------------+ | | {:.2} | {:.2f} | {:.2e} | {:.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%' | +--------------+----------------+----------------+----------------+----------------+ ``` * **When both rounding up and rounding down are possible, the one that returns result with even last digit is chosen. That makes `'{6.5:.0f}'` a `'6'` and `'{7.5:.0f}'` an `'8'`.** * **This rule only effects numbers that can be represented exactly by a float (`.5`, `.25`, …).** ### Ints ```python {90:c} # 'Z' {90:b} # '1011010' {90:X} # '5A' ``` Numbers ------- ```python = int() # Or: math.floor() = float() # Or: = complex(real=0, imag=0) # Or: ± j = fractions.Fraction(0, 1) # Or: Fraction(numerator=0, denominator=1) = decimal.Decimal() # Or: Decimal((sign, digits, exponent)) ``` * **`'int()'` and `'float()'` raise ValueError on malformed strings.** * **Decimal numbers are stored exactly, unlike most floats where `'1.1 + 2.2 != 3.3'`.** * **Floats can be compared with: `'math.isclose(, )'`.** * **Precision of decimal operations is set with: `'decimal.getcontext().prec = '`.** ### Basic Functions ```python = pow(, ) # Or: ** = abs() # = abs() = round( [, ±ndigits]) # `round(126, -1) == 130` ``` ### Math ```python from math import e, pi, inf, nan, isinf, isnan # ` == nan` is always False. from math import sin, cos, tan, asin, acos, atan # Also: degrees, radians. from math import log, log10, log2 # Log can accept base as second arg. ``` ### Statistics ```python from statistics import mean, median, variance # Also: stdev, quantiles, groupby. ``` ### Random ```python from random import random, randint, choice # Also: shuffle, gauss, triangular, seed. = random() # A float inside [0, 1). = randint(from_inc, to_inc) # An int inside [from_inc, to_inc]. = choice() # Keeps the sequence intact. ``` ### Bin, Hex ```python = ±0b # Or: ±0x = int('±', 2) # Or: int('±', 16) = int('±0b', 0) # Or: int('±0x', 0) = bin() # Returns '[-]0b'. ``` ### Bitwise Operators ```python = & # And (0b1100 & 0b1010 == 0b1000). = | # Or (0b1100 | 0b1010 == 0b1110). = ^ # Xor (0b1100 ^ 0b1010 == 0b0110). = << n_bits # Left shift. Use >> for right. = ~ # Not. Also - - 1. ``` Combinatorics ------------- * **Every function returns an iterator.** * **If you want to print the iterator, you need to pass it to the list() function first!** ```python import itertools as it ``` ```python >>> it.product([0, 1], repeat=3) [(0, 0, 0), (0, 0, 1), (0, 1, 0), (0, 1, 1), (1, 0, 0), (1, 0, 1), (1, 1, 0), (1, 1, 1)] ``` ```python >>> it.product('abc', 'abc') # a b c [('a', 'a'), ('a', 'b'), ('a', 'c'), # a x x x ('b', 'a'), ('b', 'b'), ('b', 'c'), # b x x x ('c', 'a'), ('c', 'b'), ('c', 'c')] # c x x x ``` ```python >>> it.combinations('abc', 2) # a b c [('a', 'b'), ('a', 'c'), # a . x x ('b', 'c')] # b . . x ``` ```python >>> it.combinations_with_replacement('abc', 2) # a b c [('a', 'a'), ('a', 'b'), ('a', 'c'), # a x x x ('b', 'b'), ('b', 'c'), # b . x x ('c', 'c')] # c . . x ``` ```python >>> it.permutations('abc', 2) # a b c [('a', 'b'), ('a', 'c'), # a . x x ('b', 'a'), ('b', 'c'), # b x . x ('c', 'a'), ('c', 'b')] # c x x . ``` Datetime -------- * **Module 'datetime' provides 'date' ``, 'time' ``, 'datetime' `
` and 'timedelta' `
` classes. All are immutable and hashable.** * **Time and datetime objects can be 'aware' ``, meaning they have defined timezone, or 'naive' ``, meaning they don't.** * **If object is naive, it is presumed to be in the system's timezone.** ```python from datetime import date, time, datetime, timedelta from dateutil.tz import UTC, tzlocal, gettz, datetime_exists, resolve_imaginary ``` ### Constructors ```python = date(year, month, day) # Only accepts valid dates from 1 to 9999 AD. = time(hour=0, minute=0, second=0) # Also: `microsecond=0, tzinfo=None, fold=0`.
= datetime(year, month, day, hour=0) # Also: `minute=0, second=0, microsecond=0, …`.
= timedelta(weeks=0, days=0, hours=0) # Also: `minutes=0, seconds=0, microsecond=0`. ``` * **Use `'.weekday()'` to get the day of the week as an int, with Monday being 0.** * **`'fold=1'` means the second pass in case of time jumping back for one hour.** * **Timedelta normalizes arguments to ±days, seconds (< 86 400) and microseconds (< 1M).** * **`' = resolve_imaginary()'` fixes DTs that fall into the missing hour.** ### Now ```python = D/DT.today() # Current local date or naive datetime. = DT.utcnow() # Naive datetime from current UTC time. = DT.now() # Aware datetime from current tz time. ``` * **To extract time use `'.time()'`, `'.time()'` or `'.timetz()'`.** ### Timezone ```python = UTC # UTC timezone. London without DST. = tzlocal() # Local timezone. Also gettz(). = gettz('/') # 'Continent/City_Name' timezone or None. =
.astimezone() # Datetime, converted to the passed timezone. = .replace(tzinfo=) # Unconverted object with a new timezone. ``` ### Encode ```python = D/T/DT.fromisoformat('') # Object from ISO string. Raises ValueError.
