How to handle B015 within pytest.raises blocks

[jvavrek]

Often when defining custom objects we end up testing that the __eq__ raises a particular error for unsupported comparisons. In practice, this means we have an unused comparison that triggers the B015 rule—see the MWE below. I wonder if there's a recommended way to get around this (without suppressing B015 altogether), or whether it could say be disabled within a with pytest.raises context.

Minimal working example:

# B015.py
import pytest


class MyClass:
    def __init__(self, x):
        self.x = x

    def __eq__(self, other):
        if not isinstance(other, MyClass):
            raise TypeError(f"Cannot compare with {type(other)}")
        return self.x == other.x


a = MyClass(5)
b = MyClass(5)
c = MyClass(4)

assert a == b
assert a != c
with pytest.raises(TypeError):
    a == a.x

$ flake8 B015.py 
B015.py:21:5: B015 Result of comparison is not used. This line doesn't do anything. Did you intend to prepend it with assert?

[jakkdl]

For cases like this you should just use in-line noqa https://flake8.pycqa.org/en/latest/user/violations.html#in-line-ignoring-errors

But for this specific case I'd personally just change the last line to be assert a == a.x

@cooperlees you can close this issue

[JelleZijlstra]

But for this specific case I'd personally just change the last line to be assert a == a.x

That's not right, the test is asserting that the __eq__ operation raises TypeError.

I think it's reasonable to disable this rule inside pytest.raises blocks (as well as self.assertRaises etc. for unittest).

[jakkdl]

But for this specific case I'd personally just change the last line to be assert a == a.x

That's not right, the test is asserting that the __eq__ operation raises TypeError.

no yeah, the assert would be an intentional no-op. Or something like _ = a == a.x. I guess it could slightly confuse readers though

I think it's reasonable to disable this rule inside pytest.raises blocks (as well as self.assertRaises

👍