celery/celery/utils/objects.py

# -*- coding: utf-8 -*-
"""
    celery.utils.objects
    ~~~~~~~~~~~~~~~~~~~~

    Object related utilities including introspection, etc.

"""
from __future__ import absolute_import

__all__ = ['mro_lookup']


def mro_lookup(cls, attr, stop=(), monkey_patched=[]):
    """Return the first node by MRO order that defines an attribute.

    :keyword stop: A list of types that if reached will stop the search.
    :keyword monkey_patched: Use one of the stop classes if the attr's
        module origin is not in this list, this to detect monkey patched
        attributes.

    :returns None: if the attribute was not found.

    """
    for node in cls.mro():
        if node in stop:
            try:
                attr = node.__dict__[attr]
                module_origin = attr.__module__
            except (AttributeError, KeyError):
                pass
            else:
                if module_origin not in monkey_patched:
                    return node
            return
        if attr in node.__dict__:
            return node


class FallbackContext(object):
    """The built-in ``@contextmanager`` utility does not work well
    when wrapping other contexts, as the traceback is wrong when
    the wrapped context raises.

    This solves this problem and can be used instead of ``@contextmanager``
    in this example::

        @contextmanager
        def connection_or_default_connection(connection=None):
            if connection:
                # user already has a connection, should not close
                # after use
                yield connection
            else:
                # must have new connection, and also close the connection
                # after the block returns
                with create_new_connection() as connection:
                    yield connection

    This wrapper can be used instead for the above like this::

        def connection_or_default_connection(connection=None):
            return FallbackContext(connection, create_new_connection)

    """

    def __init__(self, provided, fallback, *fb_args, **fb_kwargs):
        self.provided = provided
        self.fallback = fallback
        self.fb_args = fb_args
        self.fb_kwargs = fb_kwargs
        self._context = None

    def __enter__(self):
        if self.provided is not None:
            return self.provided
        context = self._context = self.fallback(
            *self.fb_args, **self.fb_kwargs
        ).__enter__()
        return context

    def __exit__(self, *exc_info):
        if self._context is not None:
            return self._context.__exit__(*exc_info)