New submission from Ethan Furman <[email protected]>:
PEP 487 introduced __init_subclass__ and __set_name__, and both of those were
wins for the common cases of metaclass usage.
Unfortunately, the implementation of PEP 487 with regards to __init_subclass__
has made the writing of correct metaclasses significantly harder, if not
impossible.
The cause is that when a metaclass calls type.__new__ to actually create the
class, type.__new__ calls the __init_subclass__ methods of the new class'
parents, passing it the newly created, but incomplete, class. In code:
```
class Meta(type):
#
def __new__(mcls, name, bases, namespace, **kwds):
# create new class, which will call __init_subclass__ and __set_name__
new_class = type.__new__(mcls, name, bases, namespace, **kwds)
# finish setting up class
new_class.some_attr = 9
```
As you can deduce, when the parent __init_subclass__ is called with the new
class, `some_attr` has not been added yet -- the new class is incomplete.
For Enum, this means that __init_subclass__ doesn't have access to the new
Enum's members (they haven't beet added yet).
For ABC, this means that __init_subclass__ doesn't have access to
__abstract_methods__ (it hasn't been created yet).
Because Enum is pure Python code I was able to work around it:
- remove new __init_subclass__ (if it exists)
- insert dummy class with a no-op __init_subclass__
- call type.__new__
- save any actual __init_subclass__
- add back any new __init_subclass__
- rewrite the new class' __bases__, removing the no-op dummy class
- finish creating the class
- call the parent __init_subclass__ with the now complete Enum class
I have not been able to work around the problem for ABC.
The solution would seem to be to move the calls to __init_subclass__ and
__set_names__ to type.__init__.
----------
assignee: ethan.furman
components: Interpreter Core
messages: 383946
nosy: ethan.furman, serhiy.storchaka
priority: high
severity: normal
stage: needs patch
status: open
title: __init_subclass__ should be called in __init__
type: behavior
versions: Python 3.10
_______________________________________
Python tracker <[email protected]>
<https://bugs.python.org/issue42775>
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