cc @vstinner

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And yes, it could be meaningful to use PyUnicodeWriter instead of manual buffer constructions, but we need to check if this really improves things or not before deciding whether we can do it.

Marking this as a draft.

There's performance regression:

import csv
import io
import pyperf

runner = pyperf.Runner()

INT_ROW = list(range(10))
COMPLEX_STRING_ROW = ['a,b', 'c"d', 'e\nf'] * 3 + ['ghi']

def write_the_rows(rows):
    f = io.StringIO()
    writer = csv.writer(f)
    writer.writerows(rows)

for num_rows in (10, 1_000, 10_000, ):
    int_rows = [INT_ROW] * num_rows
    complex_rows = [COMPLEX_STRING_ROW] * num_rows

    runner.bench_func(
        f'writerows {num_rows} integer rows',
        write_the_rows,
        int_rows
    )

    runner.bench_func(
        f'writerows {num_rows} complex string rows',
        write_the_rows,
        complex_rows
    )

There are two obvious issues:

• naive PyUnicodeWriter_WriteChar(), while PyUnicodeWriter_WriteSubstring() is possible sometimes.
• two pass (present also in the current version, though).

I need some time to address it, perhaps with jumping similar to gh-138214.