masque/masque/utils/autoslots.py

28 lines
1005 B
Python

from abc import ABCMeta
class AutoSlots(ABCMeta):
"""
Metaclass for automatically generating __slots__ based on superclass type annotations.
Superclasses must set `__slots__ = ()` to make this work properly.
This is a workaround for the fact that non-empty `__slots__` can't be used
with multiple inheritance. Since we only use multiple inheritance with abstract
classes, they can have empty `__slots__` and their attribute type annotations
can be used to generate a full `__slots__` for the concrete class.
"""
def __new__(cls, name, bases, dctn):
parents = set()
for base in bases:
parents |= set(base.mro())
slots = tuple(dctn.get('__slots__', tuple()))
for parent in parents:
if not hasattr(parent, '__annotations__'):
continue
slots += tuple(getattr(parent, '__annotations__').keys())
dctn['__slots__'] = slots
return super().__new__(cls, name, bases, dctn)