AssertionError: trying to find dataclass transform spec

[mishamsk]

Bug Report
mypy crashes on the example below instead of showing an error or not showing anything.

Encountered today while refactoring a pretty big codebase. Made an accidental mistake, hence the @dataclass(frozen=True)(kw_only=True), but the way Mypy fails won't allow me to find the culprit. I couldn't find a way to debug what was causing an error (--pdb option didn't work for me).

To Reproduce

from dataclasses import dataclass


@dataclass(frozen=True)(kw_only=True)
class Foo:
    bar: str

Gist URL: https://gist.github.com/mypy-play/5b29c7aec29cbab55262d61d8c440ef3
Playground URL: https://mypy-play.net/?mypy=latest&python=3.11&gist=5b29c7aec29cbab55262d61d8c440ef3

Expected Behavior
Correctly identified error in the code, or at least no crash

Actual Behavior

mypy mypy_bug.py 
Traceback (most recent call last):
  File "/Users/misha/Dev/python-playground/.venv/bin/mypy", line 8, in <module>
    sys.exit(console_entry())
             ^^^^^^^^^^^^^^^
  File "/Users/misha/Dev/python-playground/.venv/lib/python3.11/site-packages/mypy/__main__.py", line 15, in console_entry
    main()
  File "mypy/main.py", line 94, in main
  File "mypy/main.py", line 173, in run_build
  File "mypy/build.py", line 195, in build
  File "mypy/build.py", line 268, in _build
  File "mypy/build.py", line 2927, in dispatch
  File "mypy/build.py", line 3325, in process_graph
  File "mypy/build.py", line 3420, in process_stale_scc
  File "mypy/semanal_main.py", line 99, in semantic_analysis_for_scc
  File "mypy/semanal_main.py", line 439, in apply_class_plugin_hooks
  File "mypy/semanal_main.py", line 475, in apply_hooks_to_class
  File "mypy/plugins/dataclasses.py", line 930, in dataclass_class_maker_callback
    # contains additional info beyond (and possibly including) the
  File "mypy/plugins/dataclasses.py", line 948, in _get_transform_spec
    _is_kw_only))):
AssertionError: trying to find dataclass transform spec, but reason is neither dataclasses.dataclass nor decorated with typing.dataclass_transform

Your Environment

• Mypy version used: 1.5.1 (also tried most recent from git, same)
• Mypy command-line flags: None
• Mypy configuration options from mypy.ini (and other config files): None
• Python version used: 3.11.5

[sinback]

I see this problem too. I have a python3.10 project which uses a library called marshmallow-dataclass so I bet it's the same root cause as OP. I tried several older mypy releases and found the same problem in every 1.X.0 mypy version after 1.0.0 (so, it was fine in 1.0.0, but not fine from 1.1.1, which pip tells me is the first release after 1.0.0, and onwards)