Strange behaviour of C++ class in Python with SWIG

Question

I have a class in C++ with two methods (a part of a large project). The methods do a very similar job: the first one normalizes a vector and returns it, while the second returns a normalized vector without altering the original one.

vector3.h :

class Vector3
{
public:
  Vector3(double a = 0.0, double b = 0.0, double c = 0.0) 
      : x(a), y(b), z(c) {}
  Vector3(const Vector3& other) : x(other.x), y(other.y), z(other.z) {}
  double length() const { return sqrt(x*x+y*y+z*z); }
  Vector3& normalize()
  {
    float_type n = length();
    n = float_type(1.0) / ( n ? n : 1.0 );
    this->x *= n;
    this->y *= n;
    this->z *= n;
    return *this;
  }
  Vector3 normalized() const
  {
    Vector3 v(*this);
    v.normalize();
    return v;
  }
  std::string toString()
  {
    std::ostringstream strm;
    strm << '(' << x << ", " << y << ", " << z << ')' ;
    return strm.str();
  }
  Vector3 operator+(const Vector3& other) const
  {
    return Vector3(x + other.x, y + other.y, z + other.z);
  }

private:
  double x,y,z;
};

I build Python bindings for this class with SWIG (via cmake).

vector3.i :

%include 
%include 
%include 
%module vectortest
%{
#include "vector3.h"
%}
%include "vector3.h"

CMakeLists.txt :

cmake_minimum_required(VERSION 2.8)

add_executable("vtest" "vtest.cpp")

find_package(SWIG)
include(${SWIG_USE_FILE})

find_package(PythonLibs)
find_package(PythonInterp)

include_directories(${CMAKE_SOURCE_DIR})
include_directories(${PYTHON_INCLUDE_DIRS})

SET(CMAKE_SWIG_FLAGS "")
SET_SOURCE_FILES_PROPERTIES(SOURCE vector3.i PROPERTIES CPLUSPLUS ON)
SWIG_ADD_MODULE(vectortest_python python vector3.i )
SWIG_LINK_LIBRARIES(vectortest_python ${PYTHON_LIBRARIES} ${LIBS})
set_target_properties(_vectortest_python PROPERTIES PREFIX "_" OUTPUT_NAME "vectortest")

execute_process(COMMAND ${PYTHON_EXECUTABLE} -c "from distutils.sysconfig import get_python_lib; print (get_python_lib())" OUTPUT_VARIABLE PYTHON_SITE_PACKAGES OUTPUT_STRIP_TRAILING_WHITESPACE)

install(TARGETS ${SWIG_MODULE_vectortest_python_REAL_NAME} DESTINATION ${PYTHON_SITE_PACKAGES})
install(FILES ${CMAKE_BINARY_DIR}/vectortest.py DESTINATION ${PYTHON_SITE_PACKAGES})

The behavior in C++ is fine.

vtest.cpp :

#include 
#include "vector3.h"

int main()
{
  Vector3 v1(1,0,0);
  Vector3 v2(0,1,0);

  std::cout << (v1+v2).toString() << std::endl;
  std::cout << (v1+v2).normalized().toString() << std::endl;
  std::cout << (v1+v2).normalize().toString() << std::endl;

  return 0;
}

The output:

(1, 1, 0)
(0.707107, 0.707107, 0)
(0.707107, 0.707107, 0)

However, it's behavior in Python is quite strange:

vtest.py :

#!/usr/bin/python3

from vectortest import Vector3

v1 = Vector3(1,0,0)
v2 = Vector3(0,1,0)
print( (v1+v2).toString() )
print( (v1+v2).normalized().toString() )
print( (v1+v2).normalize().toString() )

outputs:

(1, 1, 0)
(0.707107, 0.707107, 0)
(0, 0.707107, 0)

The second method (normalized()) work as expected, however the first one (normalize()) does not. What is causing this? Any suggestions on how to fix this issue?

Strange behaviour of C++ class in Python with SWIG

1 Answers