Mypy crashes when trying to parse a pydantic model with optional fields

[kikones34]

Initial Checks

• I confirm that I'm using Pydantic V2

Description

I'm reporting the issue here because I think it's a problem with pydantic's mypy plugin. It was working fine before updating, more specifically it worked with pydantic-core==2.14.6 and pydantic==2.5.3.

Now, the error occurs only when both of these settings are specified in pyproject.toml:

[tool.mypy]
no_strict_optional = true
plugins = "pydantic.mypy"

When trying to parse a pydantic model with optional fields, mypy crashes as follows:

testo.py:4: error: INTERNAL ERROR -- Please try using mypy master on GitHub:
https://mypy.readthedocs.io/en/stable/common_issues.html#using-a-development-mypy-build
Please report a bug at https://github.com/python/mypy/issues
version: 1.8.0
Traceback (most recent call last):
  File "<frozen runpy>", line 198, in _run_module_as_main
  File "<frozen runpy>", line 88, in _run_code
  File "mypy\semanal.py", line 6539, in accept
  File "mypy\nodes.py", line 1142, in accept
  File "mypy\semanal.py", line 1612, in visit_class_def
  File "mypy\semanal.py", line 1697, in analyze_class
  File "mypy\semanal.py", line 1731, in analyze_class_body_common
  File "mypy\semanal.py", line 1816, in apply_class_plugin_hooks
  File "C:\Users\arico\venv2\Lib\site-packages\pydantic\mypy.py", line 187, in _pydantic_model_class_maker_callback
    return transformer.transform()
           ^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\arico\venv2\Lib\site-packages\pydantic\mypy.py", line 478, in transform
    self.add_initializer(fields, config, is_settings, is_root_model)
  File "C:\Users\arico\venv2\Lib\site-packages\pydantic\mypy.py", line 856, in add_initializer
    args = self.get_field_arguments(
           ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\arico\venv2\Lib\site-packages\pydantic\mypy.py", line 1039, in get_field_arguments
    field.to_argument(
  File "C:\Users\arico\venv2\Lib\site-packages\pydantic\mypy.py", line 346, in to_argument
    variable=self.to_var(current_info, use_alias),
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\arico\venv2\Lib\site-packages\pydantic\mypy.py", line 373, in to_var
    return Var(name, self.expand_type(current_info))
                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\arico\venv2\Lib\site-packages\pydantic\mypy.py", line 362, in expand_type
    raise _DeferAnalysis()
_DeferAnalysis:

Example Code

from pydantic import BaseModel


class MongoSettings(BaseModel):
    MONGO_PASSWORD: str | None

Python, Pydantic & OS Version

             pydantic version: 2.6.0
        pydantic-core version: 2.16.1
          pydantic-core build: profile=release pgo=true
                 install path: /home/k34/repos/internal/trio-cloud-search/venv2/lib/python3.12/site-packages/pydantic
               python version: 3.12.1 (main, Dec 10 2023, 15:07:36) [GCC 11.4.0]
                     platform: Linux-5.15.133.1-microsoft-standard-WSL2-x86_64-with-glibc2.35
             related packages: mypy-1.8.0 typing_extensions-4.9.0
                       commit: unknown

[dmontagu]

@kikones34 thanks for reporting, I've opened a PR that I believe fixes it — #8666. It would be great if you could confirm whether that resolves your issues.

[dmontagu]

Yep, this was introduced in #7411 and it's obvious that the change I made there was not handling all possible scenarios under which the _DeferAnalysis error I introduced could be raised, but also that #8666 fixes that.

[sydney-runkle]

@kikones34,

Thanks for bringing this to our attention. We'll do a patch release tomorrow including @dmontagu's fix!