Add helpers to array that return the size and strides as a std::span

[dbs4261]

Description

Add convenience functions to pybind11::array to return the shape and strides as a std::span. These functions are hidden with macros unless PYBIND11_CPP20 is defined and the include has been found. By returning an iterable container, a new array could be created more simply with array(other.dtype(), other.size_span(), other.strides_span()).

Suggested changelog entry:

Add convenience functions to pybind11::array to return the shape and strides as a std::span.

[rwgk]

My prompt for Cursor was merely:

Unit tests for the two new member functions are missing, but appart from that the PR seems fine. However, I'm not familiar with std::span. Could you please review carefully?

Next thing I know, it had generated the missing unit tests.

This is the full response:

  Implementation review

  The implementation looks correct:
  1. Correct span usage:
    • Uses std::span<const ssize_t, std::dynamic_extent> (const, dynamic extent)
    • Constructs spans with std::span(pointer, count) (valid constructor)
    • Cast from ssize_t to std::size_t is safe since ndim() is non-negative for valid arrays
  2. Proper feature gating:
    • Guarded with #ifdef PYBIND11_HAS_SPAN
    • PYBIND11_HAS_SPAN is defined only when C++20 is available and <span> is found
  3. API consistency:
    • Mirrors shape() and strides() naming
    • Returns const spans, matching the const pointer returns
  4. Use case:
    • Spans can be passed to array constructors via any_container<ssize_t>, which accepts containers with iterators


  Unit tests added

  Added tests covering:
  • 0-dimensional arrays (scalars)
  • 1D, 2D, and 3D arrays
  • Comparison with regular shape()/strides() methods
  • Using spans to construct new arrays (main use case from PR description)
  • Tests are conditionally compiled only when PYBIND11_HAS_SPAN is defined


  Minor observations

  • The cast from ssize_t to std::size_t is safe but could theoretically wrap if ndim() were negative; that would indicate a corrupted array, which is outside this function's scope.

  The PR is ready to merge once the tests pass. The implementation follows pybind11 patterns and correctly uses std::span.