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Edit File : contextlib.py \| Size : 24.18 KB
"""Utilities for with-statement contexts. See PEP 343.""" import abc import sys import _collections_abc from collections import deque from functools import wraps __all__ = ["asynccontextmanager", "contextmanager", "closing", "nullcontext", "AbstractContextManager", "AbstractAsyncContextManager", "AsyncExitStack", "ContextDecorator", "ExitStack", "redirect_stdout", "redirect_stderr", "suppress"] 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 class AbstractAsyncContextManager(abc.ABC): """An abstract base class for asynchronous context managers.""" async def __aenter__(self): """Return `self` upon entering the runtime context.""" return self @abc.abstractmethod async def __aexit__(self, exc_type, exc_value, traceback): """Raise any exception triggered within the runtime context.""" return None @classmethod def __subclasshook__(cls, C): if cls is AbstractAsyncContextManager: return _collections_abc._check_methods(C, "__aenter__", "__aexit__") 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): @wraps(func) def inner(args, kwds): with self._recreate_cm(): return func(args, *kwds) return inner class _GeneratorContextManagerBase: """Shared functionality for @contextmanager and @asynccontextmanager.""" 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. class _GeneratorContextManager(_GeneratorContextManagerBase, AbstractContextManager, ContextDecorator): """Helper for @contextmanager decorator.""" 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): # 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): 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. # # 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 raise RuntimeError("generator didn't stop after throw()") class _AsyncGeneratorContextManager(_GeneratorContextManagerBase, AbstractAsyncContextManager): """Helper for @asynccontextmanager.""" async def __aenter__(self): try: return await self.gen.__anext__() except StopAsyncIteration: raise RuntimeError("generator didn't yield") from None async def __aexit__(self, typ, value, traceback): if typ is None: try: await self.gen.__anext__() except StopAsyncIteration: return else: raise RuntimeError("generator didn't stop") else: if value is None: value = typ() # See _GeneratorContextManager.__exit__ for comments on subtleties # in this implementation try: await self.gen.athrow(typ, value, traceback) raise RuntimeError("generator didn't stop after athrow()") except StopAsyncIteration as exc: return exc is not value except RuntimeError as exc: if exc is value: return False # Avoid suppressing if a StopIteration exception # was passed to throw() and later wrapped into a RuntimeError # (see PEP 479 for sync generators; async generators also # have this behavior). But do this only if the exception wrapped # by the RuntimeError is actully Stop(Async)Iteration (see # issue29692). if isinstance(value, (StopIteration, StopAsyncIteration)): if exc.__cause__ is value: return False raise except BaseException as exc: if exc 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 def asynccontextmanager(func): """@asynccontextmanager decorator. Typical usage: @asynccontextmanager async def some_async_generator(<arguments>): <setup> try: yield <value> finally: <cleanup> This makes this: async with some_async_generator(<arguments>) as <variable>: <body> equivalent to this: <setup> try: <variable> = <value> <body> finally: <cleanup> """ @wraps(func) def helper(args, *kwds): return _AsyncGeneratorContextManager(func, args, kwds) return helper class closing(AbstractContextManager): """Context to automatically close something at the end of a block. Code like this: with closing(<module>.open(<arguments>)) as f: <block> is equivalent to this: f = <module>.open(<arguments>) try: <block> finally: f.close() """ def __init__(self, thing): self.thing = thing def __enter__(self): return self.thing def __exit__(self, exc_info): self.thing.close() class _RedirectStream(AbstractContextManager): _stream = None def __init__(self, new_target): self._new_target = new_target # We use a list of old targets to make this CM re-entrant self._old_targets = [] def __enter__(self): self._old_targets.append(getattr(sys, self._stream)) setattr(sys, self._stream, self._new_target) return self._new_target def __exit__(self, exctype, excinst, exctb): setattr(sys, self._stream, self._old_targets.pop()) class redirect_stdout(_RedirectStream): """Context manager for temporarily redirecting stdout to another file. # How to send help() to stderr with redirect_stdout(sys.stderr): help(dir) # How to write help() to a file with open('help.txt', 'w') as f: with redirect_stdout(f): help(pow) """ _stream = "stdout" class redirect_stderr(_RedirectStream): """Context manager for temporarily redirecting stderr to another file.""" _stream = "stderr" class suppress(AbstractContextManager): """Context manager to suppress specified exceptions After the exception is suppressed, execution proceeds with the next statement following the with statement. with suppress(FileNotFoundError): os.remove(somefile) # Execution still resumes here if the file was already removed """ def __init__(self, exceptions): self._exceptions = exceptions def __enter__(self): pass def __exit__(self, exctype, excinst, exctb): # Unlike isinstance and issubclass, CPython exception handling # currently only looks at the concrete type hierarchy (ignoring # the instance and subclass checking hooks). While Guido considers # that a bug rather than a feature, it's a fairly hard one to fix # due to various internal implementation details. suppress provides # the simpler issubclass based semantics, rather than trying to # exactly reproduce the limitations of the CPython interpreter. # # See http://bugs.python.org/issue12029 for more details return exctype is not None and issubclass(exctype, self._exceptions) class _BaseExitStack: """A base class for ExitStack and AsyncExitStack.""" @staticmethod def _create_exit_wrapper(cm, cm_exit): def _exit_wrapper(exc_type, exc, tb): return cm_exit(cm, exc_type, exc, tb) return _exit_wrapper @staticmethod def _create_cb_wrapper(args, *kwds): callback, args = args def _exit_wrapper(exc_type, exc, tb): callback(args, kwds) return _exit_wrapper def __init__(self): self._exit_callbacks = deque() def pop_all(self): """Preserve the context stack by transferring it to a new instance.""" new_stack = type(self)() new_stack._exit_callbacks = self._exit_callbacks self._exit_callbacks = deque() return new_stack def push(self, exit): """Registers a callback with the standard __exit__ method signature. Can suppress exceptions the same way __exit__ method can. Also accepts any object with an __exit__ method (registering a call to the method instead of the object itself). """ # We use an unbound method rather than a bound method to follow # the standard lookup behaviour for special methods. _cb_type = type(exit) try: exit_method = _cb_type.__exit__ except AttributeError: # Not a context manager, so assume it's a callable. self._push_exit_callback(exit) else: self._push_cm_exit(exit, exit_method) return exit # Allow use as a decorator. def enter_context(self, cm): """Enters the supplied context manager. If successful, also pushes its __exit__ method as a callback and returns the result of the __enter__ method. """ # We look up the special methods on the type to match the with # statement. _cm_type = type(cm) _exit = _cm_type.__exit__ result = _cm_type.__enter__(cm) self._push_cm_exit(cm, _exit) return result def callback(args, *kwds): """Registers an arbitrary callback and arguments. Cannot suppress exceptions. """ if len(args) >= 2: self, callback, args = args elif not args: raise TypeError("descriptor 'callback' of '_BaseExitStack' object " "needs an argument") elif 'callback' in kwds: callback = kwds.pop('callback') self, args = args else: raise TypeError('callback expected at least 1 positional argument, ' 'got %d' % (len(args)-1)) _exit_wrapper = self._create_cb_wrapper(callback, args, *kwds) # We changed the signature, so using @wraps is not appropriate, but # setting __wrapped__ may still help with introspection. _exit_wrapper.__wrapped__ = callback self._push_exit_callback(_exit_wrapper) return callback # Allow use as a decorator def _push_cm_exit(self, cm, cm_exit): """Helper to correctly register callbacks to __exit__ methods.""" _exit_wrapper = self._create_exit_wrapper(cm, cm_exit) _exit_wrapper.__self__ = cm self._push_exit_callback(_exit_wrapper, True) def _push_exit_callback(self, callback, is_sync=True): self._exit_callbacks.append((is_sync, callback)) # Inspired by discussions on http://bugs.python.org/issue13585 class ExitStack(_BaseExitStack, AbstractContextManager): """Context manager for dynamic management of a stack of exit callbacks. For example: with ExitStack() as stack: files = [stack.enter_context(open(fname)) for fname in filenames] # All opened files will automatically be closed at the end of # the with statement, even if attempts to open files later # in the list raise an exception. """ def __enter__(self): return self def __exit__(self, exc_details): received_exc = exc_details[0] is not None # We manipulate the exception state so it behaves as though # we were actually nesting multiple with statements frame_exc = sys.exc_info()[1] def _fix_exception_context(new_exc, old_exc): # Context may not be correct, so find the end of the chain while 1: exc_context = new_exc.__context__ if exc_context is old_exc: # Context is already set correctly (see issue 20317) return if exc_context is None or exc_context is frame_exc: break new_exc = exc_context # Change the end of the chain to point to the exception # we expect it to reference new_exc.__context__ = old_exc # Callbacks are invoked in LIFO order to match the behaviour of # nested context managers suppressed_exc = False pending_raise = False while self._exit_callbacks: is_sync, cb = self._exit_callbacks.pop() assert is_sync try: if cb(exc_details): suppressed_exc = True pending_raise = False exc_details = (None, None, None) except: new_exc_details = sys.exc_info() # simulate the stack of exceptions by setting the context _fix_exception_context(new_exc_details[1], exc_details[1]) pending_raise = True exc_details = new_exc_details if pending_raise: try: # bare "raise exc_details[1]" replaces our carefully # set-up context fixed_ctx = exc_details[1].