GH-49644: [Python] Support converting list of multi-dimensional array…

[aboderinsamuel]

Rationale for this change

Constructing a fixed-shape-tensor array from a list of individual ndarrays only
worked when each element was 1-D; ≥2-D elements failed with
ArrowInvalid: Can only convert 1-dimensional array values. The only workaround
was stacking the list into a single ndarray and using
FixedShapeTensorArray.from_numpy_ndarray.

What changes are included in this PR?

The C++ list converter PyListConverter::AppendNdarray now accepts
multi-dimensional ndarray elements for fixed-size lists (the storage of a
fixed-shape tensor) by flattening them in C order. The fixed-size-list builder
still validates that the flattened length matches the list width, so wrong sizes
error cleanly. Variable-sized lists remain restricted to 1-D values to avoid
ambiguity. As a side benefit, plain fixed_size_list also accepts
multi-dimensional ndarray elements now.

Are these changes tested?

Yes:

• test_tensor_array_from_list_of_ndarrays — construction from 2-D and 3-D
  ndarrays, null handling, storage parity with from_numpy_ndarray, and the
  size-mismatch error, across int8/int64/float32.
• test_fixed_size_list_from_multidim_ndarray — plain fixed_size_list from
  multi-dim arrays, plus a check that variable-sized lists still reject 2-D.

Are there any user-facing changes?

Yes — pa.array([multi-dim ndarrays], type=fixed_shape_tensor(...)) (and the
same for fixed_size_list) now works instead of raising. Existing 1-D behavior
and variable-sized-list behavior are unchanged.

Scoped to construction only; the reverse to_numpy shape-preservation also
raised in the issue is intentionally left as a separate follow-up.

• GitHub Issue: [Python] Support converting list of multi-dimensional arrays to FixedShapeTensor #49644

[github-actions]

⚠️ GitHub issue #49644 has been automatically assigned in GitHub to PR creator.

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

Adds support for constructing fixed-shape tensors and fixed-size lists from lists of multi-dimensional NumPy ndarrays by flattening values in C order (GH-49644).

Changes:

• Add tests covering tensor arrays built from lists of ndarrays (including nulls and shape mismatch).
• Add tests ensuring fixed-size lists accept multi-dimensional ndarray elements (and reject invalid cases).
• Update ndarray-to-list conversion to allow flattening for FIXED_SIZE_LIST while keeping variable-sized lists restricted to 1D.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
python/pyarrow/tests/test_extension_type.py	Adds coverage for building FixedShapeTensorArray from a list of ndarrays.
python/pyarrow/tests/test_array.py	Adds a regression test for fixed-size list conversion from multi-dimensional ndarrays.
python/pyarrow/src/arrow/python/python_to_arrow.cc	Implements multi-dimensional ndarray flattening for fixed-size lists during conversion.

[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.