# PEP 572: Assignment Expressions

海象表达式，用途是赋值给中间变量

# 示例一

	pattern = re.compile('s')
	data = 'ss'

	# 注意此处，会执行两次 re.match (data)
	if pattern.match(data):
	print(pattern.match(data).group(0))

	pattern = re.compile('s')
	data = 'ss'

	# 注意此处变化，使用海象表达式可以减少一次 re.match (data) 的执行
	if (match := pattern.match(data)) is not None:
	print(match.group(0))

# 示例二

	while 1:
	line = fp.readline()
	if not line:
	break
	print(line)

	while (line := fp.readline()):
	print(line)

# PEP 570: Python Positional-Only parameters

强制使用者用位置参数，原来在内置的 C 函数上有很多都用到了

	In : __builtin__.eval
	Out: <function eval(source, globals=None, locals=None, /)>

	In : __builtin__.len
	Out: <function len(obj, /)>

	In : __builtin__.divmod
	Out: <function divmod(x, y, /)>

	# 看它们的签名，最后都有一个 /，/ 用途是在 / 左面的这些参数，只能是位置参数 (不能是关键字参数):
	In : divmod(3, 2)
	Out: (1, 1)

	In : divmod(x=3, y=2)
	---------------------------------------------------------------------------
	TypeError Traceback (most recent call last)
	<ipython-input-28-6668f56246b2> in <module>
	----> 1 divmod(x=3, y=2)

	TypeError: divmod() takes no keyword arguments

如果使用关键字参数的方式，会报错。就是因为强制使用者用位置参数！

通过 PEP 570，我们写的 Python 代码也可以支持了。你可以这样写:

	def name(p1, p2, /, p_or_kw, *, kw):
	def name(p1, p2=None, /, p_or_kw=None, *, kw):
	def name(p1, p2=None, /, *, kw):
	def name(p1, p2=None, /):
	def name(p1, p2, /, p_or_kw):
	def name(p1, p2, /):

# PEP 578: Python Runtime Audit Hooks

现在可以给 Python 运行时添加审计钩子:

	In : import sys
	...: import urllib.request
	...:
	...:
	...: def audit_hook(event, args):
	...: if event in ['urllib.Request']:
	...: print(f'Network {event=} {args=}')
	...:
	...: sys.addaudithook(audit_hook)

	In : urllib.request.urlopen('https://httpbin.org/get?a=1')
	Network event='urllib.Request' args=('https://httpbin.org/get?a=1', None, {}, 'GET')
	Out: <http.client.HTTPResponse at 0x10e394310>

# Multiprocessing shared memory

可以跨进程直接访问同一内存 (共享):

	# IPython 进程 A
	In : from multiprocessing import shared_memory

	In : a = shared_memory.ShareableList([1, 'a', 0.1])

	In : a
	Out: ShareableList([1, 'a', 0.1], name='psm_d5d6ba1b') # 注意 name
	# IPython 进程 B (另外一个终端进入 IPython)
	In : from multiprocessing import shared_memory

	In : b = shared_memory.ShareableList(name='psm_d5d6ba1b') # 使用 name 就可以共享内存

	In : b
	Out: ShareableList([1, 'a', 0.1], name='psm_d5d6ba1b')

# New importlib.metadata module

使用新的 importlib.metadata 模块可以直接读取第三方包的元数据:

	In : from importlib.metadata import version, files, requires, distribution

	In : version('flask')
	Out: '1.1.1'

	In : requires('requests')
	Out:
	['chardet (<3.1.0,>=3.0.2)',
	'idna (<2.9,>=2.5)',
	'urllib3 (!=1.25.0,!=1.25.1,<1.26,>=1.21.1)',
	'certifi (>=2017.4.17)',
	"pyOpenSSL (>=0.14) ; extra == 'security'",
	"cryptography (>=1.3.4) ; extra == 'security'",
	"idna (>=2.0.0) ; extra == 'security'",
	"PySocks (!=1.5.7,>=1.5.6) ; extra == 'socks'",
	'win-inet-pton ; (sys_platform == "win32" and python_version == "2.7") and extra == \'socks\'']

	In : dist = distribution('celery')

	In : dist.version
	Out: '4.3.0'

	In : dist.metadata['Requires-Python']
	Out: '>=2.7, !=3.0., !=3.1., !=3.2., !=3.3.'

	In : dist.metadata['License']

	In : dist.entry_points
	Out:
	[EntryPoint(name='celery', value='celery.__main__:main', group='console_scripts'),
	EntryPoint(name='celery', value='celery.contrib.pytest', group='pytest11')]

	In : files('celery')[8]
	Out: PackagePath('celery/__init__.py')

	In : dist.locate_file(files('celery')[8])
	Out: PosixPath('/Users/dongweiming/test/venv/lib/python3.8/site-packages/celery/__init__.py')

# functools.cached_property

缓存属性 (cached_property) 是一个非常常用的功能，很多知名 Python 项目都自己实现过它，现在终于进入版本库了。

参考：functools.cached_property (Python 3.8) 作者：xiaoming

# functools.lru_cache 作为装饰器时可以不加参数

lru_cache 装饰器支持 max_size 和 typed2 个参数，如果对默认参数不敏感，过去只能这么用 (需要空括号):

	In : @lru_cache()
	...: def add(a, b):
	...: return a + b
	...:

从 3.8 开始可以直接作为装饰器，而不是作为返回装饰器的函数 (不加括号):

	In : @lru_cache
	...: def add(a, b):
	...: return a + b
	...:

# Asyncio REPL

参考：https://zhuanlan.zhihu.com/p/70030100

# F-strings DEBUG

参考：https://zhuanlan.zhihu.com/p/67826015

# Async Mock

单元测试模块 unittest 添加了 mock 异步代码的类:

	In : import asyncio

	In : from unittest.mock import AsyncMock, MagicMock

	In : mock = AsyncMock(return_value={'json': 123})
	In : await mock()
	Out: {'json': 123}

	In : asyncio.run(mock())
	Out: {'json': 123}

	In : async def main(args, *kwargs):
	...: return await mock(args, *kwargs)
	...:

	In : asyncio.run(main())
	Out: {'json': 123}

	In : mock = MagicMock() # AsyncMock 也可以

	In : mock.__aiter__.return_value = [1, 2, 3]

	In : async def main():
	...: return [i async for i in mock]
	...:

	In : asyncio.run(main())
	Out: [1, 2, 3]

# Generalized iterable unpacking in yield and return

参考：https://www.dongwm.com/post/iterable-unpacking/

# 本文引用：

参考文章：https://zhuanlan.zhihu.com/p/86670081
文章作者：小明

python3.8