python的上下文管理器

上下文管理器其实是with语句，这是为了简化try/finally模式，这可以保证一段代码在运行完之后，即使出现错误也能正确的运行。finally的语句用于释放重要资源，比如数据库和文件

的句柄，或者还原临时变更的对象，例如锁。

就跟前面的系列文章所述，上下文管理器也是一种协议，包含__enter__和__exit__方法。在with语句开始运行是会调用__enter__方法，结束后会调用__exit__方法。最常见的例子就是打开文件。

打开所在项目的文件

with open('List.py',encoding = 'utf8') as f:

src = f.read()

f

Out[4]: <_io.TextIOWrapper name='List.py' mode='r' encoding='utf8'>

f.closed, f.encoding

Out[5]: (True, 'utf8')

我们可以看出f这个变量依然可以用，但是文件句柄已经关闭了。as语句只不过是把值绑定到了目标变量，as语句是可选的，但是如果是打开文件或者是连接数据库，则必须绑定获得句柄。

说了这么多，这意味着我们也可以制造一个上下文管理器，只要实现了__enter__和__exit__方法。

class test():

def __enter__(self):

print('enter')

return 'enter'

def __exit__(*args):

print(args)

return 'exit'

with test() as f:

print('sdad')

raise Zero("dsad")

显示出来的结果如下：

enter

sdad

(<__main__.test object at 0x0000015C1CE351D0>, <class 'NameError'>, NameError("name 'Zero' is not defined",), <traceback object at 0x0000015C1CE27C08>)

再看看f变量：

f

Out[3]: 'enter'

我们自己定义了一个类，里面包含了__enter__和__exit__方法，在with语句开始时，你会发现显示屏上出现了'enter'，这印证了之前的话，__enter__会在with语句刚开始时执行，并且

把return的结果反馈回f变量。进入到语句块之后，会显示出'sdad'，知道报错，然后你会发现__exit__里面接受了四个变量，分别是：

--self:本身的实例

--exc_type:异常类（比如NameError）

--exc_value:异常的实例，可以通过exc_value.args获取参数

--traceback:traceback对象，Extract, format and print information about Python stack traces.也就是异常栈

更详细的信息在:https://docs.python.org/3.5/library/stdtypes.html#typecontextmanager

其实每次这样定义会很累，官方提供了contextlib模块来简化开发。

里面一共包含了：

__all__ = ["contextmanager", "closing", "ContextDecorator", "ExitStack",

"redirect_stdout", "redirect_stderr", "suppress"]

其中，contextmanager可以把简单的生成器函数变成上下文管理器。这个也是用了yield语句，但是这个却与迭代无关。

官网给出的例子如下：

from contextlib import contextmanager

@contextmanager

def tag(name):

print("<%s>" % name)

yield

print("</%s>" % name)

with tag('woshi'):

print('love')

<woshi>

love

</woshi>

结果如上通过yield语句将函数的定义体分成了两部分，yield前面的语句执行__enter__,后面的语句执行__exit__，我们可以改变test类

@contextmanager

def text():

print('enter')

yield 'enter'

print('exit')

with text() as f:

print('sd')

enter

sd

exit

class _GeneratorContextManager(ContextDecorator):

"""Helper for @contextmanager decorator."""

def __init__(self, func, args, kwds):

self.gen = func(*args, **kwds)

self.func, self.args, self.kwds = func, args, kwds

# Issue 19330: ensure context manager instances have good docstrings

doc = getattr(func, "__doc__", None)

if doc is None:

doc = type(self).__doc__

self.__doc__ = doc

# Unfortunately, this still doesn't provide good help output when

# inspecting the created context manager instances, since pydoc

# currently bypasses the instance docstring and shows the docstring

# for the class instead.

# See http://bugs.python.org/issue19404 for more details.

def _recreate_cm(self):

# _GCM instances are one-shot context managers, so the

# CM must be recreated each time a decorated function is

# called

return self.__class__(self.func, self.args, self.kwds)

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

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)

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

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:

# Likewise, avoid suppressing if a StopIteration exception

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

# (see PEP 479).

if 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 not value:

raise

从源码可以看出：

@contextmanager 实际上是把函数包装成了一个类，

__enter__：调用生成器函数，保存生成器对象，如上所述是，next(self.gen)，调用到yield关键字所在的位置，返回这个值绑定到as后面的变量。

__exit__：检查有没有异常传给exc_type，没有的话继续调用之后的代码，否则调用self.gen.throw(type, value, traceback)抛出异常。