Lib/contextlib.py

"""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", "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

    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 the exception *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:
                # 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


# Unfortunately, this was originally published as a class, so
# backwards compatibility prevents the use of the wrapper function
# approach used for the other classes
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 _RedirectStdout:
    """Helper for redirect_stdout."""

    def __init__(self, new_target):
        self._new_target = new_target
        self._old_target = self._sentinel = object()

    def __enter__(self):
        if self._old_target is not self._sentinel:
            raise RuntimeError("Cannot reenter {!r}".format(self))
        self._old_target = sys.stdout
        sys.stdout = self._new_target
        return self._new_target

    def __exit__(self, exctype, excinst, exctb):
        restore_stdout = self._old_target
        self._old_target = self._sentinel
        sys.stdout = restore_stdout

# Use a wrapper function since we don't care about supporting inheritance
# and a function gives much cleaner output in help()
def redirect_stdout(target):
    """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)
    """
    return _RedirectStdout(target)


class _SuppressExceptions:
    """Helper for suppress."""
    def __init__(self, *exceptions):
        self._exceptions = exceptions

    def __enter__(self):
        pass

    def __exit__(self, exctype, excinst, exctb):
        # Unlike isinstance and issubclass, exception handling only
        # looks at the concrete type heirarchy (ignoring the instance
        # and subclass checking hooks). However, all exceptions are
        # also required to be concrete subclasses of BaseException, so
        # if there's a discrepancy in behaviour, we currently consider it
        # the fault of the strange way the exception has been defined rather
        # than the fact that issubclass can be customised while the
        # exception checks can't.
        # See http://bugs.python.org/issue12029 for more details
        return exctype is not None and issubclass(exctype, self._exceptions)

# Use a wrapper function since we don't care about supporting inheritance
# and a function gives much cleaner output in help()
def suppress(*exceptions):
    """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
    """
    return _SuppressExceptions(*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):
            while 1:
                exc_context = new_exc.__context__
                if exc_context in (None, frame_exc):
                    break
                new_exc = exc_context
            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