When working with boost::python, it’s quickly noticeable the lack of documentation involved in integrating Python in a C++ application, however using boost::python is a lot easier then using the normal Python/C API so it’s recommended to whomever considers using Python in a C++ application.
Tricky
The downside of it is the lack of examples, and some things not working as expected (perhaps as documented, but I couldn’t find a decent documentation involving Python-in-C++)
So that lack of examples makes the whole ordeal very tedious and error-prone, as with the implementation I’m about to show.
The expected
When programming in C++ trying to access Python variables, one has access to several boost-wrappers to represent most python variable types, one of which is a dictionary (boost::python::dict)
test.py:
d = {
'key': 'value',
'another': 'yeah'
}
That’s the easy stuff, now, accessing it from C++:
test.cpp:
#include <boost/python.hpp>
#include <iostream>
int main()
{
// Initialize Python
Py_Initialize();
// Get the main namespace/module up-and-running
boost::python::object main_module = boost::python::import("__main__");
boost::python::object main_namespace = main_module.attr("__dict__");
// Run the python executable, in this case "test.py"
boost::python::exec_file(boost::python::str("test.py"), main_namespace);
// Get the 'd' variable
// This method could be shortened by using boost::python::object d = main_namespace["d"]
// However, using extract is more general, and could be used in templated functions (hint hint)
boost::python::object d = boost::python::extract<boost::python::object>(main_namespace["d"]);
// Get the dictionary, because just the "object" isn't enough to access
// Note that the previous part can actually be skipped, but it's included for sake of brevity
boost::python::dict d_dict = boost::python::extract<boost::python::dict>(d);
// Now, we're going to try and "print" everything involved
boost::python::list iterkeys = (boost::python::list)d_dict.iterkeys();
for (int i = 0; i < boost::python::len(iterkeys); i++)
{
// Because we know they're strings, we can do this
std::string key = boost::python::extract<std::string>(iterkeys[i]);
std::string value = boost::python::extract<std::string>(d_dict[iterkeys[i]]);
std::cout << "Key: " << key << std::endl;
std::cout << "Value: " << value << std::endl << std::endl;
}
}
Compile using g++ -lboost_python -lpython2.6 -I/usr/include/python2.6 test.cpp
Executing this should give:
12216 cpf@core ~/junk/cpp % ./a.out
Key: another
Value: yeah
Key: key
Value: value
The unexpected
Now, change test.py to represent:
test.py:
d = {
'key': {
'another': 'yeah'
}
}
And once again execute the file, notice the error (terminate called after throwing an instance of ‘boost::python::error_already_set’), this error is the one and only error boost::python has!
To figure out what went wrong, change test.cpp like this:
// Because we know they're strings, we can do this
std::string key = "";
std::string value = "";
try
{
key = boost::python::extract<std::string>(iterkeys[i]);
value = boost::python::extract<std::string>(d_dict[iterkeys[i]]);
} catch (boost::python::error_already_set const &)
{
PyErr_Print();
}
It should give you the error: TypeError: No registered converter was able to produce a C++ rvalue of type std::string from this Python object of type dict
Using boost::python::dict for value all the way fixes this.
Another word
During testing of a program I was writing to include Python in it and read a (lot) of variables, I had the weird occasion in which a dict only had 1 key/value pair, and using iterkeys[i] would crash saying TypeError: No to_python (by-value) converter found for C++ type: boost::python::api::proxy<boost::python::api::item_policies>
The solution is to use iterkeys.pop(0) instead of iterkeys[i]
HOWEVER: BE CAREFUL DOING THIS, THIS WILL CHANGE THE ITERKEYS ON-THE-FLY!
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