[ARROW-12482] [Doc][Python] Mention CSVStreamingReader pitfalls with type inference

• Type: Bug
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 3.0.0
• Fix Version/s: 5.0.0
• Component/s: Documentation, Python
• Labels: CSVParser CSVReader pull-request-available
• External issue URL: https://github.com/apache/arrow/issues/28247
• Assignee: Antoine Pitrou
• Reporter: Oleksandr Shevchenko
• Created: 20/Apr/21 17:14
• Updated: 11/Jan/23 08:26
• Resolved: 27/Apr/21 13:25

Description

Looks like Arrow infer type for the first batch and apply it for all subsequent batches. But information might be not enough to infer the type correctly for the whole file. For our particular case, Arrow infers some field in the schema as date32 from the first batch but the next batch has an empty field value that can’t be converted to date32.

When I increase the batch size to have such a value in the first batch Arrow set string type (not sure why not nullable date32) for such a field since it can’t be converted to date32 and the whole file is read successfully.

This problem can be easily reproduced by using the following code and attached dataset:

import pyarrow as pa
import pyarrow._csv as pa_csv
import pyarrow._fs as pa_fs

read_options: pa_csv.ReadOptions = pa_csv.ReadOptions(block_size=5_000_000)
parse_options: pa_csv.ParseOptions = pa_csv.ParseOptions(newlines_in_values=True)
convert_options: pa_csv.ConvertOptions = pa_csv.ConvertOptions(timestamp_parsers=[''])
with pa_fs.LocalFileSystem().open_input_file("dataset.csv") as file:
 reader = pa_csv.open_csv(
 file, read_options=read_options, parse_options=parse_options, convert_options=convert_options
 )
 for batch in reader:
 table_batch = pa.Table.from_batches([batch])
 table_batch

Error message:

 for batch in reader:
 File "pyarrow/ipc.pxi", line 497, in __iter__
 File "pyarrow/ipc.pxi", line 531, in pyarrow.lib.RecordBatchReader.read_next_batch
 File "pyarrow/error.pxi", line 84, in pyarrow.lib.check_status
 pyarrow.lib.ArrowInvalid: In CSV column #23: CSV conversion error to date32[day]: invalid value ''

 
When we use block_size `10_000_000` file can be read successfully since we have the problematic value in the first batch.

An error occurs when I try to attach dataset, so you can download it from Google Drive here

Issue Links

• is related to: ARROW-12510 [C++][Python][CSV] Allow quoted values to be null
• links to: GitHub Pull Request #10132