2.8.0 breaks mypy

[sparkiegeek]

Our CI pulled in pydantic-settings 2.8.0 and it triggered an internal error in mypy.

We invoke mypy using --install-types --non-interactive and are using mypy 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 "/usr/local/lib/python3.11/site-packages/pydantic/mypy.py", line 184, in _pydantic_model_class_maker_callback
    return transformer.transform()
           ^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/pydantic/mypy.py", line 527, in transform
    self.add_initializer(fields, config, is_settings, is_root_model)
  File "/usr/local/lib/python3.11/site-packages/pydantic/mypy.py", line 927, in add_initializer
    if arg_name.startswith('__') or not arg_name.startswith('_'):
       ^^^^^^^^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'startswith'

Pinning to < 2.8.0 doesn't hit the issue.

[sr-verde]

Yes, I can confirm that issue. We have this problem in one project, too. It shows up when trying to update from 2.7.1 to 2.8.0. Mypy points to this class definition:

class MongoDBURIs(BaseSettings):
    db_one: str = "mongodb://localhost:27017/db1"
    db_two: str = "mongodb://localhost:27017/db2"

[mv1005]

I had the same issue and it apparently was caused by an incompatible combination of pydantic (pulled in pinned to an older version by another package) and pydantic-settings. After upgrading pydantic, the issue no longer showed up.

Here is a sum-up of the versions I tried:

Working (existing setup):

pydantic==2.9.2
pydantic-settings==2.7.1
pydantic_core==2.23.4

BROKEN (pydantic-settings updated only):

pydantic==2.9.2
pydantic-settings==2.8.0
pydantic_core==2.23.4

Working again (everything up-to-date):

pydantic==2.10.6
pydantic-settings==2.8.0
pydantic_core==2.27.2

[hramezani]

Thanks @mv1005 for the update.

Probably we need to pin pydantic to > 2.10

What do you think @Viicos?

[Viicos]

I investigated, indeed an unsafe static type checking error fixed in pydantic/pydantic#10669 (included in 2.10) is the reason of this regression.

@hramezani, we can either bump the minimum required version, but this isn't a runtime incompatibility per se. What do you think?

[hramezani]

@Viicos what about reverting your change in 537f751?

we can include this change again in pydantic-settings 2.9 and bump the minimum version of pydantic

[hramezani]

The fix has been released on pydantic-settings 2.8.1