= DT.strptime(, '') # Datetime from str, according to format. = D/DT.fromordinal() # D/DTn from days since the Gregorian NYE 1. = DT.fromtimestamp() # Local time DTn from seconds since the Epoch. = DT.fromtimestamp(, ) # Aware datetime from seconds since the Epoch. ``` * **ISO strings come in following forms: `'YYYY-MM-DD'`, `'HH:MM:SS.mmmuuu[±HH:MM]'`, or both separated by an arbitrary character. All parts following hours are optional.** * **Python uses the Unix Epoch: `'1970-01-01 00:00 UTC'`, `'1970-01-01 01:00 CET'`, ...** ### Decode ```python = .isoformat(sep='T') # Also: `timespec='auto/hours/minutes/seconds/…'`. = .strftime('') # Custom string representation. = .toordinal() # Days since Gregorian NYE 1, ignoring time and tz. = .timestamp() # Seconds since the Epoch, from DTn in local tz. = .timestamp() # Seconds since the Epoch, from aware datetime. ``` ### Format ```python >>> dt = datetime.strptime('2015-05-14 23:39:00.00 +2000', '%Y-%m-%d %H:%M:%S.%f %z') >>> dt.strftime("%A, %dth of %B '%y, %I:%M%p %Z") "Thursday, 14th of May '15, 11:39PM UTC+02:00" ``` * **`'%Z'` only accepts `'UTC/GMT'` and local timezone's code. `'%z'` also accepts `'±HH:MM'`.** * **For abbreviated weekday and month use `'%a'` and `'%b'`.** ### Arithmetics ```python = ±
# Returned datetime can fall into missing hour. = - # Returns the difference, ignoring time jumps. = - # Ignores time jumps if they share tzinfo object. = * # Also: = abs() and = ±% . = / # How many weeks/years there are in TD. Also //. ``` Arguments --------- ### Inside Function Call ```python func() # func(0, 0) func() # func(x=0, y=0) func(, ) # func(0, y=0) ``` ### Inside Function Definition ```python def func(): ... # def func(x, y): ... def func(): ... # def func(x=0, y=0): ... def func(, ): ... # def func(x, y=0): ... ``` * **Default values are evaluated when function is first encountered in the scope.** * **Any mutation of a mutable default value will persist between invocations!** Splat Operator -------------- ### Inside Function Call **Splat expands a collection into positional arguments, while splatty-splat expands a dictionary into keyword arguments.** ```python args = (1, 2) kwargs = {'x': 3, 'y': 4, 'z': 5} func(*args, **kwargs) ``` #### Is the same as: ```python func(1, 2, x=3, y=4, z=5) ``` ### Inside Function Definition **Splat combines zero or more positional arguments into a tuple, while splatty-splat combines zero or more keyword arguments into a dictionary.** ```python def add(*a): return sum(a) ``` ```python >>> add(1, 2, 3) 6 ``` #### Legal argument combinations: ```python def f(*args): ... # f(1, 2, 3) def f(x, *args): ... # f(1, 2, 3) def f(*args, z): ... # f(1, 2, z=3) ``` ```python def f(**kwargs): ... # f(x=1, y=2, z=3) def f(x, **kwargs): ... # f(x=1, y=2, z=3) | f(1, y=2, z=3) ``` ```python def f(*args, **kwargs): ... # f(x=1, y=2, z=3) | f(1, y=2, z=3) | f(1, 2, z=3) | f(1, 2, 3) def f(x, *args, **kwargs): ... # f(x=1, y=2, z=3) | f(1, y=2, z=3) | f(1, 2, z=3) | f(1, 2, 3) def f(*args, y, **kwargs): ... # f(x=1, y=2, z=3) | f(1, y=2, z=3) ``` ```python def f(*, x, y, z): ... # f(x=1, y=2, z=3) def f(x, *, y, z): ... # f(x=1, y=2, z=3) | f(1, y=2, z=3) def f(x, y, *, z): ... # f(x=1, y=2, z=3) | f(1, y=2, z=3) | f(1, 2, z=3) ``` ### Other Uses ```python = [* [, ...]] # Or: list() [+ ...] = (*, [...]) # Or: tuple() [+ ...] = {* [, ...]} # Or: set() [| ...] = {** [, ...]} # Or: dict(** [, ...]) ``` ```python head, *body, tail = # Head or tail can be omitted. ``` Inline ------ ### Lambda ```python = lambda: # A single statement function. = lambda , : # Also accepts default arguments. ``` ### Comprehensions ```python = [i+1 for i in range(10)] # Or: [1, 2, ..., 10] = (i for i in range(10) if i > 5) # Or: iter([6, 7, 8, 9]) = {i+5 for i in range(10)} # Or: {5, 6, ..., 14} = {i: i*2 for i in range(10)} # Or: {0: 0, 1: 2, ..., 9: 18} ``` ```python >>> [l+r for l in 'abc' for r in 'abc'] ['aa', 'ab', 'ac', ..., 'cc'] ``` ### Map, Filter, Reduce ```python = map(lambda x: x + 1, range(10)) # Or: iter([1, 2, ..., 10]) = filter(lambda x: x > 5, range(10)) # Or: iter([6, 7, 8, 9]) = reduce(lambda out, x: out + x, range(10)) # Or: 45 ``` * **Reduce must be imported from the functools module.** ### Any, All ```python = any() # Is `bool()` True for any element. = all() # Is True for all elements or empty. ``` ### Conditional Expression ```python = if else # Only one expression gets evaluated. ``` ```python >>> [a if a else 'zero' for a in (0, 1, 2, 3)] # `any([0, '', [], None]) == False` ['zero', 1, 2, 3] ``` ### Named Tuple, Enum, Dataclass ```python from collections import namedtuple Point = namedtuple('Point', 'x y') # Creates a tuple's subclass. point = Point(0, 0) # Returns its instance. ``` ```python from enum import Enum Direction = Enum('Direction', 'N E S W') # Creates an enum. direction = Direction.N # Returns its member. ``` ```python from dataclasses import make_dataclass Player = make_dataclass('Player', ['loc', 'dir']) # Creates a class. player = Player(point, direction) # Returns its instance. ``` Imports ------- ```python import # Imports a built-in or '.py'. import # Imports a built-in or '/__init__.py'. import . # Imports a built-in or '/.py'. ``` * **Package is a collection of modules, but it can also define its own objects.** * **On a filesystem this corresponds to a directory of Python files with an optional init script.** * **Running `'import '` does not automatically provide access to the package's modules unless they are explicitly imported in its init script.** Closure ------- **We have/get a closure in Python when:** * **A nested function references a value of its enclosing function and then** * **the enclosing function returns the nested function.** ```python def get_multiplier(a): def out(b): return a * b return out ``` ```python >>> multiply_by_3 = get_multiplier(3) >>> multiply_by_3(10) 30 ``` * **If multiple nested functions within enclosing function reference the same value, that value gets shared.** * **To dynamically access function's first free variable use `'.__closure__[0].cell_contents'`.** ### Partial ```python from functools import partial = partial( [, , , ...]) ``` ```python >>> def multiply(a, b): ... return a * b >>> multiply_by_3 = partial(multiply, 3) >>> multiply_by_3(10) 30 ``` * **Partial is also useful in cases when function needs to be passed as an argument because it enables us to set its arguments beforehand.** * **A few examples being: `'defaultdict()'`, `'iter(, to_exclusive)'` and dataclass's `'field(default_factory=)'`.** ### Non-Local **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'.** ```python def get_counter(): i = 0 def out(): nonlocal i i += 1 return i return out ``` ```python >>> counter = get_counter() >>> counter(), counter(), counter() (1, 2, 3) ``` Decorator --------- * **A decorator takes a function, adds some functionality and returns it.** * **It can be any [callable](#callable), but is usually implemented as a function that returns a [closure](#closure).** ```python @decorator_name def function_that_gets_passed_to_decorator(): ... ``` ### Debugger Example **Decorator that prints function's name every time the function is called.** ```python from functools import wraps def debug(func): @wraps(func) def out(*args, **kwargs): print(func.__name__) return func(*args, **kwargs) return out @debug def add(x, y): return x + y ``` * **Wraps is a helper decorator that copies the metadata of the passed function (func) to the function it is wrapping (out).** * **Without it `'add.__name__'` would return `'out'`.