python / python/mypy

mypy fails to recognise dunder init method override

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bug topic-dataclass-transform
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Python
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Description

Bug Report
In an inheritance scenario with pydantic.BaseModel and generics, mypy 1.2.0 fails to recognise an overridden __init__(),
and wrongly reports an error (see "actual behavior").

To Reproduce

from __future__ import annotations

from typing import Generic, List, Type, TypeVar

from pydantic import BaseModel, PrivateAttr

B_co = TypeVar("B_co", bound=BaseModel, covariant=True)

class Container(Generic[B_co], BaseModel):
    """Container is the base class for any item-type specific containers,
    to allow bundling and processing of aggregates
    """

    _item_class: Type[B_co] = PrivateAttr()
    __root__: List[B_co]

    def __init__(self, *args: B_co) -> None:
        super().__init__(__root__=list(args))

    def __len__(self) -> int:
        return len(self.__root__)

    def __getitem__(self, index: int) -> B_co:
        return self.__root__[index]


class Item(BaseModel):
    name: str

class Items(Container[Item]):
    _item_class = Item

items = Items(Item(name="one"), Item(name="two"))
print(items)

Expected Behavior
The overridden __init__ method in the Container class has the correct signature, and mypy should not flag any issues.

Actual Behavior

ttt/ttt.py:33: error: Too many arguments for "Items"  [call-arg]
ttt/ttt.py:33: error: Too many positional arguments for "Items"  [misc]
ttt/ttt.py:33: error: Argument 1 to "Items" has incompatible type "Item"; expected "List[B_co]"  [arg-type]
Found 3 errors in 1 file (checked 1 source file)

Your Environment

  • Mypy version used: 1.2.0
  • Mypy command-line flags: none
  • Mypy configuration options from mypy.ini (and other config files):
[tool.mypy]
python_version = "3.10"
strict = true
namespace_packages = false
explicit_package_bases = false
plugins = [
  "pydantic.mypy"
]
follow_imports = "silent"
warn_redundant_casts = true
warn_unused_ignores = true
disallow_any_generics = true
check_untyped_defs = true
no_implicit_reexport = true
disallow_untyped_defs = true

[[tool.mypy.overrides]]
module = [
     "html_text.*"
]
ignore_missing_imports = true
ignore_errors = true

[tool.pydantic-mypy]
init_forbid_extra = true
init_typed = true
warn_required_dynamic_aliases = true
warn_untyped_fields = true
  • Python version used: 3.10.10
  • Other versions: pydantic=1.10.7, typing-extensions=4.5.0

Contributor guide

Open the contributing guide

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

Start by reproducing the provided example with mypy 1.2.0 and the shown configuration. Trace how mypy determines the constructor signature for the generic BaseModel subclass and how the pydantic plugin affects it. Done means the example type-checks without the three reported errors while preserving the intended constructor typing.

Written by the indexing model from the issue text.

Assessment

Tech stack
python
Domain
devtools
Issue type
Bug
Difficulty
4/5
Estimated time
3-5 days
Activity status
Stale
Clarity
Mostly clear
Newbie friendliness
35/100

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