Source code for cardinal_pythonlib.metaclasses
#!/usr/bin/env python
# cardinal_pythonlib/metaclasses.py
"""
===============================================================================
Original code copyright (C) 2009-2022 Rudolf Cardinal (rudolf@pobox.com).
This file is part of cardinal_pythonlib.
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
https://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.
===============================================================================
**Support functions to do with metaclasses. IGNORE; NOT WORKING PROPERLY.**
"""
from typing import Any, Dict, Tuple, Type
[docs]class CooperativeMeta(type):
"""
The idea is to use this as the metaclass for a class ``Derived`` that
inherits from bases ``BaseOne``, ``BaseTwo``, ..., whose metaclasses are
not of the same type.
This should avoid the error:
.. code-block:: none
TypeError: metaclass conflict: the metaclass of a derived class must
be a (non-strict) subclass of the metaclasses of all its bases
Code from:
https://stackoverflow.com/questions/6557407/triple-inheritance-causes-metaclass-conflict-sometimes.
See also:
https://code.activestate.com/recipes/204197-solving-the-metaclass-conflict/.
HOWEVER, it's not actually working. Does Python 3 have stricter checking
than Python 2?
See also
https://blog.ionelmc.ro/2015/02/09/understanding-python-metaclasses/.
"""
def __new__(
mcs: Type, name: str, bases: Tuple[Type, ...], members: Dict[str, Any]
) -> Type:
# collect up the metaclasses
metas = [type(base) for base in bases]
# prune repeated or conflicting entries
metas = [
meta
for index, meta in enumerate(metas)
if not [
later
for later in metas[index + 1 :]
if issubclass(later, meta)
]
]
# whip up the actual combined meta class; derive off all of these
meta = type(name, tuple(metas), dict(combined_metas=metas))
# make the actual object
return meta(name, bases, members)
# noinspection PyMissingConstructor
def __init__(
cls: Type, name: str, bases: Tuple[Type, ...], members: Dict[str, Any]
) -> None:
# noinspection PyUnresolvedReferences
for meta in cls.combined_metas:
meta.__init__(cls, name, bases, members)
[docs]class DebuggingCooperativeMeta(type):
"""
``CooperativeMeta``, but with :func:`print` output.
Still not working.
"""
def __new__(
mcs: Type, name: str, bases: Tuple[Type, ...], members: Dict[str, Any]
) -> Type:
metas = [type(base) for base in bases]
metas = [
meta
for index, meta in enumerate(metas)
if not [
later
for later in metas[index + 1 :]
if issubclass(later, meta)
]
]
meta = type(name, tuple(metas), dict(combined_metas=metas))
obj = meta(name, bases, members)
print(f"DebuggingCooperativeMeta.__new__: bases = {bases!r}")
print(f"DebuggingCooperativeMeta.__new__: metas = {metas!r}")
print(f"DebuggingCooperativeMeta.__new__: meta = {meta!r}")
print(f"DebuggingCooperativeMeta.__new__: obj = {obj!r}")
# make the actual object
return obj
# noinspection PyMissingConstructor
def __init__(
cls: Type, name: str, bases: Tuple[Type, ...], members: Dict[str, Any]
) -> None:
# noinspection PyUnresolvedReferences
for meta in cls.combined_metas:
print(
f"DebuggingCooperativeMeta.__init__: calling "
f"{meta.__name__}.__init__({name!r}, {bases!r}, {members!r}"
)
meta.__init__(cls, name, bases, members)
TEST_CODE_SHOULD_FAIL = """
class MetaA(type):
pass
class MetaB(type):
pass
class A(metaclass=MetaA):
pass
class B(metaclass=MetaB):
pass
class Fixed(A, B):
pass
"""
TEST_CODE_SHOULD_SUCCEED_BUT_DOES_NOT = """
from cardinal_pythonlib.metaclasses import CooperativeMeta
class MetaA(type):
pass
class MetaB(type):
pass
class A(metaclass=MetaA):
pass
class B(metaclass=MetaB):
pass
class Fixed(A, B, metaclass=CooperativeMeta):
pass
"""