mypy 1.5 thinks optional field is non-optional in a generic model

[miikka]

Initial Checks

• I confirm that I'm using Pydantic V2

Description

For the code below, mypy 1.5.0 and 1.5.1 give the following output:

pydantic_hmm.py:24: note: Revealed type is "pydantic_hmm.Data"
pydantic_hmm.py:25: error: Right operand of "and" is never evaluated  [unreachable]

This is not what I expected - the type should be Optional[Data]. When the test is run, it passes and the right-hand side of the assertion does get evaluated.

This works as expected with mypy 1.4.1. Here's its output:

pydantic_hmm.py:24: note: Revealed type is "Union[pydantic_hmm.Data, None]"

For what it's worth, Pylance gives the same revealed type as mypy 1.4.1.

I'm not sure if this is a Pydantic issue or a mypy issue, but I wasn't able to reproduce this without using BaseModel and pydantic.mypy mypy plugin so I'm reporting it here.

Example Code

from typing import TYPE_CHECKING, Generic, Optional, TypeVar

from pydantic import BaseModel


class Data(BaseModel):
    pass


T = TypeVar("T", bound=Data)


class BaseWrapper(BaseModel, Generic[T]):
    data: Optional[T] = None


class Wrapper(BaseWrapper[Data]):
    pass


def test_vrapped_data_is_none() -> None:
    wrapper = Wrapper()
    if TYPE_CHECKING:
        reveal_type(wrapper.data)
    assert wrapper.data is None and True

mypy.ini:

[mypy]
plugins = pydantic.mypy
warn_unreachable = True

Python, Pydantic & OS Version

pydantic version: 2.3.0
        pydantic-core version: 2.6.3
          pydantic-core build: profile=release pgo=false
                 install path: /Users/miikka.koskinen/Library/Caches/pypoetry/virtualenvs/xxx-mwympnZX-py3.11/lib/python3.11/site-packages/pydantic
               python version: 3.11.4 (main, Jun 20 2023, 16:59:59) [Clang 14.0.3 (clang-1403.0.22.14.1)]
                     platform: macOS-13.5-x86_64-i386-64bit
     optional deps. installed: ['typing-extensions']


mypy 1.5.1 (compiled: yes)

Selected Assignee: @Kludex

[miikka]

Checked and this is still an issue with Pydantic 2.4.2. To make reproduction easier, I made a small repo with a Poetry setup: https://github.com/miikka/repro_pydantic_7280

[Kludex]

Yeah, it does look like a bug. Thanks. 🙏

[miikka]

Just came to back to this ticket to see if I can do anything about it and it seems PR #7411 has fixed it!

Looks like this issue is essentially duplicate of #7399.