Way to Convert Python2 Unicode to std::string with swig

#define SWIG_PYTHON_2_UNICODE is notworking in python2,this is the file .i,PyUnicode_FromString can map std string to unicode

%include <std_pair.i> 
%include <std_vector.i> 
%include <std_string.i> 
%template() std::pair<std::string,int>; 
%template(PairVector) std::vector<std::pair<std::string,int> >; 
%template(StringVector) std::vector<std::string>;

%begin %{
#define SWIG_PYTHON_2_UNICODE
%}

%{ 
#include "example.h" 

%} 
 %typemap(out) std::string %{
$result = PyUnicode_FromString($1.c_str());
%}

%include "example.h" 

How to convert a python2 unicode string to c++ std::string in swig?