Python Slice / Tuple Access

[hubernikus]

I try to enable custom slicing for a class, hence I create a method as follow (simplified):

.def("__getitem__", [](const PointCloud cloud, nb::slice my_slice){
    auto final = my_slice.compute(10);
    std::cout << "Start is:" << final[0] << "\n";
  })

However, when running this, I get following error:

*.cpp: In lambda function:
*.cpp:50:38: error: no match for ‘operator[]’ (operand types are ‘nanobind::detail::tuple<long int, long int, long
int, long unsigned int>’ and ‘int’)
50 |     std::cout << "start is:" << final[0] << "\n";

I followed the docs for slices here:
https://nanobind.readthedocs.io/en/latest/api_core.html#_CPPv4NK8nanobind5slice7computeE6size_t

And tuple should be accessible using brackets / operator[]:
https://nanobind.readthedocs.io/en/latest/api_core.html#_CPPv4I0_11enable_if_tINSt15is_arithmetic_vI1TEEEENK8nanobind5tupleixEN6detail8accessorI14num_item_tupleEE1T

Any input why this happens?

[wjakob]

You must unpack the tuple withauto [...] = value.

[hubernikus]

When doing something like this:

nb::detail::tuple<int, int> tpl = {0, 1};
auto [aa, bb] = tpl;

I get following error:

 /home/ubuntu/.venv/lib/python3.12/site-packages/nanobind/include/nanobind/nb_tuple.h: In instantiation of ‘nanobind::detail::tuple<T, Ts ...>::tuple(A&&, As&& ...)
[with A = nanobind::detail::tuple<int, int>&; As = {}; T = int; Ts = {int}]’:
*.cpp:52:21:   required from here
/home/ubuntu/.venv/lib/python3.12/site-packages/nanobind/include/nanobind/nb_tuple.h:41:54: error: cannot convert ‘nanobind::detail::tuple<int, int>’ to ‘int’ in
initialization
   41 |         : Base((detail::forward_t<As>) as...), value((detail::forward_t<A>) a) {}
      |                                                      ^~~~~~~~~~~~~~~~~~~~~~~~
      |                                                      |
      |                                                      nanobind::detail::tuple<int, int>
gmake[2]: *** [CMakeFiles/pcl_common_ext.dir/build.make:76: CMakeFiles/pcl_common_ext.dir/src/python_common.cpp.o] Error 1
gmake[1]: *** [CMakeFiles/Makefile2:97: CMakeFiles/pcl_common_ext.dir/all] Error 2

[hpkfft]

I'm not sure whether the above is supposed to work or if it's a bug.
However, either of the following works:

  nb::detail::tuple<int, int> tpl = {3, 17};

  std::cout << tpl.get<0>() << "\n";
  std::cout << tpl.get<1>() << "\n";

  const auto& [aa, bb] = tpl;
  std::cout << aa << "\n";
  std::cout << bb << "\n";

@wjakob, how would you feel about a PR that adds support for std::get?

[hpkfft]

I retract the suggestion to support std::get.

std::get is not a customization point for the standard library; the function template overloads (pair, tuple, array, complex, ...) do not explicitly allow for user-defined overloads. Adding our own overload would be undefined behavior.

[hubernikus]

That did the trick! Thanks a lot.

It would be nice if the simple auto [aa, bb] = tpl; would work, too. Since this is the way to access a std::tuple, too.

And should the documentation be updated? Since the .get() method was not mentioned for tuples?

[hpkfft]

https://nanobind.readthedocs.io/en/latest/api_core.html#_CPPv4N8nanobind5tupleE is a wrapper class for a Python tuple.
It has operator[].

The class nb::detail::tuple is a tiny replacement for the C++ std::tuple. I'm not sure it's documented at all.
It's a detail. :)

[hubernikus]

Ah I see. So there is two different tuple-wrappers, one for python and one for cpp. That explains why the operator[] did not work for the cpp-wrapper, since I was reading the documentation for the python-wrapper.

Skimming through the docs, it seems the nb::detail::tuple is only used as a return of the slice.compute(). Maybe it would be useful to add some explanation on how to access the tuple-items there?

[shubhamjaju03]

This code might help you

.def("__getitem__", [](const PointCloud& cloud, nb::slice my_slice) {
    auto final = my_slice.compute(10);  // (start, stop, step)
    size_t start = std::get<0>(final);
    size_t stop  = std::get<1>(final);
    size_t step  = std::get<2>(final);

    std::cout << "Start is: " << start << "\n";
    std::cout << "Stop is:  " << stop  << "\n";
    std::cout << "Step is:  " << step  << "\n";

    // Return a subset of your PointCloud here, e.g.:
    // return cloud.slice(start, stop, step);
})

[hubernikus]

I think the std::get tuple access only works for std::tuple.
Here I have a nanobind::detail::tuple

[shubhamjaju03]

yesss its correct