Zhang Le wrote:

> Hello, 
>   I'm trying to wrap a function that takes a vector<pair<string, double> >
>   as input and return a list. The original function is
>    vector<double> eval(const vector<pair<string, double> >&);
> 
>    to use boost python, I write a wrapper function:
>    python::list py_eval(python::tuple input) {
>           here I want to use python::extract<std::string>(input[i][0])
>           and python::extract<double>(py_context[i][1])
>           to transform python input [('A', 1.0), ('B', 2.0), ...]
>           into a new vector<pair<string, double> > for calling real
>           eval()
>    }
> 
>    Compilation is ok and I get an error in python code:
>    obj.py_eval([('A', 1.0), ('B', 2.0)])
>    TypeError: No registered converter was able to produce a C++ rvalue
>    of type j from this Python object of type str
> 
>    I change the declaration to python::list input, python::object input
>    and get the same error result.
> 
>    Am I missing anything?
>   
> 

It is hard to tell by your description of the problem alone, some code 
would help. But try this:

py::list py_eval(py::list v)
{
    vector< pair<string, double>  > input;
    int i;
    int len = py::extract<int> (v.attr("__len__")());
    for (i = 0; i < len; ++i) {
        string s = py::extract<string> (v[i][0]);
        double d = py::extract<double> (v[i][1]);
        input.push_back(pair<string, double> (s, d));
    }
    vector<double>  result = eval(input);
    py::list pyresult;
    for (i = 0; i < result.size(); ++i) {
        pyresult.append(result[i]);               
    }
    return pyresult;
}                                 


HTH,
Nicodemus.


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