** ### LRU Cache **Decorator that caches function's return values. All function's arguments must be hashable.** ```python from functools import lru_cache @lru_cache(maxsize=None) def fib(n): return n if n < 2 else fib(n-2) + fib(n-1) ``` * **Default size of the cache is 128 values. Passing `'maxsize=None'` makes it unbounded.** * **CPython interpreter limits recursion depth to 1000 by default. To increase it use `'sys.setrecursionlimit()'`.** ### Parametrized Decorator **A decorator that accepts arguments and returns a normal decorator that accepts a function.** ```python from functools import wraps def debug(print_result=False): def decorator(func): @wraps(func) def out(*args, **kwargs): result = func(*args, **kwargs) print(func.__name__, result if print_result else '') return result return out return decorator @debug(print_result=True) def add(x, y): return x + y ``` * **Using only `'@debug'` 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.** Class ----- ```python class : def __init__(self, a): self.a = a def __repr__(self): class_name = self.__class__.__name__ return f'{class_name}({self.a!r})' def __str__(self): return str(self.a) @classmethod def get_class_name(cls): return cls.__name__ ``` * **Return value of repr() should be unambiguous and of str() readable.** * **If only repr() is defined, it will also be used for str().** * **Methods decorated with `'@staticmethod'` do not receive 'self' nor 'cls' as their first arg.** #### Expressions that call the str() method: ```python print() f'{}' logging.warning() csv.writer().writerow([]) raise Exception() ``` #### Expressions that call the repr() method: ```python print/str/repr([]) f'{!r}' Z = dataclasses.make_dataclass('Z', ['a']); print/str/repr(Z()) >>> ``` ### Constructor Overloading ```python class : def __init__(self, a=None): self.a = a ``` ### Inheritance ```python class Person: def __init__(self, name, age): self.name = name self.age = age class Employee(Person): def __init__(self, name, age, staff_num): super().__init__(name, age) self.staff_num = staff_num ``` ### Multiple Inheritance ```python class A: pass class B: pass class C(A, B): pass ``` **MRO determines the order in which parent classes are traversed when searching for a method or an attribute:** ```python >>> C.mro() [, , , ] ``` ### Property **Pythonic way of implementing getters and setters.** ```python class Person: @property def name(self): return ' '.join(self._name) @name.setter def name(self, value): self._name = value.split() ``` ```python >>> person = Person() >>> person.name = '\t Guido van Rossum \n' >>> person.name 'Guido van Rossum' ``` ### Dataclass **Decorator that automatically generates init(), repr() and eq() special methods.** ```python from dataclasses import dataclass, field @dataclass(order=False, frozen=False) class : : : = : list/dict/set = field(default_factory=list/dict/set) ``` * **Objects can be made sortable with `'order=True'` and immutable with `'frozen=True'`.** * **For object to be hashable, all attributes must be hashable and 'frozen' must be True.** * **Function field() is needed because `': list = []'` would make a list that is shared among all instances. Its 'default_factory' argument can be any [callable](#callable).** * **For attributes of arbitrary type use `'typing.Any'`.** #### Inline: ```python from dataclasses import make_dataclass = make_dataclass('', ) = make_dataclass('', ) = ('', [, ]) ``` #### Rest of type annotations (CPython interpreter ignores them all): ```python def func(: [= ]) -> : ... : typing.List/Set/Iterable/Sequence/Optional[] : typing.Dict/Tuple/Union[, ...] ``` ### Slots **Mechanism that restricts objects to attributes listed in 'slots' and significantly reduces their memory footprint.** ```python class MyClassWithSlots: __slots__ = ['a'] def __init__(self): self.a = 1 ``` ### Copy ```python from copy import copy, deepcopy = copy() = deepcopy() ``` Duck Types ---------- **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.** ### Comparable * **If eq() method is not overridden, it returns `'id(self) == id(other)'`, which is the same as `'self is other'`.** * **That means all objects compare not equal by default.** * **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.** * **Ne() automatically works on any object that has eq() defined.** ```python class MyComparable: def __init__(self, a): self.a = a def __eq__(self, other): if isinstance(other, type(self)): return self.a == other.a return NotImplemented ``` ### Hashable * **Hashable object needs both hash() and eq() methods and its hash value should never change.** * **Hashable objects that compare equal must have the same hash value, meaning default hash() that returns `'id(self)'` will not do.** * **That is why Python automatically makes classes unhashable if you only implement eq().** ```python class MyHashable: def __init__(self, a): self._a = a @property def a(self): return self._a def __eq__(self, other): if isinstance(other, type(self)): return self.a == other.a return NotImplemented def __hash__(self): return hash(self.a) ``` ### Sortable * **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.** * **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.** * **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.** ```python from functools import total_ordering @total_ordering class MySortable: def __init__(self, a): self.a = a def __eq__(self, other): if isinstance(other, type(self)): return self.a == other.a return NotImplemented def __lt__(self, other): if isinstance(other, type(self)): return self.a < other.a return NotImplemented ``` ### Iterator * **Any object that has methods next() and iter() is an iterator.** * **Next() should return next item or raise StopIteration.** * **Iter() should return 'self'.** ```python class Counter: def __init__(self): self.i = 0 def __next__(self): self.i += 1 return self.i def __iter__(self): return self ``` ```python >>> counter = Counter() >>> next(counter), next(counter), next(counter) (1, 2, 3) ``` #### Python has many different iterator objects: * **Sequence iterators returned by the [iter()](#iterator) function, such as list\_iterator and set\_iterator.** * **Objects returned by the [itertools](#itertools) module, such as count, repeat and cycle.** * **Generators returned by the [generator functions](#generator) and [generator expressions](#comprehensions).** * **File objects returned by the [open()](#open) function, etc.** ### Callable * **All functions and classes have a call() method, hence are callable.** * **When this cheatsheet uses `''` as an argument, it actually means `''`.** ```python class Counter: def __init__(self): self.i = 0 def __call__(self): self.i += 1 return self.i ``` ```python >>> counter = Counter() >>> counter(), counter(), counter() (1, 2, 3) ``` ### Context Manager * **Enter() should lock the resources and optionally return an object.** * **Exit() should release the resources.** * **Any exception that happens inside the with block is passed to the exit() method.** * **If it wishes to suppress the exception it must return a true value.** ```python class MyOpen: def __init__(self, filename): self.filename = filename def __enter__(self): self.file = open(self.filename) return self.file def __exit__(self, exc_type, exception, traceback): self.file.close() ``` ```python >>> with open('test.txt', 'w') as file: ... file.write('Hello World!') >>> with MyOpen('test.txt') as file: ... print(file.read()) Hello World! ``` Iterable Duck Types ------------------- ### Iterable * **Only required method is iter(). It should return an iterator of object's items.