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
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。
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。
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()")
参考文档：https://docs.python.org/3/library/contextlib.html?highlight=contextlib#module-contextlib
原文：https://www.cnblogs.com/wodeboke-y/p/10489194.html