python / python/mypy

A 'type only' module import should not count as importing a module

Open
#11,503 3 comments 2 reactions 0 assignees View on GitHub

Nobody has claimed this yet.

bug
Dominant language
Python
Stars
20.6k
Forks
3.3k
PR merge metrics
PR metrics pending

Description

given:

my-app/
├─ package/
│  ├─ __init__.py
│  ├─ things.py
├─ entry.py
├─ mod.py

entry.py:

import mod
import package

mod.func(package.things.Thing())

mod.py:

def func(it):
    ...

In typeshed/bundled stubs: mod.pyi

from package.things import Thing

def func(it: Thing) -> None: ...
OR, if this is fake imported in the .py module: mod.py
from __future__ import annotations
from typing import TYPE_CHECKING
if TYPE_CHECKING
    # do a type import so doesn't cause circular imports or slow down loading
    from package.things import Thing

def func(it: Thing) -> None:
    ...

package.things.py

class Thing:
    ...

When mypy --strict entry.py is executed, no error is generated.
When python entry.py is run: AttributeError: module 'package' has no attribute 'things'

Mypy treats the 'type import' as importing package.things, when in reality it isn't. I understand this is consistent with what TYPE_CHECKING should do, but seems really sus if you ask me.

This more seriously affects typeshed, where all imports are fake and mypy will think all fakely imported modules are really imported.

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 behavior with entry.py, mod.py, package/init.py, and package/things.py using the provided mypy --strict and Python commands. Start by tracing how the type-only import in mod.pyi or the TYPE_CHECKING block is treated, then add a regression case showing that it must not make package.things available at runtime.

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
Clearly specified
Newbie friendliness
35/100

Get new issues in your inbox

A short digest of beginner-friendly GitHub issues.