Consider special casing zip(*tuples)

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Assessment

Difficulty
5/5
Estimated time
Over a week
Newbie friendliness
32/100
Issue type
Feature
Clarity
Mostly clear
Activity status
Stale
Tech stack
python
Domain
compilers

Research direction

Start by tracing how mypy infers zip(*a) and checks multiple assignment. The issue gives the target example and proposed tuple shape, but no files, tests, or implementation entry points are identified. Done would require a precise contextual type without making zip inference unsound elsewhere, along with coverage for the shown case.

Written by the indexing model from the issue text.

Description

feature priority-1-normal topic-calls topic-plugins

This seems to be a somewhat common zip idiom:

>>> a = [('a', 1), ('b', 2), ('c', 3)]
>>> list(zip(*a))
[('a', 'b', 'c'), (1, 2, 3)]

The inferred type for zip(*a) is Iterator[Tuple[Any, ...]], which causes false negatives in multiple assignments like this:

x, y = zip(*a)

Maybe we could add a plugin feature that can provide more precise types for zip(*a) in a multiple assignment context somehow. For example, the inferred type in the above example could be Tuple[Tuple[str, ...], Tuple[int, ...]] instead of Iterator[...], but only in the context of a multiple assignment since elsewhere the inferred type is not safe to use.

Dominant language
Python
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Merged PRs (30d)
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