Python C API return more than one value / object without building a tuple

Question

I got the following problem. I have written a C-Extension to Python(2.7 / 3.2) to interface a self written software library. Unfortunately I need to return two values from the function where the last one is optional. In Python I tried

def func(x,y): 
    return x+y, x-y

test = func(13,4)

but test is a tuple. If I write

test1,test2 = func(13,4)

I got both values separated. Is there a possibility to return only one value without unpacking the tuple, i.e. the second(,.. third, ..fourth) value gets neglected?

And if such a solution existst, how does it look for the C-API? Because

return Py_BuildValue("ii",x+y,x-y);

results in a tuple as well.

What version of python are you using? If I run your first snippet I do not get a single value, I get a 2-tuple as expected (and, IMO, is only sane--there are enough issues with not knowing return types in loosely typed languages without the return types changing depending on the call site!) — Phoshi
– Phoshi, Commented Nov 5, 2013 at 12:00
I reformulated the problem a bit more to point out the problem much better. — M.K. aka Grisu
– M.K. aka Grisu, Commented Nov 5, 2013 at 12:55
I see! I presume test, _ = func(...) isn't acceptable? I don't think this is something you're going to be able to do without a little syntactic overhead, unfortunately. — Phoshi
– Phoshi, Commented Nov 5, 2013 at 13:05
Addingg the addition dummy result _ will be an option, but I am interested in a "more elegant" way. — M.K. aka Grisu
– M.K. aka Grisu, Commented Nov 5, 2013 at 14:01

glglgl · Accepted Answer · 2013-11-05 14:55:54Z

2

This question is not special about the C API; this and the Python side behave consistently.

A tuple returned from a function can

be unpacked, if its length is constant: a, _ = f()
in 3.x, be unpacked, even if its length is not constant: a, *_ = f()
be iterated over, but that is not useful here
be indexed: a = f()[0].

answered Nov 5, 2013 at 14:55

glglgl

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