New in Python3 cheatsheet¶

Table of Contents

`print` is a function¶

New in Python 3.0

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

String is unicode¶

New in Python 3.0

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

>>> s = 'Café'  # byte string
>>> s
'Caf\xc3\xa9'
>>> type(s)
<type 'str'>
>>> u = u'Café' # unicode string
>>> u
u'Caf\xe9'
>>> type(u)
<type 'unicode'>
>>> len([_c for _c in 'Café'])
5

Python 3

>>> s = 'Café'
>>> s
'Café'
>>> type(s)
<class 'str'>
>>> s.encode('utf-8')
b'Caf\xc3\xa9'
>>> s.encode('utf-8').decode('utf-8')
'Café'
>>> len([_c for _c in 'Café'])
4

New Super¶

New in Python 3.0

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

Remove `<>`¶

New in Python 3.0

Python 2

>>> a = "Python2"
>>> a <> "Python3"
True

# equal to !=
>>> a != "Python3"
True

Python 3

>>> a != "Python2"
True

Not allow `from module import *` inside function¶

New in Python 3.0

>>> def f():
...     from os import *
...
  File "<stdin>", line 1
SyntaxError: import * only allowed at module level

Add `nonlocal` keyword¶

New in Python 3.0

PEP 3104 - Access to Names in Outer Scopes

주석

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

Extended iterable unpacking¶

New in Python 3.0

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]

General unpacking¶

New in Python 3.5

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

>>> 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'}

Function annotations¶

New in Python 3.0

PEP 3107 - Function Annotations

>>> 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]

Variable annotations¶

New in Python 3.6

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()
>>> o.x
[1, 2, 3]
>>> o.y
{'foo': 'bar'}

Core support for typing module and generic types¶

New in Python 3.7

PEP 560 - Core support for typing module and generic types

Before Python 3.7

>>> from typing import Generic, TypeVar
>>> from typing import Iterable
>>> T = TypeVar('T')
>>> class C(Generic[T]): ...
...
>>> def func(l: Iterable[C[int]]) -> None:
...     for i in l:
...         print(i)
...
>>> func([1,2,3])
1
2
3

Python 3.7 or above

>>> from typing import Iterable
>>> class C:
...     def __class_getitem__(cls, item):
...         return f"{cls.__name__}[{item.__name__}]"
...
>>> def func(l: Iterable[C[int]]) -> None:
...     for i in l:
...         print(i)
...
>>> func([1,2,3])
1
2
3

Format byte string¶

New in Python 3.5

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'

fstring¶

New in Python 3.6

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'

Suppressing exception¶

New in Python 3.3

PEP 409 - Suppressing exception context

Without raise Exception from None

>>> def func():
...     try:
...         1 / 0
...     except ZeroDivisionError:
...         raise ArithmeticError
...
>>> func()
Traceback (most recent call last):
  File "<stdin>", line 3, in func
ZeroDivisionError: division by zero

During handling of the above exception, another exception occurred:

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

# debug

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

Generator delegation¶

New in Python 3.3

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(10)
...
>>> list(delegate(10))
[0, 1, 1, 2, 3, 5, 8, 13, 21, 34]

`async` and `await` syntax¶

New in Python 3.5

PEP 492 - Coroutines with async and await syntax

Before Python 3.5

>>> import asyncio
>>> @asyncio.coroutine
... def fib(n: int):
...     a, b = 0, 1
...     for _ in range(n):
...         b, a = a + b, b
...     return a
...
>>> @asyncio.coroutine
... def coro(n: int):
...     for x in range(n):
...         yield from asyncio.sleep(1)
...         f = yield from fib(x)
...         print(f)
...
>>> loop = asyncio.get_event_loop()
>>> loop.run_until_complete(coro(3))
0
1
1

Python 3.5 or above

>>> import asyncio
>>> async def fib(n: int):
...     a, b = 0, 1
...     for _ in range(n):
...         b, a = a + b, b
...     return a
...
>>> async def coro(n: int):
...     for x in range(n):
...         await asyncio.sleep(1)
...         f = await fib(x)
...         print(f)
...
>>> loop = asyncio.get_event_loop()
>>> loop.run_until_complete(coro(3))
0
1
1

Asynchronous generators¶

New in Python 3.6

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

Asynchronous comprehensions¶

New in Python 3.6

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

# 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']

Matrix multiplication¶

New in Python 3.5

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([x*y 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))
...
>>> a = Arr(9, 5, 2, 7)
>>> b = Arr(5, 5, 6, 6)
>>> a @ b  # __matmul__
124
>>> a @= b  # __imatmul__
>>> a
Arr([124])

Data Classes¶

New in Python 3.7

PEP 557 - Data Classes

>>> from dataclasses import dataclass
>>> @dataclass
... class DCls(object):
...     x: str
...     y: str
...
>>> d = DCls("foo", "bar")
>>> d
DCls(x='foo', y='bar')
>>> d = DCls(x="foo", y="baz")
>>> d
DCls(x='foo', y='baz')
>>> d.z = "bar"

# immutable

>>> from dataclasses import dataclass
>>> from dataclasses import FrozenInstanceError
>>> @dataclass(frozen=True)
... class DCls(object):
...     x: str
...     y: str
...
>>> try:
...     d.x = "baz"
... except FrozenInstanceError as e:
...     print(e)
...
cannot assign to field 'x'
>>> try:
...     d.z = "baz"
... except FrozenInstanceError as e:
...     print(e)
...
cannot assign to field 'z'

Built-in `breakpoint()`¶

New in Python 3.7

PEP 553 - Built-in breakpoint()

>>> for x in range(3):
...     print(x)
...     breakpoint()
...
0
> <stdin>(1)<module>()->None
(Pdb) c
1
> <stdin>(1)<module>()->None
(Pdb) c
2
> <stdin>(1)<module>()->None
(Pdb) c

New in Python3 cheatsheet¶

`print` is a function¶

String is unicode¶

New Super¶

Remove `<>`¶

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()`¶

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Related Topics

New in Python3 cheatsheet¶

print is a function¶

String is unicode¶

New Super¶

Remove <>¶

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()¶

`print` is a function¶

Remove `<>`¶

Not allow `from module import *` inside function¶

Add `nonlocal` keyword¶

`async` and `await` syntax¶

Built-in `breakpoint()`¶