** * **Contains() automatically works on any object that has iter() defined.** ```python class MyIterable: def __init__(self, a): self.a = a def __iter__(self): return iter(self.a) def __contains__(self, el): return el in self.a ``` ```python >>> obj = MyIterable([1, 2, 3]) >>> [el for el in obj] [1, 2, 3] >>> 1 in obj True ``` ### Collection * **Only required methods are iter() and len(). Len() should return the number of items.** * **This cheatsheet actually means `''` when it uses `''`.** * **I chose not to use the name 'iterable' because it sounds scarier and more vague than 'collection'. The only drawback of this decision is that a reader could think a certain function doesn't accept iterators when it does, since iterators are the only built-in objects that are iterable but are not collections.** ```python class MyCollection: def __init__(self, a): self.a = a def __iter__(self): return iter(self.a) def __contains__(self, el): return el in self.a def __len__(self): return len(self.a) ``` ### Sequence * **Only required methods are len() and getitem().** * **Getitem() should return an item at the passed index or raise IndexError.** * **Iter() and contains() automatically work on any object that has getitem() defined.** * **Reversed() automatically works on any object that has getitem() and len() defined.** ```python class MySequence: def __init__(self, a): self.a = a def __iter__(self): return iter(self.a) def __contains__(self, el): return el in self.a def __len__(self): return len(self.a) def __getitem__(self, i): return self.a[i] def __reversed__(self): return reversed(self.a) ``` #### Discrepancies between glossary definitions and abstract base classes: * **Glossary defines iterable as any object with iter() or getitem() and sequence as any object with getitem() and len(). It does not define collection.** * **Passing ABC Iterable to isinstance() or issubclass() checks whether object/class has method iter(), while ABC Collection checks for iter(), contains() and len().** ### ABC Sequence * **It's a richer interface than the basic sequence.** * **Extending it generates iter(), contains(), reversed(), index() and count().** * **Unlike `'abc.Iterable'` and `'abc.Collection'`, it is not a duck type. That is why `'issubclass(MySequence, abc.Sequence)'` would return False even if MySequence had all the methods defined. It however recognizes list, tuple, range, str, bytes, bytearray, memoryview and deque, because they are registered as Sequence's virtual subclasses.** ```python from collections import abc class MyAbcSequence(abc.Sequence): def __init__(self, a): self.a = a def __len__(self): return len(self.a) def __getitem__(self, i): return self.a[i] ``` #### Table of required and automatically available special methods: ```text +------------+------------+------------+------------+--------------+ | | Iterable | Collection | Sequence | abc.Sequence | +------------+------------+------------+------------+--------------+ | iter() | REQ | REQ | Yes | Yes | | contains() | Yes | Yes | Yes | Yes | | len() | | REQ | REQ | REQ | | getitem() | | | REQ | REQ | | reversed() | | | Yes | Yes | | index() | | | | Yes | | count() | | | | Yes | +------------+------------+------------+------------+--------------+ ``` * **Other ABCs that generate missing methods are: MutableSequence, Set, MutableSet, Mapping and MutableMapping.** * **Names of their required methods are stored in `'.__abstractmethods__'`.** Enum ---- ```python from enum import Enum, auto ``` ```python class (Enum): = = , = auto() ``` * **If there are no numeric values before auto(), it returns 1.** * **Otherwise it returns an increment of the last numeric value.** * **Accessing a member named after a reserved keyword causes SyntaxError.** ```python = . # Returns a member. = [''] # Returns a member or raises KeyError. = () # Returns a member or raises ValueError. = .name # Returns member's name. = .value # Returns member's value. ``` ```python list_of_members = list() member_names = [a.name for a in ] member_values = [a.value for a in ] random_member = random.choice(list()) ``` ```python def get_next_member(member): members = list(member.__class__) index = (members.index(member) + 1) % len(members) return members[index] ``` ### Inline ```python Cutlery = Enum('Cutlery', 'FORK KNIFE SPOON') Cutlery = Enum('Cutlery', ['FORK', 'KNIFE', 'SPOON']) Cutlery = Enum('Cutlery', {'FORK': 1, 'KNIFE': 2, 'SPOON': 3}) ``` #### User-defined functions cannot be values, so they must be wrapped: ```python from functools import partial LogicOp = Enum('LogicOp', {'AND': partial(lambda l, r: l and r), 'OR': partial(lambda l, r: l or r)}) ``` Exceptions ---------- ```python try: except : ``` ### Complex Example ```python try: except : except : else: finally: ``` * **Code inside the `'else'` block will only be executed if `'try'` block had no exceptions.** * **Code inside the `'finally'` block will always be executed (unless a signal is received).** * **All variables that are initialized in executed blocks are also visible in all subsequent blocks, as well as outside the try/except clause (only function blocks delimit scope).** * **To catch signals use `'signal.signal(signal_number, )'`.** ### Catching Exceptions ```python except : ... except as : ... except (, [...]): ... except (, [...]) as : ... ``` * **Also catches subclasses of the exception.** * **Use `'traceback.print_exc()'` to print the error message to stderr.** * **Use `'print()'` to print just the cause of the exception (its arguments).** * **Use `'logging.exception()'` to log the passed message, followed by the full error message of the caught exception.** ### Raising Exceptions ```python raise raise () raise ( [, ...]) ``` #### Re-raising caught exception: ```python except [as ]: ... raise ``` ### Exception Object ```python arguments = .args exc_type = .__class__ filename = .__traceback__.tb_frame.f_code.co_filename func_name = .__traceback__.tb_frame.f_code.co_name line = linecache.getline(filename, .__traceback__.tb_lineno) trace_str = ''.join(traceback.format_tb(.__traceback__)) error_msg = ''.join(traceback.format_exception(exc_type, , .__traceback__)) ``` ### Built-in Exceptions ```text BaseException +-- SystemExit # Raised by the sys.exit() function. +-- KeyboardInterrupt # Raised when the user hits the interrupt key (ctrl-c). +-- Exception # User-defined exceptions should be derived from this class. +-- ArithmeticError # Base class for arithmetic errors. | +-- ZeroDivisionError # Raised when dividing by zero. +-- AssertionError # Raised by `assert ` if expression returns false value. +-- AttributeError # Raised when an attribute is missing. +-- EOFError # Raised by input() when it hits end-of-file condition. +-- LookupError # Base class for errors when a collection can't find an item. | +-- IndexError # Raised when a sequence index is out of range. | +-- KeyError # Raised when a dictionary key or set element is missing. +-- MemoryError # Out of memory. Could be too late to start deleting vars. +-- NameError # Raised when nonexistent name (variable/func/class) is used. | +-- UnboundLocalError # Raised when local name is used before it's being defined. +-- OSError # Errors such as FileExistsError/PermissionError (see Open). +-- RuntimeError # Raised by errors that don't fall into other categories. | +-- RecursionError # Raised when the maximum recursion depth is exceeded. +-- StopIteration # Raised by next() when run on an empty iterator. +-- TypeError # Raised when an argument is of wrong type. +-- ValueError # When an argument is of right type but inappropriate value. +-- UnicodeError # Raised when encoding/decoding strings to/from bytes fails. ``` #### Collections and their exceptions: ```text +-----------+------------+------------+------------+ | | List | Set | Dict | +-----------+------------+------------+------------+ | getitem() | IndexError | | KeyError | | pop() | IndexError | KeyError | KeyError | | remove() | ValueError | KeyError | | | index() | ValueError | | | +-----------+------------+------------+------------+ ``` #### Useful built-in exceptions: ```python raise TypeError('Argument is of wrong type!') raise ValueError('Argument is of right type but inappropriate value!') raise RuntimeError('None of above!') ``` ### User-defined Exceptions ```python class MyError(Exception): pass class MyInputError(MyError): pass ``` Exit ---- **Exits the interpreter by raising SystemExit exception.