PERF101 False positive

[NeilGirdhar]

From the CPython source:

        klasses = [
            Mock, MagicMock, NonCallableMock, NonCallableMagicMock
        ]
        for Klass in list(klasses):  # PERF101 [*] Do not cast an iterable to `list` before iterating over it
            klasses.append(lambda K=Klass: K(spec=Anything))
            klasses.append(lambda K=Klass: K(spec_set=Anything))

The auto-fix rule removes list, which breaks this code.

[zanieb]

What type is klasses? How does this break the code?

[NeilGirdhar]

It's a list. Removing the copy means that the append statements below would modify the list that's being iterated over. I edited in the context.

[zanieb]

I see that makes sense. We can probably special case append calls to the inner item which would then not trigger the rule. Separately, I think this code should use copy(klasses) which is much clearer.

[NeilGirdhar]

I agree with you that copy would have been clearer. (Not my code and I can't change it.)