mypy error "Unexpected keyword argument" on pydantic dataclass InitVar field

[brianmedigate]

Initial Checks

• I have searched GitHub for a duplicate issue and I'm sure this is something new
• I have searched Google & StackOverflow for a solution and couldn't find anything
• I have read and followed the docs and still think this is a bug
• I am confident that the issue is with pydantic (not my code, or another library in the ecosystem like FastAPI or mypy)

Description

Since upgrading to pydantic 1.10.6, mypy started complaining about a kwarg being passed to the constructor of a dataclass if the field is an InitVar.

I'm guessing it might be related to #5111

$ mypy test.py
test.py:10: error: Unexpected keyword argument "foo" for "MyDataclass"  [call-arg]
Found 1 error in 1 file (checked 1 source file)

Example Code

from pydantic.dataclasses import dataclass
from dataclasses import InitVar

@dataclass
class MyDataclass:
    foo: InitVar[str]
    bar: str

MyDataclass(foo="foo", bar="bar")

Python, Pydantic & OS Version

pydantic version: 1.10.6
            pydantic compiled: True
                 install path: /home/brian/repos/medigator/venv/lib/python3.8/site-packages/pydantic
               python version: 3.8.10 (default, Nov 14 2022, 12:59:47)  [GCC 9.4.0]
                     platform: Linux-5.15.0-67-generic-x86_64-with-glibc2.29
     optional deps. installed: ['dotenv', 'email-validator', 'typing-extensions']

Affected Components

• Compatibility between releases
• Data validation/parsing
• Data serialization - .dict() and .json()
• JSON Schema
• Dataclasses
• Model Config
• Field Types - adding or changing a particular data type
• Function validation decorator
• Generic Models
• Other Model behaviour - construct(), pickling, private attributes, ORM mode
• Plugins and integration with other tools - mypy, FastAPI, python-devtools, Hypothesis, VS Code, PyCharm, etc.

[samuelcolvin]

This won't be related to #2309, rather it will be related to #5111.

@cdce8p @dmontagu any ideas?

[brianmedigate]

whoops, pasted the wrong link, thanks

[dmontagu]

@brianmedigate what is your mypy config? In particular, are you using the pydantic mypy plugin?

[brianmedigate]

yes

[mypy]
follow_imports = silent
ignore_missing_imports = True
warn_unused_ignores = True
check_untyped_defs = True
disallow_untyped_defs = True
strict_equality = True
warn_redundant_casts = True
python_version = 3.8
platform = linux
plugins = pydantic.mypy

[pydantic-mypy]
init_forbid_extra = True
init_typed = True
warn_required_dynamic_aliases = True
warn_untyped_fields = True

[dmontagu]

Okay, considering you are using the mypy plugin I think it should be readily within our power to fix this (and is probably our fault, to be honest 😅). I will try to take a look at this today or tomorrow.

[brianmedigate]

great, thank you!

[cdce8p]

Possible fix: #5162
Until now, the plugin manually applied the dataclass transform to pydantic dataclasses. This isn't necessary anymore with mypy 1.1.1 and the @dataclass_transform decorator as it also applies the same transform.

In fact, doing it twice results in the InitVar issue seen here.

[brianmedigate]

thank you @cdce8p @dmontagu and @samuelcolvin, you're awesome! 🥇