In strict mode, Mypy complains about finding attribute on union. [union-attr]

[amirsoroush]

Bug Report

Despite assigning a correct type, Mypy complains about finding an attribute when using --strict flag.

To Reproduce

var: int | None
var = 20

def fn() -> int:
    return var.bit_length()

print(fn())

Expected Behavior

Is to see "No Issue".

Actual Behavior

main.py:5: error: Item "None" of "Optional[int]" has no attribute "bit_length"  [union-attr]
Found 1 error in 1 file (checked 1 source file)

Your Environment

• Mypy version used: 1.2.0
• Mypy command-line flags: --strict
• Python version used: 3.10.6, 3.11, 3.12

[erictraut]

This is a duplicate of #2608.
is unsafe in the general case. Consider the following minor change to your code, which crashes at runtime.

var: int | None
var = 20

def fn() -> int:
    return var.bit_length()

var = None

print(fn())

[JelleZijlstra]

Agree, mypy is correct here. Even if we change the behavior in #2608, this is a global variable, where it is even harder to reliably narrow the value.

[amirsoroush]

@erictraut Reasonable. Thanks for explaining that case. So here it's better to remove the type hint for that global right?

[erictraut]

Yes, you could remove the type annotation. Another option is to add an assert within the inner scope (assert var is not None) to document your assumption for the type checker.