Actually, there are a few steps that `type.__new__` perform that are notcustomizable in metaclasses.
I had sometimes thought about mailing this here, or Python ideas, but could notcome up with a "real world" use case where the customization of thosewould be meaningful.
Let'me see if I recall all cases - two of them are the calls to`__init_subclass__` and the descriptors `__set_name__` as you put it,
I think there is a third behavior that can't be separated from `type.__new__` - butI can't remember it now
Anyway, the "thing to do" that always occurred to me about it is to add "soft" method slotsto `type` itself - so that `type.__new__` would call those on the corresponding initialization phases.
Since these are to be run only when classes are created, their impact should be negligible.
In other words, have `type` implement methods like `__run_init_subclass__`, `__run_descriptor_setname__`,(and one for the other task I can't remember now). So, all metaclass code written up to today remains valid,and these behaviors become properly customizable.Adding keyword parameters to `type.__new__`, IMHO, besides a little bit fishy as we are talking ofarguments to change the behavior of the method, would themselves compete and have to befiltered out, or otherwise special-cased in the `__init_subclass__` method itself.
I mean - let's suppose we add `__suppress_init_subclass__` as an named parameter to
`type.__new__` - what would happen with this argument in `__init_subclass__` ? Would it showup in the kwargs? Otherwise it would be the _only_ kwarg popped out and not passedto __init_subclass__, being an inconvenient exception.
Having an overridable, separate, method in type to run __init_subclass__ and __set_name__bypass these downsides.
In time, Happy holidays everyone!
js-><-On Fri, 25 Dec 2020 at 00:38, Ethan Furman <ethan@stoneleaf.us> wrote: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.
Two possible solutions I can think of:
- pass a keyword argument to type.__new__ that suppresses the call to __init_subclass__; and
- provide a way to invoke new class' parent's __init_subclass__ before returning it
or
- instead of type.__new__ doing that work, have type.__init__ do it.
Thoughts?
--
~Ethan~
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