__context__ raise exc_details[1] except BaseException: exc_details[1].__context__ = fixed_ctx raise return received_exc and suppressed_exc def close(self): """Immediately unwind the context stack.""" self.__exit__(None, None, None) # Inspired by discussions on https://bugs.python.org/issue29302 class AsyncExitStack(_BaseExitStack, AbstractAsyncContextManager): """Async context manager for dynamic management of a stack of exit callbacks. For example: async with AsyncExitStack() as stack: connections = [await stack.enter_async_context(get_connection()) for i in range(5)] # All opened connections will automatically be released at the # end of the async with statement, even if attempts to open a # connection later in the list raise an exception. """ @staticmethod def _create_async_exit_wrapper(cm, cm_exit): async def _exit_wrapper(exc_type, exc, tb): return await cm_exit(cm, exc_type, exc, tb) return _exit_wrapper @staticmethod def _create_async_cb_wrapper(args, *kwds): callback, args = args async def _exit_wrapper(exc_type, exc, tb): await callback(args, kwds) return _exit_wrapper async def enter_async_context(self, cm): """Enters the supplied async context manager. If successful, also pushes its __aexit__ method as a callback and returns the result of the __aenter__ method. """ _cm_type = type(cm) _exit = _cm_type.__aexit__ result = await _cm_type.__aenter__(cm) self._push_async_cm_exit(cm, _exit) return result def push_async_exit(self, exit): """Registers a coroutine function with the standard __aexit__ method signature. Can suppress exceptions the same way __aexit__ method can. Also accepts any object with an __aexit__ method (registering a call to the method instead of the object itself). """ _cb_type = type(exit) try: exit_method = _cb_type.__aexit__ except AttributeError: # Not an async context manager, so assume it's a coroutine function self._push_exit_callback(exit, False) else: self._push_async_cm_exit(exit, exit_method) return exit # Allow use as a decorator def push_async_callback(args, *kwds): """Registers an arbitrary coroutine function and arguments. Cannot suppress exceptions. """ if len(args) >= 2: self, callback, args = args elif not args: raise TypeError("descriptor 'push_async_callback' of " "'AsyncExitStack' object needs an argument") elif 'callback' in kwds: callback = kwds.pop('callback') self, args = args else: raise TypeError('push_async_callback expected at least 1 ' 'positional argument, got %d' % (len(args)-1)) _exit_wrapper = self._create_async_cb_wrapper(callback, args, *kwds) # We changed the signature, so using @wraps is not appropriate, but # setting __wrapped__ may still help with introspection. _exit_wrapper.__wrapped__ = callback self._push_exit_callback(_exit_wrapper, False) return callback # Allow use as a decorator async def aclose(self): """Immediately unwind the context stack.""" await self.__aexit__(None, None, None) def _push_async_cm_exit(self, cm, cm_exit): """Helper to correctly register coroutine function to __aexit__ method.""" _exit_wrapper = self._create_async_exit_wrapper(cm, cm_exit) _exit_wrapper.__self__ = cm self._push_exit_callback(_exit_wrapper, False) async def __aenter__(self): return self async def __aexit__(self, exc_details): received_exc = exc_details[0] is not None # We manipulate the exception state so it behaves as though # we were actually nesting multiple with statements frame_exc = sys.exc_info()[1] def _fix_exception_context(new_exc, old_exc): # Context may not be correct, so find the end of the chain while 1: exc_context = new_exc.__context__ if exc_context is old_exc: # Context is already set correctly (see issue 20317) return if exc_context is None or exc_context is frame_exc: break new_exc = exc_context # Change the end of the chain to point to the exception # we expect it to reference new_exc.__context__ = old_exc # Callbacks are invoked in LIFO order to match the behaviour of # nested context managers suppressed_exc = False pending_raise = False while self._exit_callbacks: is_sync, cb = self._exit_callbacks.pop() try: if is_sync: cb_suppress = cb(exc_details) else: cb_suppress = await cb(exc_details) if cb_suppress: suppressed_exc = True pending_raise = False exc_details = (None, None, None) except: new_exc_details = sys.exc_info() # simulate the stack of exceptions by setting the context _fix_exception_context(new_exc_details[1], exc_details[1]) pending_raise = True exc_details = new_exc_details if pending_raise: try: # bare "raise exc_details[1]" replaces our carefully # set-up context fixed_ctx = exc_details[1].__context__ raise exc_details[1] except BaseException: exc_details[1].__context__ = fixed_ctx raise return received_exc and suppressed_exc class nullcontext(AbstractContextManager): """Context manager that does no additional processing. Used as a stand-in for a normal context manager, when a particular block of code is only sometimes used with a normal context manager: cm = optional_cm if condition else nullcontext() with cm: # Perform operation, using optional_cm if condition is True """ def __init__(self, enter_result=None): self.enter_result = enter_result def __enter__(self): return self.enter_result def __exit__(self, *excinfo): pass Back

