GH-21761: [Python] accept pyarrow values / scalars in constructor functions ?

[AlenkaF]

Rationale for this change

Currently, pyarrow.array doesn't accept list of pyarrow Scalars and this PR adds a check to allow that.

• Closes: [Python] accept pyarrow values / scalars in constructor functions ? #21761

[github-actions]

• Closes: [Python] accept pyarrow values / scalars in constructor functions ? #21761

[jorisvandenbossche]

This seems to work?
It won't work for generators of pyarrow scalars (because they will already be consumed the first time we call _sequence_to_array. I am not sure we have to worry about that case (since this PR just adding some extra convenience anyway). But we should maybe at least test those cases to ensure it errors (and eg not silently return an empty array)

[AlenkaF]

This seems to work?

Yes, it is working for sequences with pa.Scalars.

It won't work for generators of pyarrow scalars (because they will already be consumed the first time we call _sequence_to_array.

Oh yes, that is true (missed it).

I am not sure we have to worry about that case (since this PR just adding some extra convenience anyway). But we should maybe at least test those cases to ensure it errors (and eg not silently return an empty array)

Agree, will add a test 👍

[danepitkin]

Relying on a substring within the error message seems a bit hacky. Is there any other way to check by chance?

[AlenkaF]

Oh yeah, it's very hacky! =)
I have tried multiple ways of catching the specific ArrowInvalid error as others are expected here also, but all of them have some issue. Can try to remember them all and list them, but Joris advised me to go into the Python C++ code and use ConvertToSequenceAndInferSize so this will be changed. I should convert this PR into a draft to make this clear.

Thank you for reviewing!

[AlenkaF]

Closing this PR in favour of #36162.