Mypy crashes on abstract class with generic type parameter

[bmmcq]

Crash Report

crash at the first line of :

class JobWeaver[T](abc.ABC):
    @abc.abstractmethod
    def data_type(self) -> type[T] | None: 
        ...
    @abc.abstractmethod
    def map[OT](self, args) -> "JobWeaver[OT]"
        ...

It only defines a totally abstract class with a generic type parameter.

Traceback

src/nagas_api/weaver.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: 1.18.1
Traceback (most recent call last):
  File "mypy/semanal.py", line 7350, in accept
  File "mypy/nodes.py", line 1440, in accept
  File "mypy/semanal.py", line 1775, in visit_class_def
  File "mypy/semanal.py", line 1916, in analyze_class
  File "mypy/semanal.py", line 2222, in clean_up_bases_and_infer_type_variables
AssertionError: 
src/nagas_api/weaver.py:7: : note: use --pdb to drop into pdb

Your Environment

• Mypy version used: 1.18.1

• Mypy command-line flags:
  uv run --active --group test mypy ./src --show-traceback --config-file ../pyproject.toml

• Mypy configuration options from mypy.ini (and other config files):

[tool.mypy]
ignore_missing_imports = true
check_untyped_defs = true
follow_imports = "silent"
warn_return_any = true
warn_unused_configs = true
disallow_untyped_defs = true
exclude = [
    '^test\.',
    'tests/'
]

• Python version used: 3.12

[sterliakov]

Okay, but this doesn't seem to cause a crash? Neither your original snippet (SyntaxError, normally reported) nor the fixed one (with the missing semicolon restored and import abc added). Could you try to provide a working repro case?

https://mypy-play.net/?mypy=1.18.1&python=3.13&flags=ignore-missing-imports%2Cdisallow-untyped-defs%2Cwarn-return-any&gist=e85b22f47263da6fff29585120eed653

[bmmcq]

During my attempts to reproduce this issue, I identified that the crash was caused by an import statement. When a "xxx" module uses __all__ statement to define the contents that can be included by import *, the issue is triggered when using from xxx import *. However, Mypy runs normally when importing explicitly via from xxx import (a, b, c, ...). While the exact root cause of the crash remains unclear, I have already identified a viable solution.

[sterliakov]

Oh, the star import probably brings another T into your namespace, then duplicate of #19526 or #18507. Could you please confirm that that your xxx module defines a symbol named T (either a type variable or something similar; may be imported transitively)?

[bmmcq]

Ok.

In the first Python file(fileA), several classes with type parameters are defined, and all classes defined in this file are included in the __all__ list :

# some imports
import xxx

class OperatorA[I, O](abc.ABC):
    ...

class OperatorB[T](abc.ABC):
    ...

__all__ = ["OperatorA", "OperatorB"]

Since we use Python 3.12's generic syntax, no symbols or type variables are declared using TypeVar.

The second Python file imports the first file，if it is imported in this way:

import abc
from .fileA import *

class JobWeaver[T](abc.ABC): # crash at here
    @abc.abstractmethod
    def data_type(self) -> type[T] | None: 
        ...
    @abc.abstractmethod
    def map[OT](self, args) -> "JobWeaver[OT]"
        ...

mypy crashed as described above, but if import fileA like this:

import abc
from .fileA import (OperatorA, OperatorB)

class JobWeaver[T](abc.ABC): # crash at this line
    @abc.abstractmethod
    def data_type(self) -> type[T] | None: 
        ...
    @abc.abstractmethod
    def map[OT](self, args) -> "JobWeaver[OT]"
        ...

mypy works well.

I think the second way also import some classes with type parameter "T" , but it works.

[bmmcq]

And even if I change the generic 'T' to another name， like 'Z':

import abc
from .fileA import *

class JobWeaver[Z](abc.ABC): # crash at here
    @abc.abstractmethod
    def data_type(self) -> type[Z] | None: 
        ...
    @abc.abstractmethod
    def map[OT](self, args) -> "JobWeaver[OT]"
        ...

it still crashed at same place.

[sterliakov]

Hm, this sounds more concerning, but I still can't reproduce.

$ tree
tree
.
├── foo                                                                                                                                                                                                           
│   ├── a.py                                                                                                                                                                                                      
│   ├── b.py                                                                                                                                                                                                      
│   └── __init__.py                                                                                                                                                                                               
└── pyproject.toml                                                                                                                                                                                                
                                                                                                                                                                                                                  
2 directories, 4 files 
$ cat pyproject.toml
[tool.mypy]
ignore_missing_imports = true
check_untyped_defs = true
follow_imports = "silent"
warn_return_any = true
warn_unused_configs = true
disallow_untyped_defs = true
exclude = [
    '^test\.',
    'tests/'
]
$ cat foo/a.py
# some imports
import xxx
import abc

class OperatorA[I, O](abc.ABC):
    ...

class OperatorB[T](abc.ABC):
    ...

__all__ = ["OperatorA", "OperatorB"]
$ cat foo/b.py
import abc
from .a import *

class JobWeaver[T](abc.ABC): # crash at here
    @abc.abstractmethod
    def data_type(self) -> type[T] | None: 
        ...
    @abc.abstractmethod
    def map[OT](self, args) -> "JobWeaver[OT]":
        ...
$ mypy foo --config-file pyproject.toml
foo/b.py:9: error: Function is missing a type annotation for one or more arguments  [no-untyped-def]
Found 1 error in 1 file (checked 3 source files)