Edit File : contextlib.py | Size : 24.18 KB

"""Utilities for with-statement contexts.  See PEP 343."""
import abc
import sys
import _collections_abc
from collections import deque
from functools import wraps

__all__ = ["asynccontextmanager", "contextmanager", "closing", "nullcontext",
           "AbstractContextManager", "AbstractAsyncContextManager",
           "AsyncExitStack", "ContextDecorator", "ExitStack",
           "redirect_stdout", "redirect_stderr", "suppress"]

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

class AbstractAsyncContextManager(abc.ABC):

"""An abstract base class for asynchronous context managers."""

async def __aenter__(self):
        """Return `self` upon entering the runtime context."""
        return self

@abc.abstractmethod
    async def __aexit__(self, exc_type, exc_value, traceback):
        """Raise any exception triggered within the runtime context."""
        return None

@classmethod
    def __subclasshook__(cls, C):
        if cls is AbstractAsyncContextManager:
            return _collections_abc._check_methods(C, "__aenter__",
                                                   "__aexit__")
        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):
        @wraps(func)
        def inner(*args, **kwds):
            with self._recreate_cm():
                return func(*args, **kwds)
        return inner

class _GeneratorContextManagerBase:
    """Shared functionality for @contextmanager and @asynccontextmanager."""

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.

class _GeneratorContextManager(_GeneratorContextManagerBase,
                               AbstractContextManager,
                               ContextDecorator):
    """Helper for @contextmanager decorator."""

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

class _AsyncGeneratorContextManager(_GeneratorContextManagerBase,
                                    AbstractAsyncContextManager):
    """Helper for @asynccontextmanager."""

async def __aenter__(self):
        try:
            return await self.gen.__anext__()
        except StopAsyncIteration:
            raise RuntimeError("generator didn't yield") from None

async def __aexit__(self, typ, value, traceback):
        if typ is None:
            try:
                await self.gen.__anext__()
            except StopAsyncIteration:
                return
            else:
                raise RuntimeError("generator didn't stop")
        else:
            if value is None:
                value = typ()
            # See _GeneratorContextManager.__exit__ for comments on subtleties
            # in this implementation
            try:
                await self.gen.athrow(typ, value, traceback)
                raise RuntimeError("generator didn't stop after athrow()")
            except StopAsyncIteration as exc:
                return exc is not value
            except RuntimeError as exc:
                if exc is value:
                    return False
                # Avoid suppressing if a StopIteration exception
                # was passed to throw() and later wrapped into a RuntimeError
                # (see PEP 479 for sync generators; async generators also
                # have this behavior). But do this only if the exception wrapped
                # by the RuntimeError is actully Stop(Async)Iteration (see
                # issue29692).
                if isinstance(value, (StopIteration, StopAsyncIteration)):
                    if exc.__cause__ is value:
                        return False
                raise
            except BaseException as exc:
                if exc 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

def asynccontextmanager(func):
    """@asynccontextmanager decorator.

