On Sun, Apr 9, 2017 at 8:00 AM, Robert Bradshaw <rober...@gmail.com> wrote: > On Sun, Apr 2, 2017 at 9:42 AM, Clemens Hofreither <chofreit...@gmail.com> > 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. > > _______________________________________________ > cython-devel mailing list > cython-devel@python.org > https://mail.python.org/mailman/listinfo/cython-devel >
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 _______________________________________________ cython-devel mailing list cython-devel@python.org https://mail.python.org/mailman/listinfo/cython-devel
