__add__ and __iadd__ are incompatible for list
Open
Nobody has claimed this yet.
bug
false-positive
- Dominant language
- Python
- Stars
- 20.6k
- Forks
- 3.3k
- PR merge metrics
- PR metrics pending
Description
Hi everyone,
The current typeshed for list.__add__ is the following:
class list:
def __add__(self, x: List[_T]) -> List[_T]: ...
def __iadd__(self: _S, x: Iterable[_T]) -> _S: ...
I am trying to change this to:
class list:
def __add__(self, x: List[_S]) -> List[Union[_T, _S]]: ...
# __iadd__ unchanged
Since __add__ creates a copy, it is safe to concatenate lists of different types.
However, I am getting the following mypy error when running typeshed tests:
stdlib/2and3/builtins.pyi:984: error: Signatures of "__iadd__" and "__add__" are incompatible
Is this a bug in mypy? That seems perfectly safe to me.
Thanks.
Contributor guide
First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
- Fork the repository and make your change on a branch.
- Open a pull request that references the issue number.
Research direction
Start with stdlib/2and3/builtins.pyi and reproduce the typeshed test error for the proposed add and iadd signatures. Trace the signature-compatibility check that reports the error, then determine whether the behavior is a mypy bug; done means the compatibility test reflects the intended result.
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
- 35/100