示例
@contextlib.contextmanager
def result(a):
print('before')
yield
print('after')
外层装饰源码
包装func函数，真实调用func()时，返回的为_GeneratorContextManager对象
def contextmanager(func):
@wraps(func)
def helper(*args, **kwds):
return _GeneratorContextManager(func, args, kwds)
return helper
_GeneratorContextManager对象
该对象实现上下文管理器，继承父类_GeneratorContextManagerBase，抽象类AbstractContextManager，ContextDecorator
父类_GeneratorContextManagerBase
用来初始化某些_GeneratorContextManager对象需要的属性，如被包装的生成器对象，调用生成器时的参数，对象的doc文档
class _GeneratorContextManagerBase:
"""Shared functionality for @contextmanager and @asynccontextmanager."""
def __init__(self, func, args, kwds):
self.gen = func(*args, **kwds) """保存被装饰的生成器对象"""
self.func, self.args, self.kwds = func, args, kwds
# Issue 19330: ensure context manager instances have good docstrings
doc = getattr(func, "__doc__", None) """用生成器的doc作为实例的doc，如果没有，就用类自己的doc作为实例doc"""
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.
抽象类AbstractContextManager
定义类需要实现上下文管理方法
定义判断是否为AbstractContextManager子类的方法AbstractContextManager，即如果一个类实现了__enter__ and__exit__, 没有继承定义判断是否为AbstractContextManager子类的方法AbstractContextManager，issubclass(classname,AbstractContextManager)也为真
class AbstractContextManager(abc.ABC):
"""An abstract base class for context managers."""
def __enter__(self):
"""Return `self` upon entering the runtime context."""
return self
@abc.abstractmethod
def __exit__(self, exc_type, exc_value, traceback):
"""Raise any exception triggered within the runtime context."""
return None
@classmethod
def __subclasshook__(cls, C):
if cls is AbstractContextManager:
return _collections_abc._check_methods(C, "__enter__", "__exit__")
return NotImplemented
装饰类ContextDecorator
继承这个类，可以直接作ContextDecorator对象作为装饰器，为函数执行提供上下文管理（被contextmanager装饰的函数，可以作为装饰器 & with语句使用）
class ContextDecorator(object):
"A base class or mixin that enables context managers to work as decorators."
def _recreate_cm(self):
"""Return a recreated instance of self.
Allows an otherwise one-shot context manager like
_GeneratorContextManager to support use as
a decorator via implicit recreation.
This is a private interface just for _GeneratorContextManager.
See issue #11647 for details.
"""
return self
def __call__(self, func):
@wraps(func)
def inner(*args, **kwds):
with self._recreate_cm():
return func(*args, **kwds)
return inner
示例
class MyContextManager(ContextDecorator):
"Test MyContextManager."
def __enter__(self):
print('enter')
return self
def __exit__(self, exc_type, exc_val, exc_tb):
print('exit')
@MyContextManager()
def report(a):
print(a, 'report function')
return a
执行report(1), 输出：
eneter
1 report function
exit
1
_GeneratorContextManager类
对象的使用：
@contextlib.contextmanager
def gcm_func(a):
print('before')
print('gcm_func', a)
yield
print('after')
#使用方式1：gcm_func直接作为上下文管理器：
with gcm_func(1):
print('-- in with ---')
输出：
before
gcm_func 1
-- in with ---
after
#使用方式2： gcm_func作为函数的上下文管理
@gcm_func(1)
def with_func(a):
print('-- in with ---')
with_func(1)
with_func(1)
"""
注意：ContextDecorator中__call__定义了每次调用with_func前，会调用_recreate_cm生成新的_GeneratorContextManager对象作为上下文管理器，所以这边可以调用2次
否则在第一次with_func(1)就已经清空gcm_func生成器并删除with_func属性
"""
输出：
同方式1
class _GeneratorContextManager(_GeneratorContextManagerBase,
AbstractContextManager,
ContextDecorator):
"""Helper for @contextmanager decorator."""
def _recreate_cm(self):
"""
_GeneratorContextManager实例上下文管理一次就无法再次调用
如果_GeneratorContextManager实例用作装饰器，每次调用时需要重新生成实例
"""
# _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):
"""被装饰函数的参数只有在初始化实例时有用"""
# do not keep args and kwds alive unnecessarily
# they are only needed for recreation, which is not possible anymore
del self.args, self.kwds, self.func
try:
return next(self.gen)
except StopIteration:
raise RuntimeError("generator didn't yield") from None
def __exit__(self, type, value, traceback):
"""
type: with语句内抛出的异常类
value: with语句内抛出的异常信息
traceback: with语句内抛出的异常堆栈
"""
a=str(type)
if type is None:
"""
with语句内没有报错，往yield停止的部分继续执行，并会抛出异常
如果往下走没有遇到stop异常，也就是contextmanager函数有两个yield，会报错"""
try:
next(self.gen)
except StopIteration:
return False
else:
raise RuntimeError("generator didn't stop")
else:
"""
如果with中抛出了异常，在yield处抛出异常
"""
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:
"""如果抛出的异常在yield中被except，不再抛出，而是往下走会抛出生成器的stop异常"""
# 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:
"""如果with中有异常，抛出with中的异常给yield后，如果后续的语句有异常，判断异常是否为属于上下文管理器的异常"""
# Don't re-raise the passed in exception.(issue27122)
"""如果是在上下文管理器中except raise异常，不要再抛出"""
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).
"""忽略with中抛出的stop异常，不在这里抛出异常"""
if type is StopIteration and exc.__cause__ is value:
return False
"""如果是后续语句中其他异常，属于上下文管理器的异常，抛出"""
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.
#
# This cannot use 'except BaseException as exc' (as in the
# async implementation) to maintain compatibility with
# Python 2, where old-style class exceptions are not caught
# by 'except BaseException'.
if sys.exc_info()[1] is value:
return False
raise
"""处理异常之后，往下走还有yield"""
raise RuntimeError("generator didn't stop after throw()")