Python之contextlib库及源码分析
Utilities forwith-statement contexts
__all__ = ["contextmanager", "closing", "AbstractContextManager",
"ContextDecorator", "ExitStack", "redirect_stdout",
"redirect_stderr", "suppress"]
AbstractContextManager(abc.ABC)
上下文管理抽象类，子类必须实现__enter__(self)、__exit__(self)
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: if (any("__enter__" in B.__dict__ for B in C.__mro__) and any("__exit__" in B.__dict__ for B in C.__mro__)): return True return NotImplemented
ContextDecorator(object)
上下文管理基类或mixin类，该类可以像装饰器一样工作，提供你需要实现的任何辅助功能
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
_GeneratorContextManager(ContextDecorator, AbstractContextManager)
contextmanager装饰器的包装函数提供以下方法：
_recreate_cm(重新生成新对像)，self.__class__(*args, **kwds)
__enter__上下文管理进入函数
__exit__上下文管理退出函数
可以根据ContextDecorator实现任何想实现的辅助功能
class _GeneratorContextManager(ContextDecorator, AbstractContextManager): """Helper for @contextmanager decorator.""" def __init__(self, func, args, kwds): self.gen = func(*args, **kwds) logger.info("get generator by with:{}".format(self.gen)) self.func, self.args, self.kwds = func, args, kwds # Issue 19330: ensure context manager instances have good docstrings doc = getattr(func, "__doc__", None) #得到函数文档，第三个参数为默认参数 logger.info("doc:{}".format(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. 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): logger.info("__enter__:you can add you method") @ContextDecorator() def testfun(*args, **kwds): logger.info("@ContextDecorator():testfun test success") testfun("hello") try: return next(self.gen) except StopIteration: raise RuntimeError("generator didn't yield") from None def __exit__(self, type, value, traceback): logger.info("__exit__") logger.info("type:{}".format(type)) 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: # Don't re-raise the passed in exception. (issue27112) 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 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
装饰器contextmanager
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
用例：
#coding = utf-8 import abc from functools import wraps import logging logging.basicConfig(level=logging.INFO, filename="logging.txt", filemode="w+", \ format='%(asctime)s - %(name)s - %(levelname)s - %(message)s') logger = logging.getLogger(__name__) 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: if (any("__enter__" in B.__dict__ for B in C.__mro__) and any("__exit__" in B.__dict__ for B in C.__mro__)): return True return NotImplemented 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): #logger.info("ContextDecorator func:{}".format(func)) @wraps(func) def inner(*args, **kwds): #with self._recreate_cm(): logger.info("you can do something in decorator") return func(*args, **kwds) return inner class _GeneratorContextManager(ContextDecorator, AbstractContextManager): """Helper for @contextmanager decorator.""" def __init__(self, func, args, kwds): self.gen = func(*args, **kwds) logger.info("get generator by with:{}".format(self.gen)) self.func, self.args, self.kwds = func, args, kwds # Issue 19330: ensure context manager instances have good docstrings doc = getattr(func, "__doc__", None) #得到函数文档，第三个参数为默认参数 logger.info("doc:{}".format(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. 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): logger.info("__enter__:you can add you method") @ContextDecorator() def testfun(*args, **kwds): logger.info("@ContextDecorator():testfun test success") testfun("hello") try: return next(self.gen) except StopIteration: raise RuntimeError("generator didn't yield") from None def __exit__(self, type, value, traceback): logger.info("__exit__") logger.info("type:{}".format(type)) 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: # Don't re-raise the passed in exception. (issue27112) 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 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 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 def file_open(path): ''' file open test''' try: f_obj = open(path,"w") yield f_obj except OSError: print("We had an error!") finally: print("Closing file") f_obj.close() if __name__ == "__main__": with file_open("contextlibtest.txt") as fobj: fobj.write("Testing context managers") logger.info("write file success")
关键输出：
2018-03-22 15:36:43,249 - __main__ - INFO - get generator by with:<generator object file_open at 0x01DE4870> 2018-03-22 15:36:43,249 - __main__ - INFO - doc: file open test 2018-03-22 15:36:43,249 - __main__ - INFO - __enter__:you can add you method 2018-03-22 15:36:43,249 - __main__ - INFO - you can do something in decorator 2018-03-22 15:36:43,249 - __main__ - INFO - @ContextDecorator():testfun test success 2018-03-22 15:36:43,249 - __main__ - INFO - write file success 2018-03-22 15:36:43,249 - __main__ - INFO - __exit__ 2018-03-22 15:36:43,249 - __main__ - INFO - type:None