** ```python import sys sys.exit() # Exits with exit code 0 (success). sys.exit() # Prints to stderr and exits with 1. sys.exit() # Exits with passed exit code. ``` Print ----- ```python print(, ..., sep=' ', end='\n', file=sys.stdout, flush=False) ``` * **Use `'file=sys.stderr'` for messages about errors.** * **Use `'flush=True'` to forcibly flush the stream.** ### Pretty Print ```python from pprint import pprint pprint(, width=80, depth=None, compact=False, sort_dicts=True) ``` * **Levels deeper than 'depth' get replaced by '...'.** Input ----- **Reads a line from user input or pipe if present.** ```python = input(prompt=None) ``` * **Trailing newline gets stripped.** * **Prompt string is printed to the standard output before reading input.** * **Raises EOFError when user hits EOF (ctrl-d/ctrl-z⏎) or input stream gets exhausted.** Command Line Arguments ---------------------- ```python import sys scripts_path = sys.argv[0] arguments = sys.argv[1:] ``` ### Argument Parser ```python from argparse import ArgumentParser, FileType p = ArgumentParser(description=) p.add_argument('-', '--', action='store_true') # Flag. p.add_argument('-', '--', type=) # Option. p.add_argument('', type=, nargs=1) # First argument. p.add_argument('', type=, nargs='+') # Remaining arguments. p.add_argument('', type=, nargs='*') # Optional arguments. args = p.parse_args() # Exits on error. value = args. ``` * **Use `'help='` to set argument description that will be displayed in help message.** * **Use `'default='` to set the default value.** * **Use `'type=FileType()'` for files. Accepts 'encoding', but 'newline' is None.** Open ---- **Opens the file and returns a corresponding file object.** ```python = open(, mode='r', encoding=None, newline=None) ``` * **`'encoding=None'` means that the default encoding is used, which is platform dependent. Best practice is to use `'encoding="utf-8"'` whenever possible.** * **`'newline=None'` means all different end of line combinations are converted to '\n' on read, while on write all '\n' characters are converted to system's default line separator.** * **`'newline=""'` means no conversions take place, but input is still broken into chunks by readline() and readlines() on every '\n', '\r' and '\r\n'.** ### Modes * **`'r'` - Read (default).** * **`'w'` - Write (truncate).** * **`'x'` - Write or fail if the file already exists.** * **`'a'` - Append.** * **`'w+'` - Read and write (truncate).** * **`'r+'` - Read and write from the start.** * **`'a+'` - Read and write from the end.** * **`'t'` - Text mode (default).** * **`'b'` - Binary mode (`'br'`, `'bw'`, `'bx'`, …).** ### Exceptions * **`'FileNotFoundError'` can be raised when reading with `'r'` or `'r+'`.** * **`'FileExistsError'` can be raised when writing with `'x'`.** * **`'IsADirectoryError'` and `'PermissionError'` can be raised by any.** * **`'OSError'` is the parent class of all listed exceptions.** ### File Object ```python .seek(0) # Moves to the start of the file. .seek(offset) # Moves 'offset' chars/bytes from the start. .seek(0, 2) # Moves to the end of the file. .seek(±offset, ) # Anchor: 0 start, 1 current position, 2 end. ``` ```python = .read(size=-1) # Reads 'size' chars/bytes or until EOF. = .readline() # Returns a line or empty string/bytes on EOF. = .readlines() # Returns a list of remaining lines. = next() # Returns a line using buffer. Do not mix. ``` ```python .write() # Writes a string or bytes object. .writelines() # Writes a coll. of strings or bytes objects. .flush() # Flushes write buffer. Runs every 4096/8192 B. ``` * **Methods do not add or strip trailing newlines, even writelines().** ### Read Text from File ```python def read_file(filename): with open(filename, encoding='utf-8') as file: return file.readlines() ``` ### Write Text to File ```python def write_to_file(filename, text): with open(filename, 'w', encoding='utf-8') as file: file.write(text) ``` Paths ----- ```python from os import getcwd, path, listdir, scandir from glob import glob ``` ```python = getcwd() # Returns the current working directory. = path.join(, ...) # Joins two or more pathname components. = path.abspath() # Returns absolute path. ``` ```python = path.basename() # Returns final component of the path. = path.dirname() # Returns path without the final component. = path.splitext() # Splits on last period of the final component. ``` ```python = listdir(path='.') # Returns filenames located at the path. = glob('') # Returns paths matching the wildcard pattern. ``` ```python = path.exists() # Or: .exists() = path.isfile() # Or: .is_file() = path.isdir() # Or: .is_dir() ``` ```python = os.stat() # Or: .stat() = .st_mtime/st_size/… # Modification time, size in bytes, ... ``` ### DirEntry **Unlike listdir(), scandir() returns DirEntry objects that cache isfile, isdir and on Windows also stat information, thus significantly increasing the performance of code that requires it.** ```python = scandir(path='.') # Returns DirEntry objects located at the path. = .path # Returns the whole path as a string. = .name # Returns final component as a string. = open() # Opens the file and returns a file object. ``` ### Path Object ```python from pathlib import Path ``` ```python = Path( [, ...]) # Accepts strings, Paths and DirEntry objects. = / [/ ...] # First or second path must be a Path object. ``` ```python = Path() # Returns relative cwd. Also Path('.'). = Path.cwd() # Returns absolute cwd. Also Path().resolve(). = Path.home() # Returns user's home directory (absolute). = Path(__file__).resolve() # Returns script's path if cwd wasn't changed. ``` ```python = .parent # Returns Path without the final component. = .name # Returns final component as a string. = .stem # Returns final component without extension. = .suffix # Returns final component's extension. = .parts # Returns all components as strings. ``` ```python = .iterdir() # Returns directory contents as Path objects. = .glob('') # Returns Paths matching the wildcard pattern. ``` ```python = str() # Returns path as a string. = open() # Also .read/write_text/bytes(). ``` OS Commands ----------- ```python import os, shutil, subprocess ``` ```python os.chdir() # Changes the current working directory. os.mkdir(, mode=0o777) # Creates a directory. Permissions are in octal. os.makedirs(, mode=0o777) # Creates all path's dirs. Also: `exist_ok=False`. ``` ```python shutil.copy(from, to) # Copies the file. 'to' can exist or be a dir. shutil.copytree(from, to) # Copies the directory. 