Binder should reason about complex expressions

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Assessment

Difficulty
5/5
Estimated time
Over a week
Newbie friendliness
25/100
Issue type
Feature
Clarity
Mostly clear
Activity status
Stale
Tech stack
python
Domain
compilers, devtools

Research direction

Start by reproducing the reported example with mypy and read the Binder's handling of type narrowing for indexed and chained expressions. The issue's requested behavior is complete when the guard on a[i] narrows the later a[i] expression enough to remove the Optional[str] error, with coverage for the example and the mentioned p[i].arg case.

Written by the indexing model from the issue text.

Description

false-positive feature priority-1-normal topic-type-narrowing

Here's a very simple example:

from typing import *
a = ['x', None]  # type: List[Optional[str]]
for i in range(len(a)):
    if a[i]:
        print(i, '<' + a[i] + '>')

We get an error on the last line:

__tmp__.py:5: error: Unsupported operand types for + ("str" and "Optional[str]")

Of course I can refactor this by using

for i in range(len(a)):
    ai = a[i]
    if ai:
        print(i, '<' + ai + '>')

but that's just busy-work and doesn't necessarily make the code more readable.

(I found this in some mypy test code where the actual expression was p[i].arg so this probably needs to become fairly powerful.)

Dominant language
Python
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Avg merge
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Merged PRs (30d)
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