mypy
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python
(regression) Interaction between `__new__` and `Generic`s
Bug Report
Consider the following code [mypy-play.net]:
from __future__ import annotations
from typing import *
T = TypeVar('T')
class Parent(Sequence[T]):
'''a sequence type which is implemented one of two ways,
according to the input parameters
'''
def __new__(cls, model: Sequence[T]) -> Parent[T]:
if cls is Parent:
if len(model) % 2:
return Odds(model)
return Evens(model)
return super().__new__(cls)
_data: Sequence[T]
@overload
def __getitem__(self, index: int, /) -> T: ...
@overload
def __getitem__(self, index: slice, /) -> Sequence[T]: ...
def __getitem__(self, index: Union[int, slice], /
) -> Union[Sequence[T], T]: # pragma: no cover
return self._data[index]
def __len__(self) -> int:
return len(self._data)
class Evens(Parent[T]):
'''the implementation of 'Parent' used when the input has an even length
'''
def __new__(cls, model: Sequence[T]) -> Evens[T]:
ret = super().__new__(cls, model)
# mypy: error: Argument 1 to "__new__" of "Parent" has
# incompatible type "Type[Evens[T]]";
# expected "Type[Parent[T]]"
# [arg-type]
# error: Argument 2 to "__new__" of "Parent" has
# incompatible type "Sequence[T]"; expected
# "Sequence[T]"
# [arg-type]
assert isinstance(ret, Evens)
return ret
def __init__(self, model: Sequence[T]):
self._data = model[::2]
class Odds(Parent[T]):
'''the implementation of 'Parent' used when the input has an odd length
'''
def __new__(cls, model: Sequence[T]) -> Odds[T]:
# mypy: two (3) errors, analogous to those of
# 'Evens.__new__()', above
ret = super().__new__(cls, model)
assert isinstance(ret, Odds)
return ret
def __init__(self, model: Sequence[T]):
self._data = model[1::2]
def test_function() -> None:
# this test passes; the code does work!
evens = Parent(list(range(10)))
assert isinstance(evens, Parent)
assert isinstance(evens, Evens)
assert list(evens) == [0, 2, 4, 6, 8]
odds = Parent(list(range(11)))
assert isinstance(odds, Parent)
assert isinstance(odds, Odds)
assert list(odds) == [1, 3, 5, 7, 9]
To Reproduce
See above.
Actual Behavior
main.py:40: error: Argument 1 to "__new__" of "Parent" has incompatible type "Type[Evens[T]]"; expected "Type[Parent[T]]" [arg-type]
main.py:40: error: Argument 2 to "__new__" of "Parent" has incompatible type "Sequence[T]"; expected "Sequence[T]" [arg-type]
main.py:61: error: Argument 1 to "__new__" of "Parent" has incompatible type "Type[Odds[T]]"; expected "Type[Parent[T]]" [arg-type]
main.py:61: error: Argument 2 to "__new__" of "Parent" has incompatible type "Sequence[T]"; expected "Sequence[T]" [arg-type]
Found 4 errors in 1 file (checked 1 source file)
Expected Behavior
There should be no errors. (In fact, this worked correctly until mypy 0.950.)
The first and third errors make little sense since Type[Evens[T]] is a special case of Type[Parent[T]].
The second and fourth errors make even less sense, since Sequence[T] is exactly the same as Sequence[T].
If there is actually some kind of problem here, the error messages need to be much more helpful than they currently are.
Your Environment
- Mypy version used: master, 0.981
- Mypy command-line flags: --strict --warn-unreachable --show-error-codes
- Mypy configuration options from
mypy.ini(and other config files): N/A - Python version used: 3.10, 3.8
Note that this is specific to __new__, other (explicit) class methods work correctly. I didn't check, but it may be caused by https://github.com/python/mypy/pull/12590 cc @JukkaL FWIW PR looks correct, probably it just exposes some old missing special casing for __new__. Likely should be an easy fix.
FWIW I've confirmed this issue persists on the development version of mypy (2023-06-26).
It looks like @kourbou fixed this with 1dd8e7f (#16670) when they fixed #16668:
commit 1dd8e7fe6
Author: Kouroche Bouchiat <[email protected]>
Date: Sun Dec 17 21:32:57 2023 +0100
Substitute type variables in return type of static methods (#16670)
`add_class_tvars` correctly instantiates type variables in the return
type for class methods but not for static methods. Check if the analyzed
member is a static method in `analyze_class_attribute_access` and
substitute the type variable in the return type in `add_class_tvars`
accordingly.
Fixes #16668.
Thanks, @kourbou!
Is it worth adding a test for this case?