python / python/mypy

mypy doesn't like when I use type variables to subscript generic type

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Description

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()

carbon

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

Contributor guide

Open the contributing guide

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

Start with the reproducer in tests/mypy_typealias_problem.py and run the shown mypy command against the three build_generic_type_from_str variants. Read the handling of generic type application and TypeVar expressions, then compare the diagnostics with the expected behavior described in the issue. Done means reaching a documented behavior decision and adding or updating a regression test if the behavior is confirmed as a bug.

Written by the indexing model from the issue text.

Assessment

Tech stack
python
Domain
devtools
Issue type
Bug
Difficulty
4/5
Estimated time
3-5 days
Activity status
Stale
Clarity
Mostly clear
Newbie friendliness
25/100

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