# Copyright 2020 Google LLC
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#      http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.

# Useful trick adapted from https://stackoverflow.com/a/50381071/12254339
# Allows one to define abstract instance attributes

import abc


class DummyAttribute:
    pass


def abstract_attribute(obj=None):
    if obj is None:
        obj = DummyAttribute()
    obj.__is_abstract_attribute__ = True
    return obj


class ABCMeta(abc.ABCMeta):
    def __call__(cls, *args, **kwargs):
        instance = super(ABCMeta, cls).__call__(*args, **kwargs)
        abstract_attributes = {
            name
            for name in dir(instance)
            if getattr(getattr(instance, name), '__is_abstract_attribute__', False)
        }
        if abstract_attributes:
            raise NotImplementedError(
                "Can't instantiate abstract class {} with"
                " abstract attributes: {}".format(
                    cls.__name__,
                    ', '.join(abstract_attributes)
                )
            )
        return instance