GH-47602: [Python] Make Schema hashable even when it has metadata

[jonded94]

Rationale for this change

In Python, pyarrow.Schema before was not hashable when it has metadata set.

>>> import pyarrow
>>> schema = pyarrow.schema([], metadata={b"1": b"1"})
>>> hash(schema)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "pyarrow/types.pxi", line 2921, in pyarrow.lib.Schema.__hash__
TypeError: unhashable type: 'dict'

This is because the metadata (which is a dict) was tried to be hashed as-is, which doesn't work.

What changes are included in this PR?

Slightly change how hashes are computed for Schema, by converting the dict[str, str] to the frozenset of key- and value tuples.

For reference, this is faster than computing the hash of a sorted tuple of key- and value tuples (https://stackoverflow.com/a/6014481/10070873).

Are these changes tested?

Yes.

Are there any user-facing changes?

Besides that Schema now correctly is hashable, no.

• GitHub Issue: [Python] Schema not hashable when metadata is set #47602

[github-actions]

Thanks for opening a pull request!

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Then could you also rename the pull request title in the following format?

GH-${GITHUB_ISSUE_ID}: [${COMPONENT}] ${SUMMARY}

or

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See also:

• Other pull requests
• Contribution Guidelines - Contributing Overview

[github-actions]

⚠️ GitHub issue #47602 has been automatically assigned in GitHub to PR creator.

[jonded94]

@AlenkaF , @raulcd or @rok , sorry for pinging, but could you give this a review? 🥺

Tests seem fine, and this would unblock some functionality we need on our end.

[AlenkaF]

Thank you for fixing the bug and adding the test for it!
The change looks good to me. I only have one minor nit otherwise the PR is ready to go.

Pinging @raulcd for one extra review.

[AlenkaF]

@github-actions crossbow submit -g python

[github-actions]

Revision: ad52c12

Submitted crossbow builds: ursacomputing/crossbow @ actions-6cf16e3659

Task	Status
example-python-minimal-build-fedora-conda
example-python-minimal-build-ubuntu-venv
test-conda-python-3.10
test-conda-python-3.10-hdfs-2.9.2
test-conda-python-3.10-hdfs-3.2.1
test-conda-python-3.10-pandas-1.3.4-numpy-1.21.2
test-conda-python-3.11
test-conda-python-3.11-dask-latest
test-conda-python-3.11-dask-upstream_devel
test-conda-python-3.11-hypothesis
test-conda-python-3.11-pandas-latest-numpy-latest
test-conda-python-3.11-spark-master
test-conda-python-3.12
test-conda-python-3.12-cpython-debug
test-conda-python-3.12-pandas-latest-numpy-1.26
test-conda-python-3.12-pandas-latest-numpy-latest
test-conda-python-3.13
test-conda-python-3.13-pandas-nightly-numpy-nightly
test-conda-python-3.13-pandas-upstream_devel-numpy-nightly
test-conda-python-emscripten
test-cuda-python-ubuntu-22.04-cuda-11.7.1
test-debian-12-python-3-amd64
test-debian-12-python-3-i386
test-fedora-42-python-3
test-ubuntu-22.04-python-3
test-ubuntu-22.04-python-313-freethreading
test-ubuntu-24.04-python-3

[jonded94]

The crossbow stuff for some reason coredumped for ubuntu-22.04 (https://github.com/ursacomputing/crossbow/actions/runs/17939291515/job/51011788800, https://github.com/ursacomputing/crossbow/actions/runs/17939291336/job/51011788225). But I wouldn't see how that is connected to this change? 🤔

[AlenkaF]

These three failures are not connected.
(We are dealing with quite some job issues that we need to fix)

[jonded94]

@raulcd @rok review please? 🥺

Apparently people over at Ray are already cherry-picking this, could we merge this? 😬

[raulcd]

LGTM
Sorry for taking a little to review, I've been busy with other issues.
Thanks for the ping

[conbench-apache-arrow]

After merging your PR, Conbench analyzed the 4 benchmarking runs that have been run so far on merge-commit ef718a7.

There were no benchmark performance regressions. 🎉

The full Conbench report has more details. It also includes information about 2 possible false positives for unstable benchmarks that are known to sometimes produce them.