mypy crashes with pydantic plugin and BasicSettings class

[dimanu-py]

Crash Report

I've enabled pydantic mypy plugin as I've introduced a class in my project that uses BaseSettings class from pydantic.

Now when I run mypy I get the following error: AsertionError: All arguments mus be fully typed. As all fields are typed (see class in how to reproduced section), I don't understand where this error is coming from.

Traceback

src/shared/infra/settings.py:7: 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: 1.15.0
Traceback (most recent call last):
  File "mypy/semanal.py", line 7240, in accept
  File "mypy/nodes.py", line 1177, in accept
  File "mypy/semanal.py", line 1728, in visit_class_def
  File "mypy/semanal.py", line 1944, in analyze_class
  File "mypy/semanal.py", line 1991, in analyze_class_body_common
  File "mypy/semanal.py", line 2076, in apply_class_plugin_hooks
  File "/home/dmartinez/Developer/diego/social-network/.venv/lib/python3.13/site-packages/pydantic/mypy.py", line 159, in _pydantic_model_class_maker_callback
    transformer.transform()
    ~~~~~~~~~~~~~~~~~~~~~^^
  File "/home/dmartinez/Developer/diego/social-network/.venv/lib/python3.13/site-packages/pydantic/mypy.py", line 455, in transform
    self.add_initializer(fields, config, is_settings, is_root_model)
    ~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/dmartinez/Developer/diego/social-network/.venv/lib/python3.13/site-packages/pydantic/mypy.py", line 866, in add_initializer
    add_method(self._api, self._cls, '__init__', args=args, return_type=NoneType())
    ~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/dmartinez/Developer/diego/social-network/.venv/lib/python3.13/site-packages/pydantic/mypy.py", line 1257, in add_method
    assert arg.type_annotation, 'All arguments must be fully typed.'
           ^^^^^^^^^^^^^^^^^^^
AssertionError: All arguments must be fully typed.
src/shared/infra/settings.py:7: : note: use --pdb to drop into pdb

To Reproduce

In my mypy.ini file I've added the following:

plugins = pydantic.mypy

And the class that I created is this one:

class Settings(BaseSettings):
    model_config: ClassVar[SettingsConfigDict] = SettingsConfigDict(env_file=(".env", ".env.prod"), extra="ignore")
    postgres_user: str = Field(alias="POSTGRES_USER")
    postgres_password : str = Field(alias="POSTGRES_PASSWORD")
    postgres_db: str = Field(alias="POSTGRES_DB")
    postgres_host: str = Field(alias="POSTGRES_HOST")
    postgres_port: str = Field(alias="POSTGRES_PORT")

    @property
    def postgres_url(self) -> str:
        return f"postgresql://{self.postgres_user}:{self.postgres_password}@{self.postgres_host}:{self.postgres_port}/{self.postgres_db}"

Your Environment

• Mypy version used: 1.15.0
• Mypy command-line flags: no flags
• Mypy configuration options from mypy.ini (and other config files):

this is my mypy.ini config file

[mypy]
files = src, tests
python_version = 3.13
mypy_path = .
disable_error_code = override,attr-defined
check_untyped_defs = true
disallow_any_explicit = false

# None and Optional handling
no_implicit_optional = true

# Configuring warnings
warn_redundant_casts = true
warn_unused_ignores = false
warn_no_return = true
warn_return_any = true
warn_unreachable = true

# Miscellaneous strictness flags
implicit_reexport = true
strict_equality = true

# Configuring error messages
show_error_context = true
show_column_numbers = true
show_error_codes = true
pretty = true
show_absolute_path = false

disallow_untyped_defs = true

plugins = pydantic.mypy

[mypy-expects.*]
ignore_missing_imports = True
[mypy-doublex.*]
ignore_missing_imports = True
[mypy-src.*]
ignore_missing_imports = True
[mypy-tests.*]
disallow_untyped_defs = False
[mypy-doublex_expects.*]
ignore_missing_imports = True

• Python version used: 3.13
• Operating system and version: Ubuntu 24

[sterliakov]

This sounds like an issue with pydantic plugin, not with mypy itself: the assertion comes from plugin code.

[srulre]

I had a similar issue, but it seems to be working after upgrading to pydantic-settings 2.8.1

[dimanu-py]

I had a similar issue, but it seems to be working after upgrading to pydantic-settings 2.8.1

I'll check it out, thanks for the update!!

[hauntsaninja]

Issues when running with the pydantic plugin are best reported at the pydantic repo