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From: Ian Kelly <ian.g.kelly@gmail.com>
Newsgroups: comp.lang.python
Subject: Re: I'm missing something here...
Date: Tue, 12 Jan 2016 09:03:24 -0700
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On Mon, Jan 11, 2016 at 6:04 PM, Skip Montanaro
<skip.montanaro@gmail.com> wrote:
> Sorry, I should have been explicit. prob_dates (the actual argument of the
> call) is a set. As far as I know pylint does no type inference, so pylint
> can't tell if the LHS and RHS of the |= operator are appropriate, nor can
> it tell if it has an update() method.
>
> Before writing, I had more-or-less concluded I had hit a bug, but in my
> experience when I hit something I think is a bug, it's not. It's me.
> Terry's reply convinced me that I had hit something.
>
> Something else just occurred to me. I should have tried disassembling the
> two versions of the function. Here's the output near prob_dates.update()
> call:
>
>  14
>
>    62 LOAD_FAST                3 (prob_dates)
>              65 LOAD_ATTR                6 (update)
>              68 LOAD_GLOBAL              7 (compare_prices)
>              71 LOAD_CONST               3 ('E:%s')
>              74 LOAD_FAST                5 (sym)
>              77 BINARY_MODULO
>              78 LOAD_FAST                0 (cx1)
>              81 LOAD_FAST                1 (cx2)
>              84 LOAD_FAST                2 (cx3)
>              87 CALL_FUNCTION            4
>              90 CALL_FUNCTION            1
>
> Here's how the |= version disassembles in that region:
>
>  20
>
>      62 LOAD_FAST                3 (prob_dates)
>              65 LOAD_GLOBAL              6 (compare_prices)
>              68 LOAD_CONST               3 ('E:%s')
>              71 LOAD_FAST                5 (sym)
>              74 BINARY_MODULO
>              75 LOAD_FAST                0 (cx1)
>              78 LOAD_FAST                1 (cx2)
>              81 LOAD_FAST                2 (cx3)
>              84 CALL_FUNCTION            4
>              87 INPLACE_OR
>              88 STORE_FAST               3 (prob_dates)
>
> I think what's throwing pylint is that last
> STORE_FAST. That tells pylint the argument is ignored.

I may be wrong, but I believe pylint just looks at the AST, not the opcodes.