'to' must not exist. ``` ```python os.rename(from, to) # Renames/moves the file or directory. os.replace(from, to) # Same, but overwrites 'to' if it exists. ``` ```python os.remove() # Deletes the file. os.rmdir() # Deletes the empty directory. shutil.rmtree() # Deletes the directory. ``` * **Paths can be either strings, Paths or DirEntry objects.** * **Functions report OS related errors by raising either OSError or one of its [subclasses](#exceptions-1).** ### Shell Commands ```python = os.popen('') # Executes command in sh/cmd. Returns its stdout pipe. = .read(size=-1) # Reads 'size' chars or until EOF. Also readline/s(). = .close() # Closes the pipe. Returns None on success. ``` #### Sends '1 + 1' to the basic calculator and captures its output: ```python >>> subprocess.run('bc', input='1 + 1\n', capture_output=True, text=True) CompletedProcess(args='bc', returncode=0, stdout='2\n', stderr='') ``` #### Sends test.in to the basic calculator running in standard mode and saves its output to test.out: ```python >>> from shlex import split >>> os.popen('echo 1 + 1 > test.in') >>> subprocess.run(split('bc -s'), stdin=open('test.in'), stdout=open('test.out', 'w')) CompletedProcess(args=['bc', '-s'], returncode=0) >>> open('test.out').read() '2\n' ``` JSON ---- **Text file format for storing collections of strings and numbers.** ```python import json = json.dumps() # Converts object to JSON string. = json.loads() # Converts JSON string to object. ``` ### Read Object from JSON File ```python def read_json_file(filename): with open(filename, encoding='utf-8') as file: return json.load(file) ``` ### Write Object to JSON File ```python def write_to_json_file(filename, an_object): with open(filename, 'w', encoding='utf-8') as file: json.dump(an_object, file, ensure_ascii=False, indent=2) ``` Pickle ------ **Binary file format for storing Python objects.** ```python import pickle = pickle.dumps() # Converts object to bytes object. = pickle.loads() # Converts bytes object to object. ``` ### Read Object from File ```python def read_pickle_file(filename): with open(filename, 'rb') as file: return pickle.load(file) ``` ### Write Object to File ```python def write_to_pickle_file(filename, an_object): with open(filename, 'wb') as file: pickle.dump(an_object, file) ``` CSV --- **Text file format for storing spreadsheets.** ```python import csv ``` ### Read ```python = csv.reader() # Also: `dialect='excel', delimiter=','`. = next() # Returns next row as a list of strings. = list() # Returns a list of remaining rows. ``` * **File must be opened with a `'newline=""'` argument, or newlines embedded inside quoted fields will not be interpreted correctly!** * **To print the spreadsheet to the console use [Tabulate](#table) library.** * **For XML and binary Excel files (xlsx, xlsm and xlsb) use [Pandas](#dataframe-plot-encode-decode) library.** * **Reader accepts any iterator of strings, not just files.** ### Write ```python = csv.writer() # Also: `dialect='excel', delimiter=','`. .writerow() # Encodes objects using `str()`. .writerows() # Appends multiple rows. ``` * **File must be opened with a `'newline=""'` argument, or '\r' will be added in front of every '\n' on platforms that use '\r\n' line endings!** ### Parameters * **`'dialect'` - Master parameter that sets the default values. String or a Dialect object.** * **`'delimiter'` - A one-character string used to separate fields.** * **`'quotechar'` - Character for quoting fields that contain special characters.** * **`'doublequote'` - Whether quotechars inside fields are/get doubled or escaped.** * **`'skipinitialspace'` - Is space character at the start of the field stripped by the reader.** * **`'lineterminator'` - How writer terminates rows. Reader is hardcoded to '\n', '\r', '\r\n'.** * **`'quoting'` - 0: As necessary, 1: All, 2: All but numbers which are read as floats, 3: None.** * **`'escapechar'` - Character for escaping quotechars if doublequote is False.** ### Dialects ```text +------------------+--------------+--------------+--------------+ | | excel | excel-tab | unix | +------------------+--------------+--------------+--------------+ | delimiter | ',' | '\t' | ',' | | quotechar | '"' | '"' | '"' | | doublequote | True | True | True | | skipinitialspace | False | False | False | | lineterminator | '\r\n' | '\r\n' | '\n' | | quoting | 0 | 0 | 1 | | escapechar | None | None | None | +------------------+--------------+--------------+--------------+ ``` ### Read Rows from CSV File ```python def read_csv_file(filename, dialect='excel'): with open(filename, encoding='utf-8', newline='') as file: return list(csv.reader(file, dialect)) ``` ### Write Rows to CSV File ```python def write_to_csv_file(filename, rows, dialect='excel'): with open(filename, 'w', encoding='utf-8', newline='') as file: writer = csv.writer(file, dialect) writer.writerows(rows) ``` SQLite ------ **A server-less database engine that stores each database into a separate file.** ```python import sqlite3 = sqlite3.connect() # Opens existing or new file. Also ':memory:'. .close() # Closes the connection. ``` ### Read ```python = .execute('') # Can raise a subclass of sqlite3.Error. = .fetchone() # Returns next row. Also next(). = .fetchall() # Returns remaining rows. Also list(). ``` ### Write ```python .execute('') # Can raise a subclass of sqlite3.Error. .commit() # Saves all changes since the last commit. .rollback() # Discards all changes since the last commit. ``` #### Or: ```python with : # Exits the block with commit() or rollback(), .execute('') # depending on whether any exception occurred. ``` ### Placeholders ```python .execute('', ) # Replaces '?'s in query with values. .execute('', ) # Replaces ':'s with values. .executemany('', ) # Runs execute() multiple times. ``` * **Passed values can be of type str, int, float, bytes, None, bool, datetime.date or datetime.datetime.** * **Bools will be stored and returned as ints and dates as [ISO formatted strings](#encode).** ### Example **Values are not actually saved in this example because `'conn.commit()'` is omitted!** ```python >>> conn = sqlite3.connect('test.db') >>> conn.execute('CREATE TABLE person (person_id INTEGER PRIMARY KEY, name, height)') >>> conn.execute('INSERT INTO person VALUES (NULL, ?, ?)', ('Jean-Luc', 187)).lastrowid 1 >>> conn.execute('SELECT * FROM person').fetchall() [(1, 'Jean-Luc', 187)] ``` ### SqlAlchemy ```python # $ pip3 install sqlalchemy from sqlalchemy import create_engine, text = create_engine('') # Url: 'dialect://user:password@host/dbname'. = .connect() # Creates a connection. Also .close(). = .execute(text(''), …) # Replaces ':'s with keyword arguments. with .begin(): ... # Exits the block with commit or rollback. ``` ```text +------------+--------------+-----------+-----------------------------------+ | Dialects | pip3 install | import | Dependencies | +------------+--------------+-----------+-----------------------------------+ | mysql | mysqlclient | MySQLdb | www.pypi.org/project/mysqlclient | | postgresql | psycopg2 | psycopg2 | www.psycopg.org/docs/install.html | | mssql | pyodbc | pyodbc | apt install g++ unixodbc-dev | | oracle | cx_oracle | cx_Oracle | Oracle Instant Client | +------------+--------------+-----------+-----------------------------------+ ``` Bytes ----- **Bytes object is an immutable sequence of single bytes. Mutable version is called bytearray.