SWIG/python array inside structure

Question

I've got a structure defined inside header.h that looks like :

typedef struct {
....
    int      icntl[40];
    double   cntl[15];
    int      *irn, *jcn;
....

When I init an object with this structure, I have access to integers/doubles but not arrays.

>> st.icntl
<Swig Object of type 'int *' at 0x103ce37e0>
>> st.icntl[0]
Traceback (most recent call last):
  File "test_mumps.py", line 19, in <module>
    print s.icntl[0]
TypeError: 'SwigPyObject' object is not subscriptable

How to have access to the values in read/write?

Answer 1

The easiest way to do this is to wrap your arrays inside a struct, which can then provide extra methods to meet the "subscriptable" requirements.

I've put together a small example. It assumes you're using C++, but the equivalent C version is fairly trivial to construct from this, it just requires a bit of repetition.

First up, the C++ header that has the struct we want to wrap and a template that we use for wrapping fixed size arrays:

template <typename Type, size_t N>
struct wrapped_array {
  Type data[N];
};

typedef struct {
    wrapped_array<int, 40> icntl;
    wrapped_array<double, 15> cntl;
    int      *irn, *jcn;
} Test;

Our corresponding SWIG interface then looks something like:

%module test

%{
#include "test.h"
#include <exception>
%}

%include "test.h"
%include "std_except.i"

%extend wrapped_array {
  inline size_t __len__() const { return N; }

  inline const Type& __getitem__(size_t i) const throw(std::out_of_range) {
    if (i >= N || i < 0)
      throw std::out_of_range("out of bounds access");
    return self->data[i];
  }

  inline void __setitem__(size_t i, const Type& v) throw(std::out_of_range) {
    if (i >= N || i < 0)
      throw std::out_of_range("out of bounds access");
    self->data[i] = v;
  }
}

%template (intArray40) wrapped_array<int, 40>;
%template (doubleArray15) wrapped_array<double, 15>;

The trick there is that we've used %extend to supply __getitem__ which is what Python uses for subscript reads and __setitem__ for the writes. (We could also have supplied a __iter__ to make the type iteratable). We also gave the specific wraped_arrays we want to use unique names to make SWIG wrap them in the output.

With the supplied interface we can now do:

>>> import test
>>> foo = test.Test()
>>> foo.icntl[30] = -654321
>>> print foo.icntl[30]
-654321
>>> print foo.icntl[40]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "test.py", line 108, in __getitem__
    def __getitem__(self, *args): return _test.intArray40___getitem__(self, *args)
IndexError: out of bounds access

You might also find this approach useful/interesting as an alternative.