python/mypy

Mypy doesn't recognize that truth value testing for sequences like `and my_list` result in a boolean type

Closed this issue · 2 comments

Yazan-Sharaya commented

Bug Report

Mypy fails to recognize that truth value testing results in boolean for sequences.

To Reproduce

my_list = [1]
should_update: bool = True and my_list

Actual Behavior

Mypy thinks the type of the new variable is whatever the type of the sequence is, list[int] in this case.
error: Incompatible types in assignment (expression has type "list[int]", variable has type "bool") [assignment]

Your Environment

  • Mypy version used: 1.12.0
  • Mypy command-line flags: None
  • Mypy configuration options from mypy.ini (and other config files): None
  • Python version used: 3.12.7
JelleZijlstra commented

Mypy is correct here. The and operator evaluates to its right-hand side if the left-hand side is truthy. You can try it in the REPL:

>>> my_list = [1]
... should_update: bool = True and my_list
... 
>>> should_update
[1]
Yazan-Sharaya commented

You are right, I don't know how I didn't catch that.
Thanks for the quick response!