Attempt to return a Python class for the input class reference.

    If `classref` is a class or None, return it. If `classref` is a
    python classpath (e.g., "foo.bar.MyClass") import the class and return
    it.

    Args:
        classref: A fully-qualified Python path to class, or a Python class.

    Returns:
        A class.

def resolve_class(classref):
    """Attempt to return a Python class for the input class reference.

    If `classref` is a class or None, return it. If `classref` is a
    python classpath (e.g., "foo.bar.MyClass") import the class and return
    it.

    Args:
        classref: A fully-qualified Python path to class, or a Python class.

    Returns:
        A class.
    """
    if classref is None:
        return None
    elif isinstance(classref, six.class_types):
        return classref
    elif isinstance(classref, six.string_types):
        return import_class(classref)
    else:
        raise ValueError("Unable to resolve class for '%s'" % classref)