How to use float ** in Python with Swig?

Question

Hi there,

I am writing swig bindings for some c functions. One of these functions takes a float*. I am already using cpointer.i for the normal pointers and looked into carrays.i, but I did not find a way to declare a float*. What do you recommend?

interface file:

extern int read_data(const char *file,int *n_,int *m_,float **data_,int **classes_);

Answer 1

This answer is a repost of one to a related question Framester posted about using ctypes instead of swig. I've included it here in case any web-searches turn up a link to his original question.

I've used ctypes for several projects now and have been quite happy with the results. I don't think I've personally needed a pointer-to-pointer wrapper yet but, in theory, you should be able to do the following:

from ctypes import * 

your_dll = cdll.LoadLibrary("your_dll.dll") 

PFloat = POINTER(c_float) 
PInt   = POINTER(c_int) 

p_data    = PFloat() 
p_classes = PInt() 
buff      = create_string_buffer(1024) 
n1        = c_int( 0 ) 
n2        = c_int( 0 ) 

ret = your_dll.read_data( buff, byref(n1), byref(n2), byref(p_data), byref(p_classes) ) 

print('Data:    ', p_data.contents) 
print('Classes: ', p_classes.contents)