mypy doesn't like when I use type variables to subscript generic type
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Descripción
Bug Report
Hey, I'm trying to figure out if this mypy output is a bug or if I'm doing something incorrectly. I need to pass a type variable to a function to properly deserialize my model class. Versions 1 & 2 of my build_generic_* function work correctly, as visible in the code output, but mypy doesn't like what I'm doing there saying that I can't use a type variable this way. I created versions of this function both with Type and TypeAlias annotation just to check if mypy will be fine with either one.
from typing import Generic, Type, TypeAlias, TypeVar
from pydantic import BaseModel
from pydantic.generics import GenericModel
class SomeModel(BaseModel):
a: int
T = TypeVar("T", bound=BaseModel)
class SomeGenericModel(GenericModel, Generic[T]):
some_model_instance: T
def build_generic_type_from_str_1(some_generic_model_str: str,
t_type: Type[T]) -> SomeGenericModel[T]:
return SomeGenericModel[t_type].parse_raw(some_generic_model_str)
def build_generic_type_from_str_2(some_generic_model_str: str,
t_type: TypeAlias) -> SomeGenericModel[T]:
return SomeGenericModel[t_type].parse_raw(some_generic_model_str)
def build_generic_type_from_str_3(
some_generic_model_str: str) -> SomeGenericModel[T]:
return SomeGenericModel.parse_raw(some_generic_model_str)
def main():
print(
build_generic_type_from_str_1('{"some_model_instance": {"a": 1}}',
SomeModel))
print(
build_generic_type_from_str_2('{"some_model_instance": {"a": 1}}',
SomeModel))
print(build_generic_type_from_str_3('{"some_model_instance": {"a": 1}}'))
if __name__ == "__main__":
main()

Output from running this code & mypy:
$ python tests/mypy_typealias_problem.py
some_model_instance=SomeModel(a=1)
some_model_instance=SomeModel(a=1)
some_model_instance=BaseModel()
$ mypy tests/mypy_typealias_problem.py
tests/mypy_typealias_problem.py:19: error: Variable "t_type" is not valid as a type
tests/mypy_typealias_problem.py:19: note: See https://mypy.readthedocs.io/en/stable/common_issues.html#variables-vs-type-aliases
tests/mypy_typealias_problem.py:24: error: Variable "t_type" is not valid as a type
tests/mypy_typealias_problem.py:24: note: See https://mypy.readthedocs.io/en/stable/common_issues.html#variables-vs-type-aliases
Found 2 errors in 1 file (checked 1 source file)
For comparison, pyright seems to suggest that the second approach with TypeAlias is incorrect.
$ pyright tests/mypy_typealias_problem.py
WARNING: there is a new pyright version available (v1.1.269 -> v1.1.274).
Please install the new version or set PYRIGHT_PYTHON_FORCE_VERSION to `latest`
No configuration file found.
pyproject.toml file found at /Users/dominikderen/dev/qomplx/argo-workflows/iostation-responders-client.
Loading pyproject.toml file at /Users/dominikderen/dev/qomplx/argo-workflows/iostation-responders-client/pyproject.toml
Assuming Python version 3.10
Assuming Python platform Darwin
Auto-excluding **/node_modules
Auto-excluding **/__pycache__
Auto-excluding **/.*
stubPath /Users/dominikderen/dev/qomplx/argo-workflows/iostation-responders-client/typings is not a valid directory.
Searching for source files
Found 1 source file
pyright 1.1.269
/Users/dominikderen/dev/qomplx/argo-workflows/iostation-responders-client/tests/mypy_typealias_problem.py
/Users/dominikderen/dev/qomplx/argo-workflows/iostation-responders-client/tests/mypy_typealias_problem.py:24:29 - error: Expected class type but received "TypeAlias" (reportGeneralTypeIssues)
/Users/dominikderen/dev/qomplx/argo-workflows/iostation-responders-client/tests/mypy_typealias_problem.py:23:74 - warning: TypeVar "T" appears only once in generic function signature (reportInvalidTypeVarUse)
/Users/dominikderen/dev/qomplx/argo-workflows/iostation-responders-client/tests/mypy_typealias_problem.py:28:58 - warning: TypeVar "T" appears only once in generic function signature (reportInvalidTypeVarUse)
/Users/dominikderen/dev/qomplx/argo-workflows/iostation-responders-client/tests/mypy_typealias_problem.py:38:39 - error: Argument of type "Type[SomeModel]" cannot be assigned to parameter "t_type" of type "TypeAlias" in function "build_generic_type_from_str_2"
"Type[ModelMetaclass]" is incompatible with "Type[TypeAlias]" (reportGeneralTypeIssues)
2 errors, 2 warnings, 0 informations
Completed in 0.77sec
Versions:
Any suggestions on this would be greatly appreciated!
To Reproduce
Run the script provided above.
Expected Behavior
Either of the two approaches should not raise a mypy validation error.
Actual Behavior
Mypy is making case 1 & 2 as errors.
Your Environment
-
Mypy version used: mypy 0.971 (compiled: yes)
-
Mypy command-line flags: none, just providing a directory.
-
Mypy configuration options from
mypy.ini(and other config files): none. -
Python version used: 3.10.5
-
Operating system and version: MacOS Monterey 12.5.1
-
Pyright: 1.1.269
Guía de contribución
Primeros pasos
- Lee el issue completo y luego la guía de contribución del proyecto.
- Comenta en el issue que vas a ocuparte — evita que dos personas hagan lo mismo.
- Haz un fork del repositorio y trabaja en una rama.
- Abre un pull request que haga referencia al número del issue.
Línea de trabajo
Comienza con el reproductor en tests/mypy_typealias_problem.py y ejecuta el comando de mypy mostrado contra las tres variantes de build_generic_type_from_str. Lee el tratamiento de la aplicación de tipos genéricos y de las expresiones TypeVar; después, compara los diagnósticos con el comportamiento esperado descrito en el issue. Se considera terminado cuando se haya llegado a una decisión documentada sobre el comportamiento y se haya añadido o actualizado una prueba de regresión si se confirma que el comportamiento es un bug.
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