[Python] Expose RecordBatchFileReader::CountRows in Python

[adrien-grl]

Describe the enhancement requested

Problem / Motivation

In PyArrow, pyarrow.ipc.RecordBatchFileReader the only way I found to gather the total number of rows of contained in a Feather file is to do something along the lines of:

num_rows = sum(reader.get_batch(i).num_rows for i in range(reader.num_record_batches))

This is not very efficient when it seems you can directly count the rows using the metadata (as in RecordBatchFileReader::CountRows) (if I understand the code correctly?)

The current way of doing is intractable when reading from remote file systems.

Proposed solution

Expose RecordBatchFileReader::CountRows in Python.

References

arrow/cpp/src/arrow/ipc/reader.h

Line 204 in 76f7815

virtual Result<int64_t> CountRows() = 0;

Thank you for reading my suggestion and all the amazing work!!

Component(s)

Python

[AlenkaF]

Thank you for opening the issue @adrien-grl !
From a quick look of the code I would agree. This would be nice to have in PyArrow and looks like a straightforward binding to add with all the info in the above description already figuring out important steps.

Would you be interested in submitting a PR?

[GujaLomsadze]

Hi @AlenkaF, @adrien-grl.
Seems like this issue has been stale for quite a bit. I would love to take this.

[pitrou]

Issue resolved by pull request 50646
#50646