[Cython] Out of order side effects of argument evaluation in function calls (ticket #654)

Vitja Makarov vitja.makarov at gmail.com
Fri Mar 11 15:04:24 CET 2011

2011/3/11 Stefan Behnel <stefan_ml at behnel.de>:
> Hi,
> ticket 654 describes a code generation problem where the arguments to a cdef
> function are not being evaluated in the order they are written down in the
> code.
> http://trac.cython.org/cython_trac/ticket/654
> This introduces problems when the arguments have side effects or are not
> simple, e.g. they are function calls themselves or are taken from object
> attributes or live in a closure. For example,
>    f(g(a), a.x, h(a))
> will produce different results depending on the evaluation order if g or h
> change the value of a.x.
> However, apparently, the order of evaluation is only guaranteed by Python,
> not by C. Now, the question is: what are the right semantics for Cython
> here: follow Python or C?

+1 for Python, Cython is mostly Python than C, btw I don't think that
it makes much sense.

> Personally, I think it would be nice to keep up Python's semantics, but when
> I implemented this I broke quite some code in Sage (you may have noticed
> that the sage-build project in Hudson has been red for a while). There are
> things in C and especially in C++ that cannot be easily copied into a
> temporary variable in order to make sure they are evaluated before the
> following arguments. This is not a problem for Python function calls where
> all arguments end up being copied (and often converted) anyway. It is a
> problem for C function calls, though.
> What do you think about this?

>    f(g(a), a.x, h(a))

Why could not this be translated into:

tmp1 = g(a)
tmp2 = a.x
tmp3 = h(a)

f(tmp1, tmp2, tmp3)


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