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Infer more specific type after equality or membership check #6027
Description
from typing import Union
def foo(bar: Union[str, int]):
if bar in ["a", "b", "c"]:
# to a human, bar is clearly a str. mypy still thinks it's a Union[str, int]
return bar.upper()
else:
return bar$ mypy test.py
test.py:5: error: Item "int" of "Union[str, int]" has no attribute "upper"
The same thing happens if the condition is, for example, bar == "a".
- Are you reporting a bug, or opening a feature request?
Feature request
- What is the behavior/output you expect?
Mypy to understand the effect of conditions like bar == "a" or bar in ["a"], like it already does with isinstance checks
Activity
[-]Type is too generic after equality or membership check[/-][+]Not inferring more specific type after equality or membership check[/+][-]Not inferring more specific type after equality or membership check[/-][+]Infer more specific type after equality or membership check[/+]ilevkivskyi commentedon Dec 7, 2018
This is not as safe as it looks because of tricky things like:
We can probably special-case some builtin types, but this looks like relatively low priority.
msullivan commentedon Jan 28, 2020
I think we're probably not going to do this