Typical usage:

@asynccontextmanager
        async def some_async_generator(<arguments>):
            <setup>
            try:
                yield <value>
            finally:
                <cleanup>

This makes this:

async with some_async_generator(<arguments>) as <variable>:
            <body>

equivalent to this:

<setup>
        try:
            <variable> = <value>
            <body>
        finally:
            <cleanup>
    """
    @wraps(func)
    def helper(*args, **kwds):
        return _AsyncGeneratorContextManager(func, args, kwds)
    return helper

class closing(AbstractContextManager):
    """Context to automatically close something at the end of a block.

Code like this:

with closing(<module>.open(<arguments>)) as f:
            <block>

is equivalent to this:

f = <module>.open(<arguments>)
        try:
            <block>
        finally:
            f.close()

"""
    def __init__(self, thing):
        self.thing = thing
    def __enter__(self):
        return self.thing
    def __exit__(self, *exc_info):
        self.thing.close()

class _RedirectStream(AbstractContextManager):

_stream = None

def __init__(self, new_target):
        self._new_target = new_target
        # We use a list of old targets to make this CM re-entrant
        self._old_targets = []

def __enter__(self):
        self._old_targets.append(getattr(sys, self._stream))
        setattr(sys, self._stream, self._new_target)
        return self._new_target

def __exit__(self, exctype, excinst, exctb):
        setattr(sys, self._stream, self._old_targets.pop())

class redirect_stdout(_RedirectStream):
    """Context manager for temporarily redirecting stdout to another file.

# How to send help() to stderr
        with redirect_stdout(sys.stderr):
            help(dir)

# How to write help() to a file
        with open('help.txt', 'w') as f:
            with redirect_stdout(f):
                help(pow)
    """

_stream = "stdout"

class redirect_stderr(_RedirectStream):
    """Context manager for temporarily redirecting stderr to another file."""

_stream = "stderr"

class suppress(AbstractContextManager):
    """Context manager to suppress specified exceptions

After the exception is suppressed, execution proceeds with the next
    statement following the with statement.

with suppress(FileNotFoundError):
             os.remove(somefile)
         # Execution still resumes here if the file was already removed
    """

def __init__(self, *exceptions):
        self._exceptions = exceptions

def __enter__(self):
        pass

def __exit__(self, exctype, excinst, exctb):
        # Unlike isinstance and issubclass, CPython exception handling
        # currently only looks at the concrete type hierarchy (ignoring
        # the instance and subclass checking hooks). While Guido considers
        # that a bug rather than a feature, it's a fairly hard one to fix
        # due to various internal implementation details. suppress provides
        # the simpler issubclass based semantics, rather than trying to
        # exactly reproduce the limitations of the CPython interpreter.
        #
        # See http://bugs.python.org/issue12029 for more details
        return exctype is not None and issubclass(exctype, self._exceptions)

class _BaseExitStack:
    """A base class for ExitStack and AsyncExitStack."""

@staticmethod
    def _create_exit_wrapper(cm, cm_exit):
        def _exit_wrapper(exc_type, exc, tb):
            return cm_exit(cm, exc_type, exc, tb)
        return _exit_wrapper

@staticmethod
    def _create_cb_wrapper(*args, **kwds):
        callback, *args = args
        def _exit_wrapper(exc_type, exc, tb):
            callback(*args, **kwds)
        return _exit_wrapper

def __init__(self):
        self._exit_callbacks = deque()

def pop_all(self):
        """Preserve the context stack by transferring it to a new instance."""
        new_stack = type(self)()
        new_stack._exit_callbacks = self._exit_callbacks
        self._exit_callbacks = deque()
        return new_stack

def push(self, exit):
        """Registers a callback with the standard __exit__ method signature.

