python / python/mypy

Wrong type inference with "yield from" from an Iterator class

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bug good-second-issue
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Description

Bug Report

When using "yield from" for an iterable instance the type inference if wrong. Normal for-loop iteration over the same iterable does work though.

To Reproduce

from typing import Iterator, Self


class goes_to_11(Iterator[int]):
    def __init__(self):
        self.elevens = iter((11,))

    def __next__(self) -> int:
        return next(self.elevens)

    def __iter__(self) -> Self:
        return self


def bunch_of_11() -> Iterator[tuple[int, ...]]:
    yield from goes_to_11()

    for integer in goes_to_11():
        yield integer


for value in bunch_of_11():
    print(type(value), value)

Expected Behavior

Mypy should complain about both the "yield from" line and the "yield integer" line, but for the former it doesn't.

Actual Behavior

mypy --strict buggy.py
buggy.py:19: error: Incompatible types in "yield" (actual type "int", expected type "tuple[int, ...]")  [misc]
Found 1 error in 1 file (checked 1 source file)

Your Environment

Tested on Linux Python 3.12.3 using mypy wheel mypy-1.11.0+dev.b88fdbd32fe0a45d40531a6504317aa3fd48489e-cp312-cp312-manylinux_2_17_x86_64.manylinux2014_x86_64.whl

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

Start by running the provided buggy.py reproduction with mypy --strict and compare the yield from line with the following for loop. Trace the type inference for the Iterator[int] instance; done means mypy reports the same incompatible-yield error for both paths.

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
42/100

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