** ```python = b'' # Only accepts ASCII characters and \x00-\xff. = [] # Returns an int in range from 0 to 255. = [] # Returns bytes even if it has only one element. = .join() # Joins elements using bytes as a separator. ``` ### Encode ```python = bytes() # Ints must be in range from 0 to 255. = bytes(, 'utf-8') # Or: .encode('utf-8') = .to_bytes(n_bytes, …) # `byteorder='little/big', signed=False`. = bytes.fromhex('') # Hex pairs can be separated by whitespaces. ``` ### Decode ```python = list() # Returns ints in range from 0 to 255. = str(, 'utf-8') # Or: .decode('utf-8') = int.from_bytes(, …) # `byteorder='little/big', signed=False`. '' = .hex() # Returns hex pairs. Accepts `sep=`. ``` ### Read Bytes from File ```python def read_bytes(filename): with open(filename, 'rb') as file: return file.read() ``` ### Write Bytes to File ```python def write_bytes(filename, bytes_obj): with open(filename, 'wb') as file: file.write(bytes_obj) ``` Struct ------ * **Module that performs conversions between a sequence of numbers and a bytes object.** * **System’s type sizes, byte order, and alignment rules are used by default.** ```python from struct import pack, unpack = pack('', [, ...]) # Packages arguments into bytes object. = unpack('', ) # Use iter_unpack() for iterator of tuples. ``` ```python >>> pack('>hhl', 1, 2, 3) b'\x00\x01\x00\x02\x00\x00\x00\x03' >>> unpack('>hhl', b'\x00\x01\x00\x02\x00\x00\x00\x03') (1, 2, 3) ``` ### Format #### For standard type sizes and manual alignment (padding) start format string with: * **`'='` - System's byte order (usually little-endian).** * **`'<'` - Little-endian.** * **`'>'` - Big-endian (also `'!'`).** #### Besides numbers, pack() and unpack() also support bytes objects as part of the sequence: * **`'c'` - A bytes object with a single element. For pad byte use `'x'`.** * **`'s'` - A bytes object with n elements.** #### Integer types. Use a capital letter for unsigned type. Minimum and standard sizes are in brackets: * **`'b'` - char (1/1)** * **`'h'` - short (2/2)** * **`'i'` - int (2/4)** * **`'l'` - long (4/4)** * **`'q'` - long long (8/8)** #### Floating point types: * **`'f'` - float (4/4)** * **`'d'` - double (8/8)** Array ----- **List that can only hold numbers of a predefined type. Available types and their minimum sizes in bytes are listed above. Sizes and byte order are always determined by the system.** ```python from array import array = array('', ) # Array from collection of numbers. = array('', ) # Array from bytes object. = array('', ) # Treats array as a sequence of numbers. = bytes() # Or: .tobytes() .write() # Writes array to the binary file. ``` Memory View ----------- * **A sequence object that points to the memory of another object.** * **Each element can reference a single or multiple consecutive bytes, depending on format.** * **Order and number of elements can be changed with slicing.** * **Casting only works between char and other types and uses system's sizes.** * **Byte order is always determined by the system.** ```python = memoryview() # Immutable if bytes, else mutable. = [] # Returns an int or a float. = [] # Mview with rearranged elements. = .cast('') # Casts memoryview to the new format. .release() # Releases the object's memory buffer. ``` ### Decode ```python = bytes() # Returns a new bytes object. = .join() # Joins mviews using bytes object as sep. = array('', ) # Treats mview as a sequence of numbers. .write() # Writes mview to the binary file. ``` ```python = list() # Returns a list of ints or floats. = str(, 'utf-8') # Treats mview as a bytes object. = int.from_bytes(, …) # `byteorder='little/big', signed=False`. '' = .hex() # Treats mview as a bytes object. ``` Deque ----- **A thread-safe list with efficient appends and pops from either side. Pronounced "deck".** ```python from collections import deque = deque(, maxlen=None) ``` ```python .appendleft() # Opposite element is dropped if full. .extendleft() # Collection gets reversed. = .popleft() # Raises IndexError if empty. .rotate(n=1) # Rotates elements to the right. ``` Threading --------- * **CPython interpreter can only run a single thread at a time.** * **That is why using multiple threads won't result in a faster execution, unless at least one of the threads contains an I/O operation.** ```python from threading import Thread, RLock, Semaphore, Event, Barrier from concurrent.futures import ThreadPoolExecutor, as_completed ``` ### Thread ```python = Thread(target=) # Use `args=` to set the arguments. .start() # Starts the thread. = .is_alive() # Checks if the thread has finished executing. .join() # Waits for the thread to finish. ``` * **Use `'kwargs='` to pass keyword arguments to the function.** * **Use `'daemon=True'`, or the program will not be able to exit while the thread is alive.** ### Lock ```python = RLock() # Lock that can only be released by acquirer. .acquire() # Waits for the lock to be available. .release() # Makes the lock available again. ``` #### Or: ```python with : # Enters the block by calling acquire(), ... # and exits it with release(). ``` ### Semaphore, Event, Barrier ```python = Semaphore(value=1) # Lock that can be acquired by 'value' threads. = Event() # Method wait() blocks until set() is called. = Barrier(n_times) # Wait() blocks until it's called n_times. ``` ### Thread Pool Executor * **Object that manages thread execution.** * **An object with the same interface called ProcessPoolExecutor provides true parallelism by running a separate interpreter in each process. All arguments must be [pickable](#pickle).** ```python = ThreadPoolExecutor(max_workers=None) # Or: `with ThreadPoolExecutor() as : …` .shutdown(wait=True) # Blocks until all threads finish executing. ``` ```python = .map(, , ...) # A multithreaded and non-lazy map(). = .submit(, , ...) # Starts a thread and returns its Future object. = .done() # Checks if the thread has finished executing. = .result() # Waits for thread to finish and returns result. = as_completed() # Each Future is yielded as it completes. ``` ### Queue **A thread-safe FIFO queue. For LIFO queue use LifoQueue.** ```python from queue import Queue = Queue(maxsize=0) ``` ```python .put() # Blocks until queue stops being full. .put_nowait() # Raises queue.Full exception if full. = .get() # Blocks until queue stops being empty. = .get_nowait() # Raises queue.Empty exception if empty. ``` Operator -------- **Module of functions that provide the functionality of operators.** ```python import operator as op = op.add/sub/mul/truediv/floordiv/mod(, ) # +, -, *, /, //, % = op.and_/or_/xor(, ) # &, |, ^ = op.eq/ne/lt/le/gt/ge(, ) # ==, !=, <, <=, >, >= = op.itemgetter/attrgetter/methodcaller( [, ...]) # [index/key], .name, .name() ``` ```python elementwise_sum = map(op.add, list_a, list_b) sorted_by_second = sorted(, key=op.itemgetter(1)) sorted_by_both = sorted(, key=op.itemgetter(1, 0)) product_of_elems = functools.reduce(op.mul, ) union_of_sets = functools.reduce(op.or_, ) first_element = op.methodcaller('pop', 0)() ``` * **Binary operators require objects to have and(), or(), xor() and invert() special methods, unlike logical operators that work on all types of objects.** * **Also: `' = &|^ '` and `' = &|^ '`.** Introspection ------------- **Inspecting code at runtime.** ### Variables ```python = dir() # Names of local variables (incl. functions). = vars() # Dict of local variables. Also locals(). = globals() # Dict of global variables. ``` ### Attributes ```python = dir() # Names of object's attributes (incl. methods). = vars() # Dict of writable attributes. Also .__dict__. = hasattr(, '') # Checks if getattr() raises an AttributeError. value = getattr(, '') # Raises AttributeError if attribute is missing. setattr(, '', value) # Only works on objects with '__dict__' attribute. delattr(, '') # Same. Also `del .