Can suppress exceptions the same way __exit__ method can.
        Also accepts any object with an __exit__ method (registering a call
        to the method instead of the object itself).
        """
        # We use an unbound method rather than a bound method to follow
        # the standard lookup behaviour for special methods.
        _cb_type = type(exit)

try:
            exit_method = _cb_type.__exit__
        except AttributeError:
            # Not a context manager, so assume it's a callable.
            self._push_exit_callback(exit)
        else:
            self._push_cm_exit(exit, exit_method)
        return exit  # Allow use as a decorator.

def enter_context(self, cm):
        """Enters the supplied context manager.

If successful, also pushes its __exit__ method as a callback and
        returns the result of the __enter__ method.
        """
        # We look up the special methods on the type to match the with
        # statement.
        _cm_type = type(cm)
        _exit = _cm_type.__exit__
        result = _cm_type.__enter__(cm)
        self._push_cm_exit(cm, _exit)
        return result

def callback(*args, **kwds):
        """Registers an arbitrary callback and arguments.

Cannot suppress exceptions.
        """
        if len(args) >= 2:
            self, callback, *args = args
        elif not args:
            raise TypeError("descriptor 'callback' of '_BaseExitStack' object "
                            "needs an argument")
        elif 'callback' in kwds:
            callback = kwds.pop('callback')
            self, *args = args
        else:
            raise TypeError('callback expected at least 1 positional argument, '
                            'got %d' % (len(args)-1))

_exit_wrapper = self._create_cb_wrapper(callback, *args, **kwds)

# We changed the signature, so using @wraps is not appropriate, but
        # setting __wrapped__ may still help with introspection.
        _exit_wrapper.__wrapped__ = callback
        self._push_exit_callback(_exit_wrapper)
        return callback  # Allow use as a decorator

def _push_cm_exit(self, cm, cm_exit):
        """Helper to correctly register callbacks to __exit__ methods."""
        _exit_wrapper = self._create_exit_wrapper(cm, cm_exit)
        _exit_wrapper.__self__ = cm
        self._push_exit_callback(_exit_wrapper, True)

def _push_exit_callback(self, callback, is_sync=True):
        self._exit_callbacks.append((is_sync, callback))

# Inspired by discussions on http://bugs.python.org/issue13585
class ExitStack(_BaseExitStack, AbstractContextManager):
    """Context manager for dynamic management of a stack of exit callbacks.

For example:
        with ExitStack() as stack:
            files = [stack.enter_context(open(fname)) for fname in filenames]
            # All opened files will automatically be closed at the end of
            # the with statement, even if attempts to open files later
            # in the list raise an exception.
    """

def __enter__(self):
        return self

def __exit__(self, *exc_details):
        received_exc = exc_details[0] is not None

# We manipulate the exception state so it behaves as though
        # we were actually nesting multiple with statements
        frame_exc = sys.exc_info()[1]
        def _fix_exception_context(new_exc, old_exc):
            # Context may not be correct, so find the end of the chain
            while 1:
                exc_context = new_exc.__context__
                if exc_context is old_exc:
                    # Context is already set correctly (see issue 20317)
                    return
                if exc_context is None or exc_context is frame_exc:
                    break
                new_exc = exc_context
            # Change the end of the chain to point to the exception
            # we expect it to reference
            new_exc.__context__ = old_exc

# Callbacks are invoked in LIFO order to match the behaviour of
        # nested context managers
        suppressed_exc = False
        pending_raise = False
        while self._exit_callbacks:
            is_sync, cb = self._exit_callbacks.pop()
            assert is_sync
            try:
                if cb(*exc_details):
                    suppressed_exc = True
                    pending_raise = False
                    exc_details = (None, None, None)
            except:
                new_exc_details = sys.exc_info()
                # simulate the stack of exceptions by setting the context
                _fix_exception_context(new_exc_details[1], exc_details[1])
                pending_raise = True
                exc_details = new_exc_details
        if pending_raise:
            try:
                # bare "raise exc_details[1]" replaces our carefully
                # set-up context
                fixed_ctx = exc_details[1].__context__
                raise exc_details[1]
            except BaseException:
                exc_details[1].__context__ = fixed_ctx
                raise
        return received_exc and suppressed_exc

def close(self):
        """Immediately unwind the context stack."""
        self.__exit__(None, None, None)

# Inspired by discussions on https://bugs.python.org/issue29302
class AsyncExitStack(_BaseExitStack, AbstractAsyncContextManager):
    """Async context manager for dynamic management of a stack of exit
    callbacks.

