Generic Queryset INTERNALERROR

[pelme]

Bug report

Starting from django-stubs 5.0.3, the following example crashes with an internal mypy error:

from django.db import models
import typing as t

TMyBase = t.TypeVar("TMyBase", bound="MyBase")

class MyBase(models.Model):
	class Meta:
		abstract = True


class MyQuerySet(models.QuerySet[TMyBase]):
	pass

Output:

❯ pip freeze
asgiref==3.8.1
Django==5.0.4
django-stubs==5.0.3
django-stubs-ext==5.1.1
mypy==1.11.2
mypy-extensions==1.0.0
sqlparse==0.5.3
types-PyYAML==6.0.12.20241230
typing_extensions==4.12.2

❯ mypy --no-incremental --show-traceback .
./tstapp/models.py:11: 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: 1.11.2
Traceback (most recent call last):
  File "mypy/semanal.py", line 7087, in accept
  File "mypy/nodes.py", line 819, in accept
  File "mypy/semanal.py", line 874, in visit_func_def
  File "mypy/semanal.py", line 919, in analyze_func_def
  File "mypy/semanal.py", line 6742, in defer
AssertionError: Must not defer during final iteration
./tstapp/models.py:11: : note: use --pdb to drop into pdb

How is that should be

Running the same example with django-stubs 5.0.2:

❯ pip freeze
asgiref==3.8.1
Django==5.0.4
django-stubs==5.0.2
django-stubs-ext==5.1.1
mypy==1.10.1
mypy-extensions==1.0.0
sqlparse==0.5.3
types-PyYAML==6.0.12.20241230
typing_extensions==4.12.2

❯ mypy --show-traceback .
Success: no issues found in 13 source files

Example project

You can reproduce this by downloading this project (contains the model definition above) and running it locally:
tst.zip

Install django-stubs 5.0.2 without the problem:

pip install 'django-stubs[compatible-mypy]==5.0.2'

Install django-stubs 5.0.3 with the problem

pip install 'django-stubs[compatible-mypy]==5.0.2'

Workaround

The crash can be worked around by putting TMyBase after the MyBase definition.

System information

• OS:
• python version: 3.12.7
• django version: I tested with 5.0.4 and 5.1.4 with the same result

[jamesbeith]

I was going to post a similar report. Using Python 3.13 and...

mypy==1.14.1
django==5.1.5
django-stubs==5.1.2

I was updating a QuerySet to use PEP 695 – Type Parameter Syntax, for example.

class MyQuerySet[T: "MyModel"](models.QuerySet[T]):
    pass

That worked fine until I tried to update the QuerySet used for my custom user model which subclassed Django's AbstractBaseUser, then I was getting a similar AssertionError: Must not defer during final iteration error.

I managed to resolve that error by including a default as per PEP 696 – Type Defaults for Type Parameters, so.

class MyQuerySet[T: "MyModel" = "MyModel"](models.QuerySet[T]):
    pass

@pelme I wonder, do you still get the error when you include a default for your TypeVar, for example.

TMyBase = t.TypeVar("TMyBase", bound="MyBase", default="MyBase")

Note, you'll need to be using Python 3.13 to set a default, or using the backport by importing TypeVar from the typing-extensions package.

I believe #2461 is related too.

[ngnpope]

Bisected to 8eeed9b.