Module Extension C/CPI Question

Jack Diederich jack at
Wed Aug 10 18:10:15 CEST 2005

On Tue, Aug 09, 2005 at 09:15:17PM -0400, Jeremy Moles wrote:
> When using the C API and writing extension modules, how do you normally
> pass a structure up into the python module? For instance, if I have a
> structure:
> typedef struct Foo {
> 	int x;
> 	int y;
> 	int z;
> 	char xyz[100];
> } Foo;
> Is there an "accepted" way of propagating this upstream? I was thinking
> one of these two:
> 1. Returning a enormous tuple of the struct's members using
> Py_BuildValue("(iiis)", f.x, f.y, f.z, which would later be
> converted to a similar object in Python for clients of my module to use.
> 2. Creating a PyObject and calling PyObject_SetAttrString(O, "x",
> Py_BuildValue("i", f.x)) for each member, finally returning a generic
> PyObject using the "O" flag in BuildValue.

The easiest thing to do is to return a dictionary
  myd = PyDict_New();
  assert(myd != NULL);
  PyDict_SetItemString(myd, "x", PyInt_FromLong((long)mystruct->x));
  PyDict_SetItemString(myd, "y", PyInt_FromLong((long)mystruct->y));
  PyDict_SetItemString(myd, "z", PyInt_FromLong((long)mystruct->z));
  return myd

Creating a full fledged object by hand isn't hard and if you are accessing
a lot of these it is worth it (instead of creating new dictionaries all
the time).  But try this first as a quick and dirty, upgrade to a full
object if it is still too slow.

To see examples of C objects check the python source under Objects/
(warning, SourceForge CVS is currently being upgraded and is flaky)



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