For example:
        async with AsyncExitStack() as stack:
            connections = [await stack.enter_async_context(get_connection())
                for i in range(5)]
            # All opened connections will automatically be released at the
            # end of the async with statement, even if attempts to open a
            # connection later in the list raise an exception.
    """

@staticmethod
    def _create_async_exit_wrapper(cm, cm_exit):
        async def _exit_wrapper(exc_type, exc, tb):
            return await cm_exit(cm, exc_type, exc, tb)
        return _exit_wrapper

@staticmethod
    def _create_async_cb_wrapper(*args, **kwds):
        callback, *args = args
        async def _exit_wrapper(exc_type, exc, tb):
            await callback(*args, **kwds)
        return _exit_wrapper

async def enter_async_context(self, cm):
        """Enters the supplied async context manager.

If successful, also pushes its __aexit__ method as a callback and
        returns the result of the __aenter__ method.
        """
        _cm_type = type(cm)
        _exit = _cm_type.__aexit__
        result = await _cm_type.__aenter__(cm)
        self._push_async_cm_exit(cm, _exit)
        return result

def push_async_exit(self, exit):
        """Registers a coroutine function with the standard __aexit__ method
        signature.

Can suppress exceptions the same way __aexit__ method can.
        Also accepts any object with an __aexit__ method (registering a call
        to the method instead of the object itself).
        """
        _cb_type = type(exit)
        try:
            exit_method = _cb_type.__aexit__
        except AttributeError:
            # Not an async context manager, so assume it's a coroutine function
            self._push_exit_callback(exit, False)
        else:
            self._push_async_cm_exit(exit, exit_method)
        return exit  # Allow use as a decorator

def push_async_callback(*args, **kwds):
        """Registers an arbitrary coroutine function and arguments.

Cannot suppress exceptions.
        """
        if len(args) >= 2:
            self, callback, *args = args
        elif not args:
            raise TypeError("descriptor 'push_async_callback' of "
                            "'AsyncExitStack' object needs an argument")
        elif 'callback' in kwds:
            callback = kwds.pop('callback')
            self, *args = args
        else:
            raise TypeError('push_async_callback expected at least 1 '
                            'positional argument, got %d' % (len(args)-1))

_exit_wrapper = self._create_async_cb_wrapper(callback, *args, **kwds)

# We changed the signature, so using @wraps is not appropriate, but
        # setting __wrapped__ may still help with introspection.
        _exit_wrapper.__wrapped__ = callback
        self._push_exit_callback(_exit_wrapper, False)
        return callback  # Allow use as a decorator

async def aclose(self):
        """Immediately unwind the context stack."""
        await self.__aexit__(None, None, None)

def _push_async_cm_exit(self, cm, cm_exit):
        """Helper to correctly register coroutine function to __aexit__
        method."""
        _exit_wrapper = self._create_async_exit_wrapper(cm, cm_exit)
        _exit_wrapper.__self__ = cm
        self._push_exit_callback(_exit_wrapper, False)

async def __aenter__(self):
        return self

async def __aexit__(self, *exc_details):
        received_exc = exc_details[0] is not None

if cb_suppress:
                    suppressed_exc = True
                    pending_raise = False
                    exc_details = (None, None, None)
            except:
                new_exc_details = sys.exc_info()
                # simulate the stack of exceptions by setting the context
                _fix_exception_context(new_exc_details[1], exc_details[1])
                pending_raise = True
                exc_details = new_exc_details
        if pending_raise:
            try:
                # bare "raise exc_details[1]" replaces our carefully
                # set-up context
                fixed_ctx = exc_details[1].__context__
                raise exc_details[1]
            except BaseException:
                exc_details[1].__context__ = fixed_ctx
                raise
        return received_exc and suppressed_exc

class nullcontext(AbstractContextManager):
    """Context manager that does no additional processing.

Used as a stand-in for a normal context manager, when a particular
    block of code is only sometimes used with a normal context manager:

cm = optional_cm if condition else nullcontext()
    with cm:
        # Perform operation, using optional_cm if condition is True
    """

def __init__(self, enter_result=None):
        self.enter_result = enter_result

def __enter__(self):
        return self.enter_result

def __exit__(self, *excinfo):
        pass

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