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TypeError: Some type variables (+T) are not listed in Generic[~T] #94607
Description
Bug report
In Python 3.11.0b3 subclasses of generic classes seems to have started requiring explicitly filling in all the super-classes type vars, even though the subclass isn't supposed to be generic in the same variable as the super type.
The behavior is inconsistent and only throws an error if the subclass is also generic.
Minimal reproduction
from typing import Generic
from typing import TypeVar
# --- base
T_co = TypeVar("T_co", covariant=True)
class Base(Generic[T_co]):
...
# --- non-generic subclass, this doesn't throw an error
class NonGenericSub(Base):
...
# --- subclass generic, in _another_ variable
T = TypeVar("T")
# this throws an error
class Sub(Base, Generic[T]):
...This gives this error:
Traceback (most recent call last):
File "/Users/annevoab/projects/phantom-types/reproduce.py", line 22, in <module>
class Sub(Base, Generic[T]):
^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/Users/annevoab/.pyenv/versions/3.11.0b3/lib/python3.11/typing.py", line 1869, in __init_subclass__
raise TypeError(f"Some type variables ({s_vars}) are"
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
TypeError: Some type variables (+T_co) are not listed in Generic[~T]
Workaround
By explicitly passing object to fill in the typevar of the super class, the error is surpressed.
class Sub(Base[object], Generic[T]):
...Your environment
- CPython versions tested on: Python 3.11.0b3
- Operating system and architecture: Macos 12.3.1 / Darwin Kernel Version 21.4.0: Fri Mar 18 00:45:05 PDT 2022; root:xnu-8020.101.4~15/RELEASE_X86_64 x86_64
Activity
JelleZijlstra commentedon Jul 6, 2022
Confirmed, and marking as a release blocker since this is a regression. I'll take a look later today. Also cc @Fidget-Spinner @serhiy-storchaka in case you have any insights.
Fidget-Spinner commentedon Jul 6, 2022
@JelleZijlstra bisect points me to commit b6a5d85.
CC @serhiy-storchaka
Fidget-Spinner commentedon Jul 6, 2022
Sorry @JelleZijlstra I hope I didn't create duplicate work, but I have a PR open at #94610.
JelleZijlstra commentedon Jul 6, 2022
Thanks @Fidget-Spinner! I haven't done anything yet; I was planning to try and come up with a solution after work today, but now I'll get too review your PR instead.
gh-94607: Fix subclassing generics (GH-94610)
pythongh-94607: Fix subclassing generics (pythonGH-94610)
gh-94607: Fix subclassing generics (GH-94610)