GH-36038: [Python] Implement __reduce__ on ExtensionType class

[AlenkaF]

Rationale for this change

ExtensionType subclasses can't be pickled if __reduce__ method isn't implemented separately.

What changes are included in this PR?

Add __reduce__ method to ExtensionType class.

Are these changes tested?

Yes, test is added to python/pyarrow/tests/test_extension_type.py.

Are there any user-facing changes?

No.

• Closes: [Python] Implement __reduce__ on ExtensionType class #36038

[jorisvandenbossche]

Looks good!

Do you know if we have test coverage for the array itself also being pickleable?

[AlenkaF]

Do you know if we have test coverage for the array itself also being pickleable?

I don't think so. Will check and add if necessary 👍

[AlenkaF]

There is a test for ExtensionArray with PyExtensionType called test_ext_array_pickling, but no test for an ExtensionArray with ExtensionType. So I added test_generic_ext_array_pickling.

[AlenkaF]

@jorisvandenbossche I plan to merge this unless there is some issue with changes from the last commit 7d72817.

[jorisvandenbossche]

Thanks!

[conbench-apache-arrow]

Conbench analyzed the 6 benchmark runs on commit f865fbd7.

There were 5 benchmark results indicating a performance regression:

• Commit Run on ursa-thinkcentre-m75q at 2023-06-28 23:48:00Z
  • params=/ConvertToSparseCSFTensorInt32, source=cpp-micro, suite=arrow-tensor-conversion-benchmark
  • params=/ConvertToSparseCSFTensorInt64, source=cpp-micro, suite=arrow-tensor-conversion-benchmark
• and 3 more (see the report linked below)

The full Conbench report has more details.