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contextlib.py
"""Utilities for with-statement contexts. See PEP 343.""" import abc import os import sys import _collections_abc from collections import deque from functools import wraps from types import MethodType, GenericAlias __all__ = ["asynccontextmanager", "contextmanager", "closing", "nullcontext", "AbstractContextManager", "AbstractAsyncContextManager", "AsyncExitStack", "ContextDecorator", "ExitStack", "redirect_stdout", "redirect_stderr", "suppress", "aclosing", "chdir"] class AbstractContextManager(abc.ABC): """An abstract base class for context managers.""" __class_getitem__ = classmethod(GenericAlias) 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.""" __class_getitem__ = classmethod(GenericAlias) 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 AsyncContextDecorator(object): "A base class or mixin that enables async context managers to work as decorators." def _recreate_cm(self): """Return a recreated instance of self. """ return self def __call__(self, func): @wraps(func) async def inner(*args, **kwds): async with self._recreate_cm(): return await 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. def _recreate_cm(self): # _GCMB 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) class _GeneratorContextManager( _GeneratorContextManagerBase, AbstractContextManager, ContextDecorator, ): """Helper for @contextmanager decorator.""" 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, typ, value, traceback): if typ is None: try: next(self.gen) except StopIteration: return False else: try: raise RuntimeError("generator didn't stop") finally: self.gen.close() else: if value is None: # Need to force instantiation so we can reliably # tell if we get the same exception back value = typ() try: self.gen.throw(typ, 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: exc.__traceback__ = traceback 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 actually Stop(Async)Iteration (see # issue29692). if ( isinstance(value, StopIteration) and exc.__cause__ is value ): value.__traceback__ = traceback return False raise except BaseException as exc: # 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 exc is not value: raise exc.__traceback__ = traceback return False try: raise RuntimeError("generator didn't stop after throw()") finally: self.gen.close() class _AsyncGeneratorContextManager( _GeneratorContextManagerBase, AbstractAsyncContextManager, AsyncContextDecorator, ): """Helper for @asynccontextmanager decorator.""" async def __aenter__(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 await anext(self.gen) except StopAsyncIteration: raise RuntimeError("generator didn't yield") from None async def __aexit__(self, typ, value, traceback): if typ is None: try: await anext(self.gen) except StopAsyncIteration: return False else: try: raise RuntimeError("generator didn't stop") finally: await self.gen.aclose() else: if value is None: # Need to force instantiation so we can reliably # tell if we get the same exception back value = typ() try: await self.gen.athrow(typ, value, traceback) except StopAsyncIteration 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: exc.__traceback__ = traceback return False # Avoid suppressing if a Stop(Async)Iteration exception # was passed to athrow() 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 actually Stop(Async)Iteration (see # issue29692). if ( isinstance(value, (StopIteration, StopAsyncIteration)) and exc.__cause__ is value ): value.__traceback__ = traceback return False raise except BaseException as exc: # 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 exc is not value: raise exc.__traceback__ = traceback return False try: raise RuntimeError("generator didn't stop after athrow()") finally: await self.gen.aclose() 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 aclosing(AbstractAsyncContextManager): """Async context manager for safely finalizing an asynchronously cleaned-up resource such as an async generator, calling its ``aclose()`` method. Code like this: async with aclosing(<module>.fetch(<arguments>)) as agen: <block> is equivalent to this: agen = <module>.fetch(<arguments>) try: <block> finally: await agen.aclose() """ def __init__(self, thing): self.thing = thing async def __aenter__(self): return self.thing async def __aexit__(self, *exc_info): await self.thing.aclose() 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): return MethodType(cm_exit, cm) @staticmethod def _create_cb_wrapper(callback, /, *args, **kwds): 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. cls = type(cm) try: _enter = cls.__enter__ _exit = cls.__exit__ except AttributeError: raise TypeError(f"'{cls.__module__}.{cls.__qualname__}' object does " f"not support the context manager protocol") from None result = _enter(cm) self._push_cm_exit(cm, _exit) return result def callback(self, callback, /, *args, **kwds): """Registers an arbitrary callback and arguments. Cannot suppress exceptions. """ _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) 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 None or exc_context is old_exc: # Context is already set correctly (see issue 20317) return if 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): return MethodType(cm_exit, cm) @staticmethod def _create_async_cb_wrapper(callback, /, *args, **kwds): 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. """ cls = type(cm) try: _enter = cls.__aenter__ _exit = cls.__aexit__ except AttributeError: raise TypeError(f"'{cls.__module__}.{cls.__qualname__}' object does " f"not support the asynchronous context manager protocol" ) from None result = await _enter(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(self, callback, /, *args, **kwds): """Registers an arbitrary coroutine function and arguments. Cannot suppress exceptions. """ _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) 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 None or exc_context is old_exc: # Context is already set correctly (see issue 20317) return if 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, AbstractAsyncContextManager): """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 async def __aenter__(self): return self.enter_result async def __aexit__(self, *excinfo): pass class chdir(AbstractContextManager): """Non thread-safe context manager to change the current working directory.""" def __init__(self, path): self.path = path self._old_cwd = [] def __enter__(self): self._old_cwd.append(os.getcwd()) os.chdir(self.path) def __exit__(self, *excinfo): os.chdir(self._old_cwd.pop())
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