def authenticate(**credentials):
    """
    If the given credentials are valid, return a User object.
    """
    for backend in get_backends():
        try:
            user = backend.authenticate(**credentials)
        except __HOLE__:
            # This backend doesn't accept these credentials as arguments. Try the next one.
            continue
        except PermissionDenied:
            # This backend says to stop in our tracks - this user should not be allowed in at all.
            return None
        if user is None:
            continue
        # Annotate the user object with the path of the backend.
        user.backend = "%s.%s" % (backend.__module__, backend.__class__.__name__)
        return user

    # The credentials supplied are invalid to all backends, fire signal
    user_login_failed.send(sender=__name__,
            credentials=_clean_credentials(credentials))
dataset/ETHPy150Open cloudera/hue/desktop/core/ext-py/Django-1.6.10/django/contrib/auth/__init__.py/authenticate