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error: Right operand of "or" is never evaluated - although it is evaluated at some point #14120
Description
Bug Report
Mypy is giving out error: Right operand of "or" is never evaluated on a statement that is evaluated. It is part of a for loop and it is not evaluated on the first pass of the loop, however it is evaluated on each consecutive pass of the for loop.
I understand why the right side is not evaluated on the first pass, but I feel that it should not error out on this code as it is evaluated at some point. Or at least the error message should be different.
To Reproduce
This is simplified example of the code I'm seeing this error in.
from __future__ import annotations
from typing import Optional
class Role:
def __init__(self, rank: int):
self.rank = rank
def __str__(self):
return f'Role with rank {self.rank}'
def is_more_important_than(self, other_role: Role) -> bool:
return self.rank > other_role.rank
def find_highest_role(some_roles: list[Role]) -> Optional[Role]:
highest_role = None
for role in some_roles:
if not highest_role or role.is_more_important_than(highest_role): # error is here
highest_role = role
return highest_role
some_roles = [
Role(10),
Role(1),
Role(6),
Role(7),
Role(11),
Role(25),
]
print(find_highest_role(some_roles))Expected Behavior
Success.
Actual Behavior
Found 1 error in 1 file (checked 1 source file)
Your Environment
- Mypy version used:
mypy 0.982 (compiled: yes) - Mypy command-line flags:
- Mypy configuration options from
mypy.ini(and other config files):
warn_redundant_casts = true
warn_unreachable = true
warn_unused_configs = true
warn_unused_ignores = true
- Python version used: Python 3.9
Activity
[-]error: Right operand of "or" is never evaluated - although it is[/-][+]error: Right operand of "or" is never evaluated - although it is evaluated at some point[/+]tmke8 commentedon Nov 17, 2022
It works if you define the variable like this:
because otherwise mypy thinks the variable has the type
None. But it's still weird that mypy didn't complain about the assignment within the loop if it thought that the type isNone...pcustic commentedon Nov 17, 2022
Yes, you are correct, that works, thanks! I was sure I tried that but.. 🤔
Yeah, I would also say it should complain about that.
DevilXD commentedon Nov 24, 2022
It seems like MyPy just doesn't want to warn about the inferred type being
Noneat first, and then silently changing it toUnion[Role, None]after the assignment, no matter which options I try to use on the playground. There should be at least a possibility of generating a "need type annotation" warning on the untyped local variable. From the documentation, it seems like--local-partial-typesshould cause a warning like this to be generated, but it doesn't do so in this case, for some reason.@pcustic As a general tip for the future, if you want to avoid getting "weird" errors like this, always check if the variable involved in the error you're getting is properly typed.
A5rocks commentedon Jan 22, 2025
Fixed by #18433