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python-cheatsheet Documentation

Release 0.1.

crazyguitar

May 01, 2022

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CONTENTS 1

CHAPTER

ONE

WHAT’S NEW IN PYTHON 3

The official document, What’s New In Python, displays all of the most important changes. However, if you’re too busy to read the whole changes, this part provides a brief glance of new features in Python 3.

1.1 New in Python

Table of Contents

• New in Python - print is a function - String is unicode - Division Operator - New dict implementation - Keyword-Only Arguments - New Super - Remove <> - BDFL retirement - Not allow from module import * inside function - Add nonlocal keyword - Extended iterable unpacking - General unpacking - Function annotations - Variable annotations - Core support for typing module and generic types - Format byte string - fstring - Suppressing exception - Generator delegation - async and await syntax

- Asynchronous generators - Asynchronous comprehensions - Matrix multiplication - Data Classes - Built-in breakpoint() - The walrus operator - Positional-only parameters - Dictionary Merge

1.1.1 print is a function

New in Python 3.

• PEP 3105 - Make print a function

Python 2

>>> print "print is a statement" print is a statement >>> for x in range(3): ... print x, ... 0 1 2

Python 3

>>> print("print is a function") print is a function >>> print() >>> for x in range(3): ... print(x, end=' ') ... else: ... print() ... 0 1 2

1.1.2 String is unicode

New in Python 3.

• PEP 3138 - String representation in Python 3000
• PEP 3120 - Using UTF-8 as the default source encoding
• PEP 3131 - Supporting Non-ASCII Identifiers

Python 2

4 Chapter 1. What’s New In Python 3

1.1.4 New dict implementation

New in Python 3.

• PEP 468 - Preserving the order of **kwargs in a function
• PEP 520 - Preserving Class Attribute Definition Order
• bpo 27350 - More compact dictionaries with faster iteration

Before Python 3.

>>> import sys >>> sys.getsizeof({str(i):i for i in range(1000)}) 49248

>>> d = {'timmy': 'red', 'barry': 'green', 'guido': 'blue'} >>> d # without order-preserving {'barry': 'green', 'timmy': 'red', 'guido': 'blue'}

Python 3.

• Memory usage is smaller than Python 3.
• Preserve insertion ordered

>>> import sys >>> sys.getsizeof({str(i):i for i in range(1000)}) 36968

>>> d = {'timmy': 'red', 'barry': 'green', 'guido': 'blue'} >>> d # preserve insertion ordered {'timmy': 'red', 'barry': 'green', 'guido': 'blue'}

1.1.5 Keyword-Only Arguments

New in Python 3.

• PEP 3102 - Keyword-Only Arguments

>>> def f(a, b, *, kw): ... print(a, b, kw) ... >>> f(1, 2, 3) Traceback (most recent call last): File "", line 1, in TypeError: f() takes 2 positional arguments but 3 were given >>> f(1, 2) Traceback (most recent call last): (continues on next page)

6 Chapter 1. What’s New In Python 3

(continued from previous page) File "", line 1, in TypeError: f() missing 1 required keyword-only argument: 'kw' >>> f(1, 2, kw=3) 1 2 3

1.1.6 New Super

New in Python 3.

• PEP 3135 - New Super

Python 2

>>> class ParentCls(object): ... def foo(self): ... print "call parent" ... >>> class ChildCls(ParentCls): ... def foo(self): ... super(ChildCls, self).foo() ... print "call child" ... >>> p = ParentCls() >>> c = ChildCls() >>> p.foo() call parent >>> c.foo() call parent call child

Python 3

>>> class ParentCls(object): ... def foo(self): ... print("call parent") ... >>> class ChildCls(ParentCls): ... def foo(self): ... super().foo() ... print("call child") ... >>> p = ParentCls() >>> c = ChildCls() >>> p.foo() call parent >>> c.foo() call parent call child

1.1. New in Python3 7

1.1.10 Add nonlocal keyword

New in Python 3.

PEP 3104 - Access to Names in Outer Scopes

Note: nonlocal allow assigning directly to a variable in an outer (but non-global) scope

>>> def outf(): ... o = "out" ... def inf(): ... nonlocal o ... o = "change out" ... inf() ... print(o) ... >>> outf() change out

1.1.11 Extended iterable unpacking

New in Python 3.

• PEP 3132 - Extended Iterable Unpacking

>>> a, *b, c = range(5) >>> a, b, c (0, [1, 2, 3], 4) >>> for a, *b in [(1, 2, 3), (4, 5, 6, 7)]: ... print(a, b) ... 1 [2, 3] 4 [5, 6, 7]

1.1.12 General unpacking

New in Python 3.

• PEP 448 - Additional Unpacking Generalizations

Python 2

>>> def func(*a, *k): ... print(a) ... print(k) ... >>> func([1,2,3,4,5], **{"foo": "bar"}) (1, 2, 3, 4, 5) {'foo': 'bar'}

Python 3

1.1. New in Python3 9

>>> print(*[1, 2, 3], 4, [5, 6]) 1 2 3 4 5 6 >>> [range(4), 4] [0, 1, 2, 3, 4] >>> {"foo": "Foo", "bar": "Bar", *{"baz": "baz"}} {'foo': 'Foo', 'bar': 'Bar', 'baz': 'baz'} >>> def func(a, *k): ... print(a) ... print(k) ... >>> func([1], *[4,5], **{"foo": "FOO"}, **{"bar": "BAR"}) (1, 4, 5) {'foo': 'FOO', 'bar': 'BAR'}

1.1.13 Function annotations

New in Python 3.

