contextlib2.py

"""contextlib2 - backports and enhancements to the contextlib module"""

import abc
import sys
import warnings
from collections import deque
from functools import wraps

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

# Backwards compatibility
__all__ += ["ContextStack"]


# Backport abc.ABC
if sys.version_info[:2] >= (3, 4):
    _abc_ABC = abc.ABC
else:
    _abc_ABC = abc.ABCMeta('ABC', (object,), {'__slots__': ()})


# Backport classic class MRO
def _classic_mro(C, result):
    if C in result:
        return
    result.append(C)
    for B in C.__bases__:
        _classic_mro(B, result)
    return result


# Backport _collections_abc._check_methods
def _check_methods(C, *methods):
    try:
        mro = C.__mro__
    except AttributeError:
        mro = tuple(_classic_mro(C, []))

    for method in methods:
        for B in mro:
            if method in B.__dict__:
                if B.__dict__[method] is None:
                    return NotImplemented
                break
        else:
            return NotImplemented
    return True


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):
        """Check whether subclass is considered a subclass of this ABC."""
        if cls is AbstractContextManager:
            return _check_methods(C, "__enter__", "__exit__")
        return NotImplemented


class ContextDecorator(object):
    """A base class or mixin that enables context managers to work as decorators."""

    def refresh_cm(self):
        """Returns the context manager used to actually wrap the call to the
        decorated function.

        The default implementation just returns *self*.

        Overriding this method allows otherwise one-shot context managers
        like _GeneratorContextManager to support use as decorators via
        implicit recreation.

        DEPRECATED: refresh_cm was never added to the standard library's
                    ContextDecorator API
        """
        warnings.warn("refresh_cm was never added to the standard library",
                      DeprecationWarning)
        return self._recreate_cm()

    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")

    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
                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 _HAVE_EXCEPTION_CHAINING 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.
                #
                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(object):

    _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(object):
    """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)


# Context manipulation is Python 3 only
_HAVE_EXCEPTION_CHAINING = sys.version_info[0] >= 3
if _HAVE_EXCEPTION_CHAINING:
    def _make_context_fixer(frame_exc):
        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
        return _fix_exception_context

    def _reraise_with_existing_context(exc_details):
        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
else:
    # No exception context in Python 2
    def _make_context_fixer(frame_exc):
        return lambda new_exc, old_exc: None

    # Use 3 argument raise in Python 2,
    # but use exec to avoid SyntaxError in Python 3
    def _reraise_with_existing_context(exc_details):
        exc_type, exc_value, exc_tb = exc_details
        exec("raise exc_type, exc_value, exc_tb")

# Handle old-style classes if they exist
try:
    from types import InstanceType
except ImportError:
    # Python 3 doesn't have old-style classes
    _get_type = type
else:
    # Need to handle old-style context managers on Python 2
    def _get_type(obj):
        obj_type = type(obj)
        if obj_type is InstanceType:
            return obj.__class__  # Old-style class
        return obj_type  # New-style class


# 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 = _get_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 = _get_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]
        _fix_exception_context = _make_context_fixer(frame_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:
            _reraise_with_existing_context(exc_details)
        return received_exc and suppressed_exc


# Preserve backwards compatibility
class ContextStack(ExitStack):
    """Backwards compatibility alias for ExitStack"""

    def __init__(self):
        warnings.warn("ContextStack has been renamed to ExitStack",
                      DeprecationWarning)
        super(ContextStack, self).__init__()

    def register_exit(self, callback):
        return self.push(callback)

    def register(self, callback, *args, **kwds):
        return self.callback(callback, *args, **kwds)

    def preserve(self):
        return self.pop_all()


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