python / python/mypy

Lazy circular import with bottom import cannot determine type if the module name is alphabetically after the base class module

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bug topic-import-cycles
Dominant language
Python
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Description

To Reproduce

mkdir mypyissue
cd mypyissue
touch __init__.py a.py b.py c.py circular.py py.typed

a.py:

import typing_extensions as t
from .b import Base
class A(Base):
    def run(self) -> None:
        t.reveal_type(self._v)

b.py:

import typing as t
class Base:
    def __init__(self, v: t.Optional[str] = None) -> None:
        from .circular import circular
        self._v: t.Final = v or circular.v
from .a import A
from .c import C

c.py:

import typing_extensions as t
from .b import Base
class C(Base):
    def run(self) -> None:
        t.reveal_type(self._v)

circular.py

from .b import Base
class Circular:
    @property
    def v(self) -> str:
        return "value"
circular = Circular()

Expected Behavior

self._v to have the correct type for all children. Or at least consistently fail rather than now where somehow the module name matters.

Actual Behavior

c.py:5: error: Cannot determine type of "_v"  [has-type]
c.py:5: note: Revealed type is "Any"
a.py:5: note: Revealed type is "builtins.str"
Found 1 error in 1 file (checked 5 source files)

Notice that a.py has the correct type but not c.py.

Your Environment

  • Mypy version used: mypy 1.15.0 (compiled: yes)
  • Mypy command-line flags: Nothing
  • Mypy configuration options from mypy.ini (and other config files): Nothing
  • Python version used: 3.9.19

Additional notes

  • Pyright works in this case.
  • The first character of the file name is important. In this case, since Base is in b.py, any module named after b alphabetically (i.e., c, d, e, etc...) will fail to determine the type but not a.py. E.g., if b.py was instead d.py, then both a.py and c.py will have the correct type.
  • The bottom imports and the Base import in circular.py are needed to reproduce this. If removed, this works.

Workaround

Explicitly add the type:

self._v: t.Final[str] = v or circular.v

Contributor guide

Open the contributing guide

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

Reproduce the issue using the listed a.py, b.py, c.py, and circular.py files with mypy 1.15.0, then compare the inferred types for A and C. Trace the circular-import and bottom-import handling responsible for the alphabetical difference; done means both child classes infer _v consistently or fail consistently, with regression coverage for the example.

Written by the indexing model from the issue text.

Assessment

Tech stack
python
Domain
devtools
Issue type
Bug
Difficulty
4/5
Estimated time
3-5 days
Activity status
Stale
Clarity
Mostly clear
Newbie friendliness
45/100

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