`. ``` ### Parameters ```python = inspect.signature() # Function's Signature object. = .parameters # Dict of Parameter objects. = .kind # Member of ParameterKind enum. = .default # Default value or .empty. = .annotation # Type or .empty. ``` Metaprogramming --------------- **Code that generates code.** ### Type **Type is the root class. If only passed an object it returns its type (class). Otherwise it creates a new class.** ```python = type('', , ) ``` ```python >>> Z = type('Z', (), {'a': 'abcde', 'b': 12345}) >>> z = Z() ``` ### Meta Class **A class that creates classes.** ```python def my_meta_class(name, parents, attrs): attrs['a'] = 'abcde' return type(name, parents, attrs) ``` #### Or: ```python class MyMetaClass(type): def __new__(cls, name, parents, attrs): attrs['a'] = 'abcde' return type.__new__(cls, name, parents, attrs) ``` * **New() is a class method that gets called before init(). If it returns an instance of its class, then that instance gets passed to init() as a 'self' argument.** * **It receives the same arguments as init(), except for the first one that specifies the desired type of the returned instance (MyMetaClass in our case).** * **Like in our case, new() can also be called directly, usually from a new() method of a child class (**`def __new__(cls): return super().__new__(cls)`**).** * **The only difference between the examples above is that my\_meta\_class() returns a class of type type, while MyMetaClass() returns a class of type MyMetaClass.** ### Metaclass Attribute **Right before a class is created it checks if it has the 'metaclass' attribute defined. If not, it recursively checks if any of his parents has it defined and eventually comes to type().** ```python class MyClass(metaclass=MyMetaClass): b = 12345 ``` ```python >>> MyClass.a, MyClass.b ('abcde', 12345) ``` ### Type Diagram ```python type(MyClass) == MyMetaClass # MyClass is an instance of MyMetaClass. type(MyMetaClass) == type # MyMetaClass is an instance of type. ``` ```text +-------------+-------------+ | Classes | Metaclasses | +-------------+-------------| | MyClass <-- MyMetaClass | | | ^ | | object <----- type <+ | | | | +--+ | | str <---------+ | +-------------+-------------+ ``` ### Inheritance Diagram ```python MyClass.__base__ == object # MyClass is a subclass of object. MyMetaClass.__base__ == type # MyMetaClass is a subclass of type. ``` ```text +-------------+-------------+ | Classes | Metaclasses | +-------------+-------------| | MyClass | MyMetaClass | | ^ | ^ | | object -----> type | | v | | | str | | +-------------+-------------+ ``` Eval ---- ```python >>> from ast import literal_eval >>> literal_eval('[1, 2, 3]') [1, 2, 3] >>> literal_eval('1 + 2') ValueError: malformed node or string ``` Coroutines ---------- * **Coroutines have a lot in common with threads, but unlike threads, they only give up control when they call another coroutine and they don’t use as much memory.** * **Coroutine definition starts with `'async'` and its call with `'await'`.** * **`'asyncio.run()'` is the main entry point for asynchronous programs.** * **Functions wait(), gather() and as_completed() start multiple coroutines at the same time.** * **Asyncio module also provides its own [Queue](#queue), [Event](#semaphore-event-barrier), [Lock](#lock) and [Semaphore](#semaphore-event-barrier) classes.** #### Runs a terminal game where you control an asterisk that must avoid numbers: ```python import asyncio, collections, curses, curses.textpad, enum, random P = collections.namedtuple('P', 'x y') # Position D = enum.Enum('D', 'n e s w') # Direction W, H = 15, 7 # Width, Height def main(screen): curses.curs_set(0) # Makes cursor invisible. screen.nodelay(True) # Makes getch() non-blocking. asyncio.run(main_coroutine(screen)) # Starts running asyncio code. async def main_coroutine(screen): moves = asyncio.Queue() state = {'*': P(0, 0), **{id_: P(W//2, H//2) for id_ in range(10)}} ai = [random_controller(id_, moves) for id_ in range(10)] mvc = [human_controller(screen, moves), model(moves, state), view(state, screen)] tasks = [asyncio.create_task(cor) for cor in ai + mvc] await asyncio.wait(tasks, return_when=asyncio.FIRST_COMPLETED) async def random_controller(id_, moves): while True: d = random.choice(list(D)) moves.put_nowait((id_, d)) await asyncio.sleep(random.triangular(0.01, 0.65)) async def human_controller(screen, moves): while True: ch = screen.getch() key_mappings = {258: D.s, 259: D.n, 260: D.w, 261: D.e} if ch in key_mappings: moves.put_nowait(('*', key_mappings[ch])) await asyncio.sleep(0.005) async def model(moves, state): while state['*'] not in (state[id_] for id_ in range(10)): id_, d = await moves.get() x, y = state[id_] deltas = {D.n: P(0, -1), D.e: P(1, 0), D.s: P(0, 1), D.w: P(-1, 0)} state[id_] = P((x + deltas[d].x) % W, (y + deltas[d].y) % H) async def view(state, screen): offset = P(curses.COLS//2 - W//2, curses.LINES//2 - H//2) while True: screen.erase() curses.textpad.rectangle(screen, offset.y-1, offset.x-1, offset.y+H, offset.x+W) for id_, p in state.items(): screen.addstr(offset.y + (p.y - state['*'].y + H//2) % H, offset.x + (p.x - state['*'].x + W//2) % W, str(id_)) await asyncio.sleep(0.005) if __name__ == '__main__': curses.wrapper(main) ```
Libraries ========= Progress Bar ------------ ```python # $ pip3 install tqdm >>> from tqdm import tqdm >>> from time import sleep >>> for el in tqdm([1, 2, 3], desc='Processing'): ... sleep(1) Processing: 100%|████████████████████| 3/3 [00:03<00:00, 1.00s/it] ``` Plot ---- ```python # $ pip3 install matplotlib import matplotlib.pyplot as plt plt.plot(, [, label=]) # Or: plt.plot() plt.legend() # Adds a legend. plt.savefig() # Saves the figure. plt.show() # Displays the figure. plt.clf() # Clears the figure. ``` Table ----- #### Prints a CSV file as an ASCII table: ```python # $ pip3 install tabulate import csv, tabulate with open('test.csv', encoding='utf-8', newline='') as file: rows = csv.reader(file) header = next(rows) table = tabulate.tabulate(rows, header) print(table) ``` Curses ------ #### Runs a basic file explorer in the terminal: ```python import curses, curses.ascii, os from curses import A_REVERSE, KEY_DOWN, KEY_UP, KEY_LEFT, KEY_RIGHT, KEY_ENTER def main(screen): ch, first, selected, paths = 0, 0, 0, os.listdir() while ch != curses.ascii.ESC: height, _ = screen.getmaxyx() screen.erase() for y, filename in enumerate(paths[first : first+height]): screen.addstr(y, 0, filename, A_REVERSE * (selected == first + y)) ch = screen.getch() selected += (ch == KEY_DOWN) - (ch == KEY_UP) selected = max(0, min(len(paths)-1, selected)) first += (first <= selected - height) - (first > selected) if ch in [KEY_LEFT, KEY_RIGHT, KEY_ENTER, 10, 13]: new_dir = '..' if ch == KEY_LEFT else paths[selected] if os.path.isdir(new_dir): os.chdir(new_dir) first, selected, paths = 0, 0, os.listdir() if __name__ == '__main__': curses.wrapper(main) ``` Logging ------- ```python # $ pip3 install loguru from loguru import logger ``` ```python logger.add('debug_{time}.log', colorize=True) # Connects a log file. logger.add('error_{time}.log', level='ERROR') # Another file for errors or higher. logger.('A logging message.') # Logs to file/s and prints to stderr. ``` * **Levels: `'debug'`, `'info'`, `'success'`, `'warning'`, `'error'`, `'critical'`.** ### Exceptions **Exception description, stack trace and values of variables are appended automatically.** ```python try: ... except : logger.exception('An error happened.') ``` ### Rotation **Argument that sets a condition when a new log file is created.** ```python rotation=||| ``` * **`''` - Max file size in bytes.** * **`''` - Max age of a file.** * **`'