hangs on imports from pydantic v2

[DetachHead]

from pydantic import BaseModel

mypy version: 1.0.0
pydantic version: 2.0.0.dev.0 (installed from github pydantic/pydantic@ef3cc49)

when running mypy with -vvv it gets stuck on:

LOG:  Processing SCC singleton (pydantic_core.core_schema) as inherently stale

[KotlinIsland]

Could this be a super massive black hole union: https://github.com/pydantic/pydantic-core/blob/bfb373c269ae2bfcc32c15e4f1e59f2cee30b2ac/pydantic_core/core_schema.py#L2547?

[JelleZijlstra]

Using the poor man's debugger of running mypy -vvvvv -c 'from pydantic import BaseModel' --show-traceback and Ctrl-C-ing, it seems to get stuck somewhere in is_subtype:

  File "mypy/types.py", line 2330, in accept
  File "mypy/subtypes.py", line 780, in visit_typeddict_type
  File "mypy/subtypes.py", line 233, in is_equivalent
  File "mypy/subtypes.py", line 175, in is_subtype
  File "mypy/subtypes.py", line 335, in _is_subtype
  File "mypy/types.py", line 2673, in accept
  File "mypy/subtypes.py", line 927, in visit_union_type
  File "mypy/subtypes.py", line 388, in _is_subtype
  File "mypy/subtypes.py", line 175, in is_subtype
  File "mypy/subtypes.py", line 335, in _is_subtype
  File "mypy/types.py", line 2673, in accept
  File "mypy/subtypes.py", line 927, in visit_union_type
  File "mypy/subtypes.py", line 388, in _is_subtype
  File "mypy/subtypes.py", line 175, in is_subtype
  File "mypy/subtypes.py", line 300, in _is_subtype
  File "mypy/subtypes.py", line 175, in is_subtype
  File "mypy/subtypes.py", line 300, in _is_subtype
  File "mypy/subtypes.py", line 175, in is_subtype
  File "mypy/subtypes.py", line 335, in _is_subtype
  File "mypy/types.py", line 2330, in accept
  File "mypy/subtypes.py", line 780, in visit_typeddict_type
  File "mypy/subtypes.py", line 233, in is_equivalent
  File "mypy/subtypes.py", line 175, in is_subtype
  File "mypy/subtypes.py", line 335, in _is_subtype
  File "mypy/types.py", line 2673, in accept
  File "mypy/subtypes.py", line 927, in visit_union_type
  File "mypy/subtypes.py", line 388, in _is_subtype
  File "mypy/subtypes.py", line 175, in is_subtype
  File "mypy/subtypes.py", line 300, in _is_subtype
  File "mypy/subtypes.py", line 175, in is_subtype
  File "mypy/subtypes.py", line 335, in _is_subtype
  File "mypy/types.py", line 2330, in accept
  File "mypy/subtypes.py", line 780, in visit_typeddict_type
  File "mypy/subtypes.py", line 240, in is_equivalent
  File "mypy/subtypes.py", line 175, in is_subtype
  File "mypy/subtypes.py", line 335, in _is_subtype
  File "mypy/types.py", line 2673, in accept
  File "mypy/subtypes.py", line 927, in visit_union_type
  File "mypy/subtypes.py", line 388, in _is_subtype
  File "mypy/subtypes.py", line 175, in is_subtype
  File "mypy/subtypes.py", line 300, in _is_subtype
  File "mypy/subtypes.py", line 174, in is_subtype
  File "/Users/jelle/.pyenv/versions/3.11.1/lib/python3.11/contextlib.py", line 284, in helper
    @wraps(func)

It's indeed plausible this has something to do with the huge Union.

[cdce8p]

Could this be a super massive black hole union: https://github.com/pydantic/pydantic-core/blob/bfb373c269ae2bfcc32c15e4f1e59f2cee30b2ac/pydantic_core/core_schema.py#L2547?

Probably. It's also used recursively in some places which doesn't help either.
I added a common base type which seems to resolve this particular issue. pydantic/pydantic-core#387

As for mypy, it seems there is a performance issue with large unions in particular. That might be something to look into.
Had a similar issue a while back but that was with custom Enum types and a large Union type. Resolved it in a similar way though by adding a base class and using that instead. home-assistant-libs/zwave-js-server-python#435

[KotlinIsland]

I had a similar issues here KotlinIsland#399

[cdce8p]

Maybe a first step could be to add a section to the Common issues page. That way users could at least find and know about it.

[samuelcolvin]

This is a duplicate of #14034 I think.