| """Utilities for with-statement contexts. See PEP 343.""" |
| |
| import sys |
| from collections import deque |
| from functools import wraps |
| |
| __all__ = ["contextmanager", "closing", "ContextDecorator", "ExitStack", |
| "redirect_stdout", "redirect_stderr", "suppress"] |
| |
| |
| 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 _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: |
| # 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 |
| |
| |
| class closing(object): |
| """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: |
| |
| _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: |
| """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) |
| |
| |
| # Inspired by discussions on http://bugs.python.org/issue13585 |
| class ExitStack(object): |
| """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 __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_cm_exit(self, cm, cm_exit): |
| """Helper to correctly register callbacks to __exit__ methods""" |
| def _exit_wrapper(*exc_details): |
| return cm_exit(cm, *exc_details) |
| _exit_wrapper.__self__ = cm |
| self.push(_exit_wrapper) |
| |
| def push(self, exit): |
| """Registers a callback with the standard __exit__ method signature |
| |
| Can suppress exceptions the same way __exit__ methods 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 its a callable |
| self._exit_callbacks.append(exit) |
| else: |
| self._push_cm_exit(exit, exit_method) |
| return exit # Allow use as a decorator |
| |
| def callback(self, callback, *args, **kwds): |
| """Registers an arbitrary callback and arguments. |
| |
| Cannot suppress exceptions. |
| """ |
| def _exit_wrapper(exc_type, exc, tb): |
| 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_wrapper) |
| return callback # 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 close(self): |
| """Immediately unwind the context stack""" |
| self.__exit__(None, None, None) |
| |
| 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: |
| cb = self._exit_callbacks.pop() |
| 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 |