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csv: apply dialect quoting and lineterminator in writer#8254

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RustPython:mainfrom
jinmay:csv-writer-dialect-quoting
Open

csv: apply dialect quoting and lineterminator in writer#8254
jinmay wants to merge 2 commits into
RustPython:mainfrom
jinmay:csv-writer-dialect-quoting

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@jinmay

@jinmay jinmay commented Jul 11, 2026 •
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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

jinmay added 2 commits July 11, 2026 15:54
@jinmay
csv: map QUOTE_MINIMAL to Necessary quote style
2afe107 
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
@jinmay
csv: apply dialect quoting and lineterminator in writer
7145002 
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
@coderabbitai

coderabbitai Bot commented Jul 11, 2026 •
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Review Change Stack

📝 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 while retaining conditional escape handling.

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

Possibly related PRs

Suggested reviewers: youknowone, ShaharNaveh

🚥 Pre-merge checks | ✅ 5
✅ Passed checks (5 passed)
Check name Status Explanation
Linked Issues check ✅ Passed The code now applies dialect quoting and lineterminator in csv.writer and fixes QUOTE_MINIMAL mapping as required.
Out of Scope Changes check ✅ Passed The diff appears focused on the CSV writer behavior described in the issue with no unrelated changes.
Docstring Coverage ✅ Passed No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Description Check ✅ Passed Check skipped - CodeRabbit’s high-level summary is enabled.
Title check ✅ Passed The title clearly describes the main writer change: applying dialect quoting and line terminators.
✨ Finishing Touches
🧪 Generate unit tests (beta)
  • Create PR with unit tests

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@github-actions

github-actions Bot commented Jul 11, 2026

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📦 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: 25)

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

@jinmay jinmay marked this pull request as ready for review July 11, 2026 15:29
@youknowone youknowone added the z-ca-2026 Tag to track Contribution Academy 2026 label Jul 12, 2026
@doma17

doma17 commented Jul 12, 2026 •
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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

youknowone commented Jul 13, 2026

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

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csv.writer ignores a dialect's quoting and lineterminator

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@jinmay @doma17 @youknowone