On Sun, Apr 9, 2017 at 8:00 AM, Robert Bradshaw wrote:
> On Sun, Apr 2, 2017 at 9:42 AM, Clemens Hofreither
> wrote:
>>
>> Hey all,
>>
>> (I hope this list isn't dead.)
>>
>> I'm reading up on the Cython codebase because I'm trying to implement
>> a small feature (more on that later). There's one thing that really
>> confuses me currently, and that's the distinction between
>> generate_post_assignment_code and generate_disposal_code. From the
>> comments, it appears that exactly one of these should be called, and
>> which one it is depends on whether a reference from the rhs was
>> absorbed into the lhs.
>>
>> So much for the theory, but in practice it seems to work differently.
>> I'm looking at NameNode.generate_assignment_code() and
>> IndexNode.generate_assignment_code(), which should do similar things
>> (generate code for an assignment) except that the lhs has different
>> structure.
>>
>> Yet they treat the cleanup of the rhs completely differently.
>> IndexNode always calls rhs.generate_disposal_code. NameNode almost
>> always calls rhs.generate_post_assignment_code.
>>
>> Can anyone shed light on why this cleanup is handled differently
>> depending on the target of the assignment?
>
>
> It's been a while since I've looked at this, but the best explanation is
> that generate_post_assignment_code is used when the rhs is "moved" into the
> lhs whereas generate_post_assignment_code is what happens for a "copy." This
> is mostly relevant for temps, e.g. when a temp holds a reference to a Python
> object, it can be transferred to the assignee (e.g. with NameNodes) or a new
> reference can be created for the assignee (e.g. with IndexNodes) which
> changes whether the temp lhs needs to be decref'ed.
>
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Thanks for your answer. I understood as much about the difference with
temporaries and copies.
My question was why the RIGHT-hand side is treated differently in
NameNode.generate_assignment_code() and
IndexNode.generate_assignment_code(). My understanding is that
NameNode/IndexNode here refers to the type of the left-hand side. But
I don't see why whether or not the RHS is a temporary should depend on
the type of the LHS. Therefore it seems confusing that the code in
these two methods uses different cleanup for the RHS.
Clemens
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