For loop variable scope issue

[Harmos274]

Bug Report

When you create two or more for x in y loop, if the xs are same-named, mypy raise a Incompatible types in assignment error.

To Reproduce

1. Code sample

# file: main.py

from typing import List

def main() -> None:
    vector: List[str] = list()
    matrix: List[List[str]] =  [["0" for i in range(len(vector))] for y in range(len(vector))]

    for line in vector: # first loop
        print(line)

    for line in matrix: # second loop
        print(line)
    
main()

2. Mypy

$> mypy ./main.py

Expected Behavior
The "line" variables exists only in the scope of their for loops, so it shouldn't be an error for mypy.

Actual Behavior

main.py:12: error: Incompatible types in assignment (expression has type "List[str]", variable has type "str")
Found 1 error in 1 file (checked 1 source file)

It seems that mypy consider the two "line" variables as one and raise an error.

My Environment

• Mypy version used: mypy 0.782
• Python version used: Python 3.8.5
• Operating system and version: Fedora 32

[gvanrossum]

This is by design, since for loop variables survive the loop.

Rename one of the variables as a remedy.

[hauntsaninja]

Note you can also get things to type check (without loss of type safety here) by sticking a line: object in there somewhere before either loop.

[lisandrotornado]

@hauntsaninja https://elwynechoing.github.io/yCpUIZIaBnAOErHPe6xjVFuAwSHzJTAuyQb_lHX_KwE/