How can I extract the start, stop and step values from a py::slice object?

[123epsilon]

I have a custom class with a __getitem__ implementation that requires custom behavior for slices. I.e I am overloading the following: __getitem__(py::slice sliceObj), how can I retrieve the start, stop, and step attributes from the py::slice object? I'd like to have them as either a py::none instance or as an int. Using the getattr() function returns me an object, but I don't know how to parse that into the type I want.

[123epsilon]

Solution:

auto parseAttr = [](const py::object &obj) -> OptionalIntValue {
  if(py::isinstance<py::none>(obj)){
    return py::none();
  }
  else if(py::isinstance<py::int_>(obj)){
    return obj.cast<int>();
  }
  else {
    throw std::invalid_argument("Invalid indices);
  }
  };
        
// py::isinstance<py::slice>(sliceObj) == true
parseAttr(getattr(sliceObj, "start"));

[reubenjohn]

Can you explain your answer a bit more?

I am trying to overload the getitem method of MyList. Do you know how to implement it?

py::class_<MyList>(m, "MyList")
          .def("__getitem__", [](const MyList& s, const py::slice& slice) {
            MyList sliced;
            // if ( slice[1] == py::None ) {}
            // sliced.insert(sliced.end(), s.begin() + slice[0], s.begin() + slice[1]);
            return sliced;
        })

[123epsilon]

My answer above is just how one would parse each attribute in the slice object to determine whether it is a None or an integer. Then you'd implement your custom behavior using that function. So for instance in the above example, you might use parseAttr as follows to get the start, stop, and step of the slice object your user passed in

auto start = parseAttr(getattr(sliceObject, "start"));
auto stop = parseAttr(getattr(sliceObject, "stop"));
auto step = parseAttr(getattr(sliceObject, "step"));

and then your implementation will depend heavily on your custom class, but I believe you should return an array based on those indices (being careful about NoneType arguments). So if you have an underlying array of data, then you might use a couple of for loops to grab the correct elements based on parseAttr.