Returning a tuple of multipe objects in Python C API

Question

I am writing a native function that will return multiple Python objects

PyObject *V = PyList_New(0);
PyObject *E = PyList_New(0);
PyObject *F = PyList_New(0);

return Py_BuildValue("ooo", V, E, F);

This compiles fine, however, when I call it from a Python program, I get an error:

SystemError: bad format char passed to Py_BuildValue

How can this be done correctly?

EDIT: The following works

PyObject *rslt = PyTuple_New(3);
PyTuple_SetItem(rslt, 0, V);
PyTuple_SetItem(rslt, 1, E);
PyTuple_SetItem(rslt, 2, F);
return rslt;

However, isn't there a shorter way to do this?

You also need to remember to decrement the reference count on V, E and F (Py_DECREF(V);, etc.) after calling Py_BuildValue, since Py_BuildValue increments the reference count of any object used with the O format specifier (see docs.python.org/c-api/arg.html#Py_BuildValue). Otherwise, the lists will never be freed, and you have a memory leak. — jchl
– jchl, Commented Aug 17, 2010 at 8:30
Thanks for the suggestion. I am returning Py_BuildValue("NNN", V, E, F) which does not increment the reference count. — D R
– D R, Commented Aug 17, 2010 at 23:04
I have a question when will V,E,F be free (equivalent Py_DECREF) after return to Python implementation? Thanks! — Chinh Nguyen Huu
– Chinh Nguyen Huu, Commented Feb 16, 2020 at 1:41

Ned Batchelder · Accepted Answer · 2010-08-16 23:29:40Z

13

I think it wants upper-case O? "OOO", not "ooo".

answered Aug 16, 2010 at 23:29

Ned Batchelder

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1 Comment

Maybe you have also to add parenthesis to make python understand that you want to build a tuple : return Py_BuildValue("(OOO)", V, E, F);

maxbachmann · Accepted Answer · 2021-02-06 12:27:04Z

2

As Ned Batcheder noted Py_BuildValue requires Uppercase and paranthesis to create the Tuple

Py_BuildValue("(OOO)", V, E, F);

Another option to achieve the same results is PyTuple_Pack

PyTuple_Pack(3, V, E, F);

answered Feb 6, 2021 at 12:27

maxbachmann

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John Machin · Accepted Answer · 2010-08-17 10:45:36Z

-6

return V, E, F

answered Aug 17, 2010 at 10:45

John Machin

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