[C++][Python] CSV: option to pad short rows with nulls instead of failing #53290

Describe the enhancement requested

Add an option to the CSV reader to read rows that have fewer fields than the number of columns, and fill the missing fields with null. pandas' C engine does this when names is given, and DuckDB has read_csv(..., null_padding = true).

Today the width is fixed by column_names, or by the first row. A shorter row is an error, and invalid_row_handler can only skip it or stop. It cannot keep the row.

import io
import pyarrow as pa
from pyarrow import csv

data = b"a,b\n1,2,3,4\n5,6,7,8\n"
names = ["w", "x", "y", "z"]

csv.read_csv(io.BytesIO(data), read_options=csv.ReadOptions(column_names=names))
# ArrowInvalid: CSV parse error: Expected 4 columns, got 2: a,b

csv.read_csv(io.BytesIO(data), read_options=csv.ReadOptions(autogenerate_column_names=True))
# ArrowInvalid: CSV parse error: Expected 2 columns, got 4: 1,2,3,4

csv.read_csv(io.BytesIO(data),
             read_options=csv.ReadOptions(column_names=names),
             parse_options=csv.ParseOptions(invalid_row_handler=lambda r: "skip"))
# {'w': [1, 5], 'x': [2, 6], 'y': [3, 7], 'z': [4, 8]}  -- the short row is dropped, not padded

Expected, with the new option:

{'w': ['a', '1', '5'], 'x': ['b', '2', '6'], 'y': [None, '3', '7'], 'z': [None, '4', '8']}

Real files have this shape. For example, the AEMO (Australian electricity market) daily CSV files mix record types: the first row is a short header record, and the data rows are wider. pandas with engine="pyarrow" cannot read them, because pandas uses Arrow's reader and Arrow takes the width from the first row. pandas marked this as an Arrow limitation: pandas-dev/pandas#71034

The same request was #27832, closed as stale in January 2026. #28421 added invalid_row_handler, but it can only skip or error.

Tested with pyarrow 26.0.0, Python 3.12, Linux.

Component(s)

C++, Python