That's still "using" the resulting value for a comparison. CPython isn't an opti...

That's still "using" the resulting value for a comparison. CPython isn't an optimizing compiler, or it would completely remove the call to upper().

    >>> def up(v):
    ...     v.upper()
    ...
    >>> dis.dis(up)
    2           0 LOAD_FAST                0 (v)
                2 LOAD_METHOD              0 (upper)
                4 CALL_METHOD              0
                6 POP_TOP
                8 LOAD_CONST               0 (None)
                10 RETURN_VALUE

    >>> def up(v):
    ...     if v.upper() == "HelloWorld":
    ...        return True
    ...
    >>> dis.dis(up)
    2           0 LOAD_FAST                0 (v)
                2 LOAD_METHOD              0 (upper)
                4 CALL_METHOD              0
                6 LOAD_CONST               1 ('HelloWorld')
                8 COMPARE_OP               2 (==)
                10 POP_JUMP_IF_FALSE       16

    3          12 LOAD_CONST               2 (True)
                14 RETURN_VALUE
            >>   16 LOAD_CONST               0 (None)
                18 RETURN_VALUE

Notice in the first example, right after CALL_METHOD the return value on the stack is just immediately POP'd away. The parent is saying that when you run `python example.py` CPython should see that the return value is never used and emit a warning. This would only happen because `upper()` was manually marked using the suggested `must_use` annotation.