笔记-python-lib-contextlib

1.      contextlib

with 语句很好用，但不想每次都写__enter_-和__exit__方法；

py标准库也为此提供了工具模块contextlib

模块提供的功能有很多，重点说一下contextmanager

2.      contextmanager

它提供了一个简单的上下文环境

简单使用：

from contextlib import contextmanager

@contextmanager

def make_open_context(filename, mode):

fp = open(filename, mode)

try:

yield fp

finally:

fp.close()

with make_open_context('i002.txt','a') as fi:

fi.write('hello ')

不用创建类和写__enter__,__exit__方法。

2.1.    代码释义

def contextmanager(func):

"""@contextmanager decorator.

Typical usage:

@contextmanager

def some_generator(<arguments>):

<setup>

try:

yield <value>

finally:

<cleanup>

This makes this:

with some_generator(<arguments>) as <variable>:

<body>

equivalent to this:

<setup>

try:

<variable> = <value>

<body>

finally:

<cleanup>

"""

@wraps(func)

def helper(*args, **kwds):

return _GeneratorContextManager(func, args, kwds)

return helper

contextmanager是一个装饰器函数，去掉注释，实际上是返回一个中间函数helper，helper返回的则是一个上下文管理器，具体的实现可以看下代码。

2.2.    _GeneratorContextManager(func, args, kwds)

前序准备工作非常简单，主要是后续清理代码比较多。

def __enter__(self):

try:

return next(self.gen)

except StopIteration:

raise RuntimeError("generator didn't yield") from None

def __exit__(self, type, value, traceback):

if type is None:

try:

next(self.gen)

except StopIteration:

return False

else:

raise RuntimeError("generator didn't stop")

else:

if value is None:

# Need to force instantiation so we can reliably

# tell if we get the same exception back

value = type()

try:

self.gen.throw(type, value, traceback)

except StopIteration as exc:

# Suppress StopIteration *unless* it's the same exception that

# was passed to throw().  This prevents a StopIteration

# raised inside the "with" statement from being suppressed.

return exc is not value

except RuntimeError as exc:

# Don't re-raise the passed in exception. (issue27122)

if exc is value:

return False

# Likewise, avoid suppressing if a StopIteration exception

# was passed to throw() and later wrapped into a RuntimeError

# (see PEP 479).

if type is StopIteration and exc.__cause__ is value:

return False

raise

except:

# only re-raise if it's *not* the exception that was

# passed to throw(), because __exit__() must not raise

# an exception unless __exit__() itself failed.  But throw()

# has to raise the exception to signal propagation, so this

# fixes the impedance mismatch between the throw() protocol

# and the __exit__() protocol.

#

if sys.exc_info()[1] is value:

return False

raise

raise RuntimeError("generator didn't stop after throw()")

3.      参考文档

参考文档：https://docs.python.org/3/library/contextlib.html?highlight=contextlib#module-contextlib

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