A SWIG multi-argument typemap apply to functions, but not to the constructor if there's multiple constructors

Question

I'm trying to make a quarter float type in C++ and wrap it to Python using SWIG. This is a work in progress. The problem is %typemap(in) (int dim_count, int* shape) does not apply to the constructor if there's more than one constructor.

If I use just a one constructor it looks like this:

Python 2.7.15rc1 (default, Nov 12 2018, 14:31:15) 
[GCC 7.3.0] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import logchar
>>> logchar.floatTensor(1.0)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "logchar.py", line 263, in __init__
    this = _logchar.new_floatTensor(dim_count)
ValueError: Expecting a list
>>> logchar.floatTensor([1])
<logchar.floatTensor; proxy of <Swig Object of type 'logchar::Tensor< float > *' at 0x7f1fa68da390> >
>>>

But with the multiple constructors, I get a mess like that:

Python 2.7.15rc1 (default, Nov 12 2018, 14:31:15) 
[GCC 7.3.0] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> from logchar import *
>>> a=quarterTensor([1])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "logchar.py", line 230, in __init__
    this = _logchar.new_quarterTensor(*args)
NotImplementedError: Wrong number or type of arguments for overloaded function 'new_quarterTensor'.
  Possible C/C++ prototypes are:
    logchar::Tensor< logchar::quarter >::Tensor(logchar::quarter)
    logchar::Tensor< logchar::quarter >::Tensor(int,int *)
    logchar::Tensor< logchar::quarter >::Tensor(logchar::Tensor< logchar::quarter > const &)

>>> num=quarter(1.0)
>>> num
1.000000
>>> a=quarterTensor(num)
>>> a._reshape([1,1,1])
>>>

In C++:

namespace logchar {
    template <typename T> class Tensor {
        public:
            //Tensor(T scalar);
                                //causes trouble
            Tensor(const int dim_count, int* shape);
            //Tensor(const Tensor<T>& from);
                                //causes trouble
            ~Tensor();
            Tensor<T> reshape(const int dim_count, int* shape) const;
            int get_dim_count() const;
            void get_shape(const int dim_count, int* shape) const;
            std::shared_ptr<T> operator*() const;
            void _reshape(const int dim_count, int* shape);
        private:
            int dim_count;
            int* shape;
            int size_flattened;
            std::shared_ptr<T> buffer;
    } ;

In SWIG:

%module logchar
%{
    #include "logchar.h"
%}

%typemap(in) (const int dim_count, int *shape) {
  if (!PyList_Check($input)) {
    PyErr_SetString(PyExc_ValueError, "Expecting a list");
    SWIG_fail;
  }
  $1 = PyList_Size($input);
  $2 = new int[$1];
  for (int i = 0; i < $1; i++) {
    PyObject *s = PyList_GetItem($input,i);
    if (!PyInt_Check(s)) {
      delete[] $2;
      PyErr_SetString(PyExc_ValueError, "List items must be integers");
      SWIG_fail;
    }
    $2[i] = PyInt_AsLong(s);
  }
 }

%typemap(freearg) int *shape {
  if ($1) {
    delete[] $1;
  }
 }

%include "logchar.h"

%template(quarterTensor) logchar::Tensor<logchar::quarter>;
%template(floatTensor) logchar::Tensor<float>;
%template(doubleTensor) logchar::Tensor<double>;

%include exception.i

%exception {
    try {
        $function
    } catch(const char* e) {
        SWIG_exception(SWIG_RuntimeError, e);
    }
}

%include "carrays.i"
%array_class(int, intArray);

The only relevant link I found. Doesn't have the answer thought. http://swig.10945.n7.nabble.com/multi-argument-typemap-and-default-parameter-in-python-td6721.html

Answer 1

The both previous answers did not solved my problem. But the solution is available in the link I said to be the only relevant. The solution is to use typechecks.

%typecheck(SWIG_TYPECHECK_INT32_ARRAY) (const int dim_count, int *shape) {
  $1 = PyList_Check($input) ? 1 : 0;
}

https://github.com/swig/swig/issues/614 - that is the similar issue.

From SWIG documentation:

To support dynamic dispatch, SWIG first defines a general purpose type hierarchy as follows:

Symbolic Name                   Precedence Value
------------------------------  ------------------
SWIG_TYPECHECK_POINTER           0  
SWIG_TYPECHECK_VOIDPTR           10 
SWIG_TYPECHECK_BOOL              15 
SWIG_TYPECHECK_UINT8             20 
SWIG_TYPECHECK_INT8              25 
SWIG_TYPECHECK_UINT16            30 
SWIG_TYPECHECK_INT16             35 
SWIG_TYPECHECK_UINT32            40 
SWIG_TYPECHECK_INT32             45 
SWIG_TYPECHECK_UINT64            50 
SWIG_TYPECHECK_INT64             55 
SWIG_TYPECHECK_UINT128           60 
SWIG_TYPECHECK_INT128            65 
SWIG_TYPECHECK_INTEGER           70 
SWIG_TYPECHECK_FLOAT             80 
SWIG_TYPECHECK_DOUBLE            90 
SWIG_TYPECHECK_COMPLEX           100 
SWIG_TYPECHECK_UNICHAR           110 
SWIG_TYPECHECK_UNISTRING         120 
SWIG_TYPECHECK_CHAR              130 
SWIG_TYPECHECK_STRING            140 
SWIG_TYPECHECK_BOOL_ARRAY        1015 
SWIG_TYPECHECK_INT8_ARRAY        1025 
SWIG_TYPECHECK_INT16_ARRAY       1035 
SWIG_TYPECHECK_INT32_ARRAY       1045 
SWIG_TYPECHECK_INT64_ARRAY       1055 
SWIG_TYPECHECK_INT128_ARRAY      1065 
SWIG_TYPECHECK_FLOAT_ARRAY       1080 
SWIG_TYPECHECK_DOUBLE_ARRAY      1090 
SWIG_TYPECHECK_CHAR_ARRAY        1130 
SWIG_TYPECHECK_STRING_ARRAY      1140

Answer 2

Here goes a solution

test.i:

%module example
%{
  #include "test.h"
%}

%typemap(in) (const int dim_count, int *shape) {
  int i;
  if (!PyList_Check($input)) {
    PyErr_SetString(PyExc_ValueError, "Expecting a list");
    return NULL;
  }
  $1 = PyList_Size($input);
  $2 = (int *) malloc(($1)*sizeof(int));
  for (i = 0; i < $1; i++) {
    PyObject *s = PyList_GetItem($input,i);
    if (!PyInt_Check(s)) {
      free($2);
      PyErr_SetString(PyExc_ValueError, "List items must be integers");
      return NULL;
    }
    $2[i] = PyInt_AsLong(s);
  }
 }

%typemap(freearg) (const int dim_count, int *shape) {
  if ($2) {
    free($2);
  }
 }

%include "test.h"

test.h

#pragma once

class B {
 public:
  B(const int dim_count, int *shape);
};

test.cpp:

#include <cstdio>
#include "test.h"

B::B(const int dim_count, int *shape) {
  printf("First element: %d\n", shape[0]);
}

setup.py:

from distutils.core import setup, Extension

setup(name="example",
      py_modules=['example'],
      ext_modules=[Extension("_example",
                     ["test.i","test.cpp"],
                     swig_opts=['-c++'],
    extra_compile_args=['--std=c++11']
                  )]
)

Build using:

python setup.py build_ext --inplace