Unpacking an iterable in a list comprehension leads to the type inferred as list[Any]
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bug
good-second-issue
- Dominant language
- Python
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Description
Bug Report
When a value of a user-defined class with __iter__ is converted into a list using [*value], the type of the list is inferred as list[Any].
To Reproduce
import typing
class Spam:
def __iter__(self, /) -> typing.Iterator[int]:
yield 1
a = Spam()
# list[int]
reveal_type(list(a))
# list[int]
reveal_type([i for i in a])
# list[int]
reveal_type([*(i for i in a)])
# list[int]
reveal_type([*a.__iter__()])
# list[Any] ???
reveal_type([*a])
b, = a
# int
reveal_type(b)
Expected Behavior
The type of [*a] is list[int]
Actual Behavior
example.py:22: note: Revealed type is "builtins.list[Any]"
Your Environment
- Mypy version used: 1.4.1
- Mypy command-line flags:
--strict - Mypy configuration options from
mypy.ini(and other config files): - Python version used:
3.11.4
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 the reproducer in the issue and trace how list displays handle starred expressions and iterable types. Add a regression test for a user-defined iter returning Iterator[int], then verify that [*a] is inferred as list[int] without breaking the other revealed types.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- python
- Domain
- compilers
- Issue type
- Bug
- Difficulty
- 4/5
- Estimated time
- 3-5 days
- Activity status
- Stale
- Clarity
- Mostly clear
- Newbie friendliness
- 45/100