• PEP 3107 - Function Annotations
• PEP 484 - Type Hints
• PEP 483 - The Theory of Type Hints

>>> import types >>> generator = types.GeneratorType >>> def fib(n: int) -> generator: ... a, b = 0, 1 ... for _ in range(n): ... yield a ... b, a = a + b, b ... >>> [f for f in fib(10)] [0, 1, 1, 2, 3, 5, 8, 13, 21, 34]

1.1.14 Variable annotations

New in Python 3.

• PEP 526 - Syntax for Variable Annotations

>>> from typing import List >>> x: List[int] = [1, 2, 3] >>> x [1, 2, 3]

>>> from typing import List, Dict >>> class Cls(object): ... x: List[int] = [1, 2, 3] ... y: Dict[str, str] = {"foo": "bar"} ... >>> o = Cls() (continues on next page)

10 Chapter 1. What’s New In Python 3

1.1.16 Format byte string

New in Python 3.

• PEP 461 - Adding % formatting to bytes and bytearray

>>> b'abc %b %b' % (b'foo', b'bar') b'abc foo bar' >>> b'%d %f ' % (1, 3.14) b'1 3.140000' >>> class Cls(object): ... def repr(self): ... return "repr" ... def str(self): ... return "str" ... 'repr' >>> b'%a' % Cls() b'repr'

1.1.17 fstring

New in Python 3.

• PEP 498 - Literal String Interpolation

>>> py = "Python3" >>> f'Awesome {py}' 'Awesome Python3' >>> x = [1, 2, 3, 4, 5] >>> f'{x}' '[1, 2, 3, 4, 5]' >>> def foo(x:int) -> int: ... return x + 1 ... >>> f'{foo(0)}' ' 1 ' >>> f'{123.567:1.3}' '1.24e+02'

1.1.18 Suppressing exception

New in Python 3.

• PEP 409 - Suppressing exception context

Without raise Exception from None

>>> def func(): ... try: ... 1 / 0 ... except ZeroDivisionError: ... raise ArithmeticError (continues on next page)

12 Chapter 1. What’s New In Python 3

(continued from previous page)

... >>> func() Traceback (most recent call last): File "", line 3, in func ZeroDivisionError: division by zero

During handling of the above exception, another exception occurred:

Traceback (most recent call last): File "", line 1, in File "", line 5, in func ArithmeticError

With raise Exception from None

>>> def func(): ... try: ... 1 / 0 ... except ZeroDivisionError: ... raise ArithmeticError from None ... >>> func() Traceback (most recent call last): File "", line 1, in File "", line 5, in func ArithmeticError

debug

>>> try: ... func() ... except ArithmeticError as e: ... print(e.context) ... division by zero

1.1.19 Generator delegation

New in Python 3.

• PEP 380 - Syntax for Delegating to a Subgenerator

>>> def fib(n: int): ... a, b = 0, 1 ... for _ in range(n): ... yield a ... b, a = a + b, b ... >>> def delegate(n: int): ... yield from fib(n) ... (continues on next page)

1.1. New in Python3 13

1.1.21 Asynchronous generators

New in Python 3.

• PEP 525 - Asynchronous Generators

>>> import asyncio >>> async def fib(n: int): ... a, b = 0, 1 ... for _ in range(n): ... await asyncio.sleep(1) ... yield a ... b, a = a + b , b ... >>> async def coro(n: int): ... ag = fib(n) ... f = await ag.asend(None) ... print(f) ... f = await ag.asend(None) ... print(f) ... >>> loop = asyncio.get_event_loop() >>> loop.run_until_complete(coro(5)) 0 1

1.1.22 Asynchronous comprehensions

New in Python 3.

• PEP 530 - Asynchronous Comprehensions

>>> import asyncio >>> async def fib(n: int): ... a, b = 0, 1 ... for _ in range(n): ... await asyncio.sleep(1) ... yield a ... b, a = a + b , b ...

async for ... else

>>> async def coro(n: int): ... async for f in fib(n): ... print(f, end=" ") ... else: ... print() ... >>> loop = asyncio.get_event_loop() >>> loop.run_until_complete(coro(5)) 0 1 1 2 3

(continues on next page)

1.1. New in Python3 15

(continued from previous page)

async for in list

>>> async def coro(n: int): ... return [f async for f in fib(n)] ... >>> loop.run_until_complete(coro(5)) [0, 1, 1, 2, 3]

await in list

>>> async def slowfmt(n: int) -> str: ... await asyncio.sleep(0.5) ... return f'{n}' ... >>> async def coro(n: int): ... return [await slowfmt(f) async for f in fib(n)] ... >>> loop.run_until_complete(coro(5)) [' 0 ', ' 1 ', ' 1 ', ' 2 ', ' 3 ']

1.1.23 Matrix multiplication

New in Python 3.

• PEP 465 - A dedicated infix operator for matrix multiplication

>>> # "@" represent matrix multiplication >>> class Arr: ... def init(self, arg): ... self._arr = arg ... def matmul(self, other): ... if not isinstance(other, Arr): ... raise TypeError ... if len(self) != len(other): ... raise ValueError ... return sum([xy for x, y in zip(self._arr, other._arr)]) ... def imatmul(self, other): ... if not isinstance(other, Arr): ... raise TypeError ... if len(self) != len(other): ... raise ValueError ... res = sum([x*y for x, y in zip(self._arr, other._arr)]) ... self._arr = [res] ... return self ... def len(self): ... return len(self._arr) ... def str(self): ... return self.repr() ... def repr(self): ... return "Arr({})".format(repr(self._arr)) ... (continues on next page)

16 Chapter 1. What’s New In Python 3