Return int or string from C++ function exposed to Python

Question

Using Boost Python, is it possible for a C++ function exposed to python to return either an integer or a string (or other type) depending on the value of the single argument passed in?

So in Python I want to do this:

from my_module import get_property_value     

# get an integer property value
i = get_property_value("some_int_property")

# get a string 
s = get_property_value("some_string_property")

C++ pseudo code (obviously won't it work like this, but you get the idea)

???? getPropertyValue(const char* propertyName)
{
  Property *p = getProperty(propertyName);
  switch(p->type)
  {
    case INTEGER: return p->as_int();
    case STRING: return p->as_string();
    ...
  }
}


BOOST_PYTHON_MODULE(my_module)
{
  boost::python::def("get_property_value", &getPropertyValue);
}

If it makes any difference, I'm using Boost 1.48 with Python 3.2.

Tanner Sansbury · Accepted Answer · 2013-09-16 16:46:30Z

3

Have the C++ function return a boost::python::object. The object constructor will attempt to converts its argument to the appropriate python type and manages a reference to it. For example, boost::python::object(42) will return a Python object with a Python type of int.

Here is a basic example:

#include <boost/python.hpp>

/// @brief Get value, returning a python object based on the provided type
///        string.
boost::python::object get_value(const std::string& type)
{
  using boost::python::object;
  if      (type == "string") { return object("string 42"); }
  else if (type == "int")    { return object(42);          }
  return object(); // None
}

BOOST_PYTHON_MODULE(example)
{
  namespace python = boost::python;
  python::def("get_value", &get_value);
}

and its usage:

>>> import example
>>> x = example.get_value("string")
>>> x
'string 42'
>>> type(x)
<type 'str'>
>>> x = example.get_value("int")
>>> x
42
>>> type(x)
<type 'int'>
>>> x = example.get_value("")
>>> x
>>> type(x)
<type 'NoneType'>

answered Sep 16, 2013 at 16:46

Tanner Sansbury

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1 Comment

Dan D Over a year ago

tks, boost::python::object saves my day

Sebastian Redl · Accepted Answer · 2013-09-16 14:57:23Z

2

I would suggest you make the C++ function return an object. It will have to do the appropriate conversions internally.

answered Sep 16, 2013 at 14:57

Sebastian Redl

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Comments

Panzercrisis · Accepted Answer · 2013-09-16 15:19:03Z

0

How's this?

string getPropertyValue(cont char* propertyName)
{
    // do some stuff
    if (someCondition)
    {
        return "^" + someInteger;
    }
    else
    {
        return someString; // if someString starts with the character '^', make
                           // it start with "^^" instead.
    }
}

Then in Python, if the return value doesn't start with "^", use it as-is as a string. Otherwise if it starts with "^^", trim one of them off and then use it as a string. Otherwise trim the one "^" off the beginning and use it as an int.

edited Sep 16, 2013 at 15:19

answered Sep 16, 2013 at 15:04

Panzercrisis

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Comments

mousomer · Accepted Answer · 2013-09-16 14:59:28Z

-1

I don't think you can do this in c (or c++). Worse, you're trying to use a function pointer - which means you can't even overload.

What you can do is build a Property Class with methods for property->as_int() and property->as_string();

answered Sep 16, 2013 at 14:59

mousomer

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