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shAn-kor · 2026-08-09T04:46:31Z

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Related to #6839

What changed

RustPython did not support the z format option introduced by PEP 682, which removes the sign from a negative value when its formatted result rounds to zero.

The formatter now parses z after the sign option and applies it to floating point and complex formatting, including the locale-aware n path. The sign is removed only when the formatted magnitude is zero, so -0.0001 formatted with z.2f becomes 0.00 while nonzero negative results keep their sign.

Invalid uses with integer, boolean, and string formats follow CPython error behavior. Complex formatting also preserves explicit + and space signs for purely imaginary values.

The existing test_negative_zero CPython test is now enabled, with additional common-layer coverage for rounding, complex values, locale formatting, and invalid format specifications.

Tests

cargo test -p rustpython-common format_negative_zero_coercion
cargo run --release -- -m test test_format
cargo run --release -- -m test test_complex
prek run --all-files
repository pre-push checks, including full clippy, workspace tests, and 102 C-API tests

AI assistance

Codex (GPT-5) assisted with comparing CPython and RustPython behavior, drafting the implementation and regression tests, and writing this PR description. I reviewed the final diff and verified the behavior locally.

Summary by CodeRabbit

New Features
- Added support for suppressing negative signs on zero-valued floating-point and complex-number output.
- Added validation for character formatting options, including precision, signs, alternate form, coercion, and valid code points.
Bug Fixes
- Standardized errors for unsupported formatting options and unknown boolean format types.
- Added clear type-specific messages when negative-zero suppression is not supported.

Assisted-by: Codex:gpt-5

Assisted-by: Codex:GPT-5

coderabbitai · 2026-08-09T04:47:05Z

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

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Run ID: 645fc07f-c995-4f09-b024-1a1cfb5c2243

📥 Commits

Reviewing files that changed from the base of the PR and between 1819677 and 4939a3a.

⛔ Files ignored due to path filters (1)

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

📒 Files selected for processing (2)

crates/common/src/format.rs
crates/vm/src/format.rs

📝 Walkthrough

Walkthrough

The formatting engine now parses the z negative-zero coercion flag. Float and complex formatting suppress negative signs for values formatted as zero. Unsupported types return a dedicated error that the VM converts to ValueError.

Changes

Negative-zero coercion

Layer / File(s)	Summary
Parse and error contract `crates/common/src/format.rs`	`FormatSpec` stores the `z` flag. `FormatSpecError` includes a type-specific rejection variant.
Formatting and validation behavior `crates/common/src/format.rs`	Float and complex formatting apply negative-zero coercion. Integer, string, boolean, and character paths reject unsupported use.
Validation and exception mapping `crates/common/src/format.rs`, `crates/vm/src/format.rs`	Tests cover parsing, coercion, locale formatting, complex values, and rejection cases. The VM converts the new error to `ValueError`.

Estimated code review effort: 3 (Moderate) | ~25 minutes

Sequence Diagram(s)

sequenceDiagram
  participant PythonCode
  participant FormatSpec
  participant FloatOrComplexFormatter
  participant VMExceptionMapper

  PythonCode->>FormatSpec: parse format specification with z
  FormatSpec->>FloatOrComplexFormatter: provide no_neg_0
  FloatOrComplexFormatter-->>PythonCode: format zero without negative sign
  FormatSpec->>VMExceptionMapper: report unsupported coercion type
  VMExceptionMapper-->>PythonCode: raise ValueError

Possibly related PRs

RustPython/RustPython#8373: Both changes modify FormatSpec parsing and numeric formatting in crates/common/src/format.rs.
RustPython/RustPython#8453: Both changes modify complex-number formatting validation in crates/common/src/format.rs.

Suggested labels: z-ca-2026

Suggested reviewers: joshuamegnauth54, youknowone

🚥 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 and concisely describes the implementation of negative-zero coercion formatting.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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github-actions · 2026-08-09T04:47:27Z

📦 Library Dependencies

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

[x] lib: cpython/Lib/reprlib.py
[x] test: cpython/Lib/test/test_reprlib.py (TODO: 1)

dependencies:

reprlib

dependent tests: (245 tests)

[x] test: cpython/Lib/test/test_funcattrs.py (TODO: 2)

dependencies:

dependent tests: (no tests depend on funcattrs)

[x] test: cpython/Lib/test/test_format.py (TODO: 3)

dependencies:

dependent tests: (no tests depend on format)

Legend:

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

youknowone

👍

Assisted-by: Codex:gpt-5

shAn-kor added 2 commits July 27, 2026 00:35

Implement negative zero coercion format option

7b1e11b

Assisted-by: Codex:gpt-5

Merge upstream/main and complete negative zero formatting parity

4939a3a

Assisted-by: Codex:GPT-5

youknowone added the z-ca-2026 Tag to track Contribution Academy 2026 label Aug 9, 2026

youknowone approved these changes Aug 9, 2026

View reviewed changes

youknowone merged commit 12a0a1e into RustPython:main Aug 9, 2026
28 checks passed

kyokuping pushed a commit to kyokuping/RustPython that referenced this pull request Aug 9, 2026

Implement negative zero coercion format option (RustPython#8475)

a71403b

Assisted-by: Codex:gpt-5

shAn-kor mentioned this pull request Aug 9, 2026

Match invalid format specifier error messages #8477

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Implement negative zero coercion formatting - #8475

Implement negative zero coercion formatting#8475
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