AssertionError: Must not defer during final iteration

[violet4]

Crash Report

this could be related to #15927, but that ticket is related to python 3.10; mine is specific to python 3.13's new TypeVar(..., default=...) argument (the default argument is why i chose python 3.13 for my project 😃).

the minimal example below in the appropriate section uses TypeVar and Generic and produces a crash. i saw the traceback requesting that i try again on the main branch, pulled the main branch and tried again, and the traceback occurred again. so here it is!

Specifically, to get the mypy main branch, i did git clone, then from inside my project i did poetry add ../../mypy

Traceback

$ poetry run mypy --strict --show-traceback craftgrid/game_node.py 
craftgrid/game_node.py:4: 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: 1.12.0+dev.fe15ee69b9225f808f8ed735671b73c31ae1bed8
Traceback (most recent call last):
  File "/home/violet/git/violet/games/craftgrid/py/.venv/bin/mypy", line 8, in <module>
    sys.exit(console_entry())
  File "/home/violet/git/violet/games/craftgrid/py/.venv/lib/python3.13/site-packages/mypy/__main__.py", line 15, in console_entry
    main()
  File "/home/violet/git/violet/games/craftgrid/py/.venv/lib/python3.13/site-packages/mypy/main.py", line 102, in main
    res, messages, blockers = run_build(sources, options, fscache, t0, stdout, stderr)
  File "/home/violet/git/violet/games/craftgrid/py/.venv/lib/python3.13/site-packages/mypy/main.py", line 186, in run_build
    res = build.build(sources, options, None, flush_errors, fscache, stdout, stderr)
  File "/home/violet/git/violet/games/craftgrid/py/.venv/lib/python3.13/site-packages/mypy/build.py", line 193, in build
    result = _build(
  File "/home/violet/git/violet/games/craftgrid/py/.venv/lib/python3.13/site-packages/mypy/build.py", line 268, in _build
    graph = dispatch(sources, manager, stdout)
  File "/home/violet/git/violet/games/craftgrid/py/.venv/lib/python3.13/site-packages/mypy/build.py", line 2950, in dispatch
    process_graph(graph, manager)
  File "/home/violet/git/violet/games/craftgrid/py/.venv/lib/python3.13/site-packages/mypy/build.py", line 3348, in process_graph
    process_stale_scc(graph, scc, manager)
  File "/home/violet/git/violet/games/craftgrid/py/.venv/lib/python3.13/site-packages/mypy/build.py", line 3443, in process_stale_scc
    mypy.semanal_main.semantic_analysis_for_scc(graph, scc, manager.errors)
  File "/home/violet/git/violet/games/craftgrid/py/.venv/lib/python3.13/site-packages/mypy/semanal_main.py", line 93, in semantic_analysis_for_scc
    process_top_levels(graph, scc, patches)
  File "/home/violet/git/violet/games/craftgrid/py/.venv/lib/python3.13/site-packages/mypy/semanal_main.py", line 220, in process_top_levels
    deferred, incomplete, progress = semantic_analyze_target(
  File "/home/violet/git/violet/games/craftgrid/py/.venv/lib/python3.13/site-packages/mypy/semanal_main.py", line 351, in semantic_analyze_target
    analyzer.refresh_partial(
  File "/home/violet/git/violet/games/craftgrid/py/.venv/lib/python3.13/site-packages/mypy/semanal.py", line 619, in refresh_partial
    self.refresh_top_level(node)
  File "/home/violet/git/violet/games/craftgrid/py/.venv/lib/python3.13/site-packages/mypy/semanal.py", line 630, in refresh_top_level
    self.accept(d)
  File "/home/violet/git/violet/games/craftgrid/py/.venv/lib/python3.13/site-packages/mypy/semanal.py", line 7087, in accept
    node.accept(self)
    ~~~~~~~~~~~^^^^^^
  File "/home/violet/git/violet/games/craftgrid/py/.venv/lib/python3.13/site-packages/mypy/nodes.py", line 1183, in accept
    return visitor.visit_class_def(self)
           ~~~~~~~~~~~~~~~~~~~~~~~^^^^^^
  File "/home/violet/git/violet/games/craftgrid/py/.venv/lib/python3.13/site-packages/mypy/semanal.py", line 1700, in visit_class_def
    self.analyze_class(defn)
    ~~~~~~~~~~~~~~~~~~^^^^^^
  File "/home/violet/git/violet/games/craftgrid/py/.venv/lib/python3.13/site-packages/mypy/semanal.py", line 1828, in analyze_class
    self.defer()
    ~~~~~~~~~~^^
  File "/home/violet/git/violet/games/craftgrid/py/.venv/lib/python3.13/site-packages/mypy/semanal.py", line 6742, in defer
    assert not self.final_iteration, "Must not defer during final iteration"
           ^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError: Must not defer during final iteration
craftgrid/game_node.py:4: : note: use --pdb to drop into pdb

