mypy rejects unpacking into same variable

[PeterJCLaw]

Bug Report

I believe the following code should be fine, however mypy rejects the case where we're unpacking into the variable which was expanded:

foo: tuple[int, ...] = ()

# Ok
bar, *quox = foo

# Incompatible types in assignment (expression has type "List[int]", variable has type "Tuple[int, ...]")  [assignment]
bar, *foo = foo

There are a number of similar issues reported, the closest being #9706, however I believe this is distinct as it reproduces when the generic type is int (though my original code was using str). This suggests a different root cause.

Your Environment

• Mypy version used: 1.0.1
• Mypy command-line flags: none
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: 3.10

[sterliakov]

I'd say it's not a bug: you really have foo defined as a tuple and assign a list to it later, why should this be allowed? This code passes with --allow-redefinition, as expected.

[PeterJCLaw]

Oh, huh. For some reason I thought that *foo would capture the values into a tuple not a list.

Happy to close, though I wonder if the error could be clearer about this?
The error position is on the RHS expression, not the LHS assignment, yet in this case the RHS expression has no control over the error and it's the LHS which will need updating to be correct.

[JelleZijlstra]

Agree that the error could be improved here. The wording indicates the RHS is a list, but it's not.