Boost::Python Extract -- Accessing Python string in C++

Question

I'm trying to access returned string values from a Python function in C++ using Boost Python.

I'm seeing this error:

TypeError: No registered converter was able to produce a C++ rvalue of type std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > from this Python object of type function

The error makes sense, the Python data type isn't matching the C++ datatype. But I'm not sure how to fix it.

The C++ code is:

    Py_Initialize();
try {
    boost::python::object main = boost::python::import("__main__");

    boost::python::object global(main.attr("__dict__"));

    boost::python::object result = exec_file("/Users/tomconlin/Documents/495_496_Research/Current_Source_Code/Source_Files/test_hello.py", global, global);

    boost::python::object test_hello = global["get_info"];

    std::string message = boost::python::extract<std::string>(test_hello);

    std::string m = message;
    std::cout << message << std::endl;
    return message;
}
catch (boost::python::error_already_set&) {
    PyErr_Print();
}

Py_Finalize();

std::string error_string = "\nAn Error Occured\n";
return error_string;

The Python Code is:

import sys
sys.path.append("/Users/tomconlin/anaconda/lib/python2.7/site-packages")

def get_info():
     version = 2 # API version
     block_io = BlockIo('2c18-7990-8d0d-7511', '7816902248', version)

     credit_card_balance = block_io.get_address_balance(addresses='2Mytv6K7BkiBRuHhnHmVTTc51xyhLRNtDgr')

     #print "Credit Card Balance Type:", type(credit_card_balance)
     address = ""
     for key in credit_card_balance:
         address += str(credit_card_balance[key])

    #print "This is the address:", address
    address = address.split()
    #print address
    print "\n\n"
    return_address = address[12].strip("u',")
    print return_address
    print "\n\n"
    return return_address

def main():
    get_info()

if __name__ == '__main__':
    from block_io import BlockIo
    main()

Answer 1

You probably want to call the function and get the return value as a string, rather than try to convert the function itself into a string.

As such, you may want to use:

boost::python::extract<std::string>(test_hello());

Instead of:

boost::python::extract<std::string>(test_hello);

Here's a generalized example. In hello.py:

def hello_from_python ():
  return 'Hello from Python!'

In example.cc:

#include <iostream>
#include <string>
#include <boost/python.hpp>

namespace py = boost::python;

int main (int argc, char * argv []) {
  // Initialize the Python interpreter.
  Py_Initialize();

  // Add the current directory to the path.
  py::import("sys").attr("path").attr("append")(".");

  // Get the "hello_from_python" function from "hello.py".
  py::object hello_from_python = py::import("hello").attr("hello_from_python");

  // Call the "hello_from_python" and extract the return value as a string.
  std::string return_value = py::extract<std::string>(hello_from_python());

  // Print out the return value.
  std::cout << "Return value: '" << return_value << "'" << std::endl;
}

Compile, then run the program:

$ ./example
Return value: 'Hello from Python!'