csv: apply dialect quoting and lineterminator in writer by jinmay · Pull Request #8254 · RustPython/RustPython

jinmay · 2026-07-11T07:14:55Z

Closes csv.writer ignores a dialect's quoting and lineterminator #8253
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Summary

csv.writer ignored the quoting and lineterminator defined by a dialect (passed by name like dialect='unix' or as a dialect object), so writing with a built-in dialect diverged from CPython. Only explicit keyword arguments took effect. to_writer now resolves both from the dialect (via get_quoting() / get_lineterminator(), mirroring the existing get_delimiter()), while an explicit keyword argument still overrides it.

While fixing this I also found the QuoteStyle::Minimal → csv_core mapping was Always (quoting every field under QUOTE_MINIMAL); it is now Necessary. This was dormant because the quote style was only applied for an explicit quoting= argument, so it had to be fixed before applying the dialect's quoting.

import csv, io
sio = io.StringIO()
csv.writer(sio, dialect='unix').writerow([1, 'abc def', 'abc'])
# before: '1,abc def,abc\r\n'
# after:  '"1","abc def","abc"\n'   (matches CPython)

Test plan

Unmarks TestDialectUnix.test_simple_writer and Test_Csv.test_write_quoting (both @unittest.expectedFailure before). cargo run --release -- -m test test_csv: 128 run, 7 skipped, SUCCESS.
extra_tests/snippets/stdlib_csv.py: passes.
cargo fmt --check / cargo clippy: clean (no new warnings).
Compared against real CPython 3.14.6 on the same machine; the outputs for the dialect='unix' and QUOTE_MINIMAL cases match.

Assisted-by: Claude Code:claude-opus-4-8

Summary by CodeRabbit

Bug Fixes
- Corrected minimal CSV quoting so values are quoted only when necessary.
- Improved CSV formatting to consistently respect the selected dialect’s line terminator and quoting settings.
- Ensured explicitly configured formatting options take precedence over dialect defaults.

coderabbitai · 2026-07-11T07:15:01Z

No actionable comments were generated in the recent review. 🎉

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📥 Commits

Reviewing files that changed from the base of the PR and between 7145002 and 13ef019.

⛔ Files ignored due to path filters (1)

Lib/test/test_csv.py is excluded by !Lib/**

📒 Files selected for processing (1)

crates/stdlib/src/csv.rs

🚧 Files skipped from review as they are similar to previous changes (1)

crates/stdlib/src/csv.rs

📝 Walkthrough

Walkthrough

CSV writer configuration now respects dialect-provided quoting and line terminators, while explicit FormatOptions values override dialect defaults. Minimal quoting maps to csv_core::QuoteStyle::Necessary.

Changes

CSV formatting behavior

Layer / File(s)	Summary
Quote mapping and option resolution `crates/stdlib/src/csv.rs`	Minimal quoting uses the necessary quote style, and helper methods resolve dialect or default values with explicit overrides.
Writer configuration `crates/stdlib/src/csv.rs`	`to_writer` applies resolved line terminator and quoting settings.

Estimated code review effort: 2 (Simple) | ~10 minutes

Possibly related PRs

RustPython/RustPython#7834: Modifies CSV quoting and dialect handling in the same module.
RustPython/RustPython#8109: Changes CSV writer quote-style handling in the same module.
RustPython/RustPython#8201: Overlaps with CSV writer quoting configuration changes.

Suggested reviewers: shaharnaveh, youknowone, moreal

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly summarizes the main change: applying dialect quoting and line terminator behavior in csv.writer.
Linked Issues check	✅ Passed	The code changes match issue `#8253` by honoring dialect quoting and lineterminator settings and preserving explicit overrides.
Out of Scope Changes check	✅ Passed	No unrelated changes are evident in the reviewed summary; the edits stay focused on csv writer dialect behavior.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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github-actions · 2026-07-11T07:15:47Z

📦 Library Dependencies

The following Lib/ modules were modified. Here are their dependencies:

[x] lib: cpython/Lib/csv.py
[x] test: cpython/Lib/test/test_csv.py (TODO: 23)

dependencies:

csv

dependent tests: (4 tests)

csv: test_csv test_genericalias
- importlib.metadata: test_importlib test_zoneinfo

Legend:

[+] path exists in CPython
[x] up-to-date, [ ] outdated

doma17 · 2026-07-12T15:07:45Z

Hello!

With this PR, RustPython writes \r,\n!, while CPython writes "\r","\n"!.

  csv.writer(sio, lineterminator="!").writerow(["\r", "\n"])

It looks like embedded CR/LF may no longer be quoted when the line terminator is not CR or LF.

youknowone · 2026-07-13T05:09:28Z

@doma17 Thank you so much!
@jinmay could you check it please?

QuoteStyle::Minimal was mapped to csv_core's Always, which quoted every field even under QUOTE_MINIMAL. Map it to Necessary so only fields that require quoting are quoted. Unmarks Test_Csv.test_write_quoting. Assisted-by: Claude Code:claude-opus-4-8

csv.writer ignored the quoting and lineterminator from a dialect (whether passed by name or object), always using the defaults. Resolve both from the dialect via get_quoting()/get_lineterminator(), mirroring get_delimiter(), so an explicit keyword argument still overrides the dialect. Unmarks TestDialectUnix.test_simple_writer. Assisted-by: Claude Code:claude-opus-4-8

jinmay · 2026-07-16T05:35:06Z

@doma17 thanks.

From what I found, the cause seems to be in csv-core: for QUOTE_MINIMAL it decides quoting by the terminator, so with a custom lineterminator it doesn't quote \r/\n. CPython always quotes them.

To fix it, I think we'd move QUOTE_MINIMAL to our native write path (like QUOTE_NONE) and unmark test_write_lineterminator. But that's a different test than this PR targeted, so the scope gets bigger than the original issue.

Fix it here, or as a follow-up?

youknowone

@jinmay @doma17 Thank you so much! If it is separated issue, could you open an issue about that? I prefer to have separated issue when it is not a blocker of other task

jinmay marked this pull request as ready for review July 11, 2026 15:29

youknowone added the z-ca-2026 Tag to track Contribution Academy 2026 label Jul 12, 2026

jinmay added 2 commits July 14, 2026 07:42

jinmay force-pushed the csv-writer-dialect-quoting branch from 7145002 to 13ef019 Compare July 13, 2026 22:42

youknowone approved these changes Jul 16, 2026

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youknowone merged commit 5154e7e into RustPython:main Jul 16, 2026
27 checks passed

jinmay mentioned this pull request Jul 17, 2026

csv.writer: embedded \r/\n not quoted with a custom lineterminator (QUOTE_MINIMAL) #8302

Closed

coderabbitai Bot mentioned this pull request Jul 18, 2026

csv: quote embedded CR/LF under QUOTE_MINIMAL #8315

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2 tasks

jinmay mentioned this pull request Jul 18, 2026

csv.writer: embedded \r/\n not quoted with a custom lineterminator (QUOTE_MINIMAL) #8317

Closed

coderabbitai Bot mentioned this pull request Jul 20, 2026

csv: support multi-character lineterminator in writer #8328

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csv: apply dialect quoting and lineterminator in writer#8254

csv: apply dialect quoting and lineterminator in writer#8254
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