To Reproduce

from typing import TypeVar, Generic


class GameNode(Generic['GameNodeType']):
    pass


GameNodeType = TypeVar('GameNodeType', bound=GameNode, default=GameNode)

Your Environment

• Mypy version used: newest from main branch as of right now, 1.12.0+dev.fe15ee69b9225f808f8ed735671b73c31ae1bed8
• Mypy command-line flags: poetry run mypy --strict --show-traceback craftgrid/game_node.py
• Mypy configuration options from mypy.ini (and other config files):
• Python version used: 3.13.0rc1
• Operating system and version: 6.6.41-gentoo-dist

[wyattscarpenter]

I would like to emphasize, in case it wasn't clear from OP, that this doesn't use any plugins (unlike every other close-to-duplicate of this issue, which seems to use pydantic or django-stubs). The crash is native. This replicates on master with no additional configuration at time of writing:

Gist URL: https://gist.github.com/mypy-play/eed8c069d307071718ae2692366f2136
Playground URL: https://mypy-play.net/?mypy=master&python=3.14&gist=eed8c069d307071718ae2692366f2136

Failed (exit code: 2) (8010 ms)

Traceback (most recent call last):
  File "<frozen runpy>", line 198, in _run_module_as_main
  File "<frozen runpy>", line 88, in _run_code
  File "/layers/google.python.pip/pip/lib/python3.12/site-packages/mypy/__main__.py", line 37, in <module>
    console_entry()
  File "/layers/google.python.pip/pip/lib/python3.12/site-packages/mypy/__main__.py", line 15, in console_entry
    main()
  File "/layers/google.python.pip/pip/lib/python3.12/site-packages/mypy/main.py", line 127, in main
    res, messages, blockers = run_build(sources, options, fscache, t0, stdout, stderr)
  File "/layers/google.python.pip/pip/lib/python3.12/site-packages/mypy/main.py", line 211, in run_build
    res = build.build(sources, options, None, flush_errors, fscache, stdout, stderr)
  File "/layers/google.python.pip/pip/lib/python3.12/site-packages/mypy/build.py", line 196, in build
    result = _build(
  File "/layers/google.python.pip/pip/lib/python3.12/site-packages/mypy/build.py", line 272, in _build
    graph = dispatch(sources, manager, stdout)
  File "/layers/google.python.pip/pip/lib/python3.12/site-packages/mypy/build.py", line 2946, in dispatch
    process_graph(graph, manager)
  File "/layers/google.python.pip/pip/lib/python3.12/site-packages/mypy/build.py", line 3346, in process_graph
    process_stale_scc(graph, scc, manager)
  File "/layers/google.python.pip/pip/lib/python3.12/site-packages/mypy/build.py", line 3441, in process_stale_scc
    mypy.semanal_main.semantic_analysis_for_scc(graph, scc, manager.errors)
  File "/layers/google.python.pip/pip/lib/python3.12/site-packages/mypy/semanal_main.py", line 95, in semantic_analysis_for_scc
    process_top_levels(graph, scc, patches)
  File "/layers/google.python.pip/pip/lib/python3.12/site-packages/mypy/semanal_main.py", line 222, in process_top_levels
    deferred, incomplete, progress = semantic_analyze_target(
  File "/layers/google.python.pip/pip/lib/python3.12/site-packages/mypy/semanal_main.py", line 399, in semantic_analyze_target
    analyzer.refresh_partial(
  File "/layers/google.python.pip/pip/lib/python3.12/site-packages/mypy/semanal.py", line 658, in refresh_partial
    self.refresh_top_level(node)
  File "/layers/google.python.pip/pip/lib/python3.12/site-packages/mypy/semanal.py", line 676, in refresh_top_level
    self.accept(d)
  File "/layers/google.python.pip/pip/lib/python3.12/site-packages/mypy/semanal.py", line 7371, in accept
    node.accept(self)
  File "/layers/google.python.pip/pip/lib/python3.12/site-packages/mypy/nodes.py", line 1445, in accept
    return visitor.visit_class_def(self)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/layers/google.python.pip/pip/lib/python3.12/site-packages/mypy/semanal.py", line 1776, in visit_class_def
    self.analyze_class(defn)
  File "/layers/google.python.pip/pip/lib/python3.12/site-packages/mypy/semanal.py", line 1929, in analyze_class
    self.defer()
  File "/layers/google.python.pip/pip/lib/python3.12/site-packages/mypy/semanal.py", line 7017, in defer
    assert not self.final_iteration, "Must not defer during final iteration"
           ^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError: Must not defer during final iteration

main.py:2: 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: 1.19.0+dev.feeb3f00a63d31cf5c7369885c4dd4a294133f59
main.py:2: : note: use --pdb to drop into pdb