CODEWITHSUNDEEP
c++pybind11
Georgia S
Georgia S

Reputation: 622

Pybind11: Passing list of tuples from Python to C++

I'm using pybind11 and I would like to pass a list of tuples (coordinates) from Python to C++. Here is the example C++ code:

parse_coords(py::list coords) {
    std::vector<PointI> coordinates(py::len(coords));
    
    for (size_t i = 1; i < py::len(coords); i++) {
        coordinates[i].x = coords[i][0];
        coordinates[i].z = coords[i][1];
    }
}

Obviously, this does not work. The error I get is cannot convert ‘pybind11::detail::item_accessor’ {aka ‘pybind11::detail::accessor<pybind11::detail::accessor_policies::generic_item>’} to ‘int’ in assignment

How can I pass a list of anything more complicated than an int, which is all the examples I've been able to find?

Upvotes: 1

Views: 4489

Answers (1)

Piotr Barejko
Piotr Barejko

Reputation: 648

You have to use py::cast. If there is no casting function registered for your custom type, or there is a type mismatch, then you are going to get an exception:

Example:

void cast_test(const py::list& l)
{
    for(auto it = l.begin(); it != l.end(); ++it)
    {
        std::cout << it->cast<int>() << std::endl;
    }
}

>>> example.cast_test([1,2,3])
1
2
3
>>> example.cast_test([1,2,"a"])
1
2
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
RuntimeError: Unable to cast Python instance to C++ type (compile in debug mode for details)
>>>

Here you can find more details about built in casts: https://pybind11.readthedocs.io/en/stable/advanced/pycpp/object.html

And here how to add custom type casters: https://pybind11.readthedocs.io/en/stable/advanced/cast/custom.html

Upvotes: 3

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