Question

SWIG seems to be generating incorrect bindings for converting a struct field of type map, resulting in a TypeError trying to set the map field to a python dictionary. Is there an error I am missing? an unsupported use-case? a bug in SWIG?

First the output

Traceback (most recent call last):
  File ".\use_test.py", line 4, in <module>
    struct.data = { 'A':1, 'B':2 }
  File "C:\Users\kmahan\Projects\SwigTest\test.py", line 150, in <lambda>
    __setattr__ = lambda self, name, value: _swig_setattr(self, MyStruct, name, value)
  File "C:\Users\kmahan\Projects\SwigTest\test.py", line 49, in _swig_setattr
    return _swig_setattr_nondynamic(self,class_type,name,value,0)
  File "C:\Users\kmahan\Projects\SwigTest\test.py", line 42, in _swig_setattr_nondynamic
    if method: return method(self,value)
TypeError: in method 'MyStruct_data_set', argument 2 of type 'std::map< std::string,unsigned int,std::less< std::string >,std::allocator< std::pair< std::string const,unsigned int > > > *'

And here is my test case:

test.i

%module test

%include "std_string.i"
%include "std_map.i"

namespace std {
    %template(StringIntMap) map<string, unsigned int>;
}

%{
#include "test.h"
%}

%include "test.h"

test.h

#ifndef _TEST_H_
#define _TEST_H_

#include <string>
#include <map>

struct MyStruct 
{
    std::map<std::string, unsigned int> data;
};

#endif //_TEST_H_

test.cpp

#include "test.h"

run_test.py

import test

struct = test.MyStruct()
struct.data = { 'A':1, 'B':2 }

print struct.data

I build test_wrapper.cpp with swig -python -c++ -o test_wrapper.cpp test.i, compile everything else, and run run_test.py.

As a workaround I can explicitly define a setter instead (
void setData(const map<string, unsigned int>& data)
) which generates different conversion code — it goes through traits_asptr instead of SWIG_ConvertPtr — and seems to work but is not very pythonic!

EDIT

Here is my .i file that pipes gets and sets of the attribute itself to C++ getters and setters. I think this is what Nathan suggested in the comment below his answer.

%module test

%include "std_string.i"
%include "std_map.i"

namespace std {
    %template(StringIntMap) map<string, unsigned int>;
}

struct MyStruct 
{
    const std::map<std::string, unsigned int>& getData() const;
    void setData(const std::map<std::string, unsigned int>&);

    %pythoncode %{
        __swig_getmethods__["data"] = getData
        __swig_setmethods__["data"] = setData
        if _newclass:
            data = _swig_property(getData, setData)
    %}
};

Answer 1

When you’re setting struct.data, it’s expecting a test.StringIntMap, not a python dict.

The easiest thing is for you to do this:

struct.data = test.StringIntMap({ 'A':1, 'B':2 })