Match invalid format specifier error messages by shAn-kor · Pull Request #8477 · RustPython/RustPython

shAn-kor · 2026-08-09T21:21:42Z

Related to Upgrade Python libraries and test from Python 3.14 #6839
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Summary

This is a follow-up to #8475. RustPython still returned the generic Invalid format specifier message for malformed format specifications, while CPython includes both the original specifier and the runtime object type.

Add a VM-level format-spec parsing helper that maps only InvalidFormatSpecifier to CPython's detailed ValueError message. Integer, boolean, float, complex, and string formatting now use the helper, including subclasses whose runtime type names must appear in the error.

With the error text matching CPython, this enables the existing test_better_error_message_format, test_unicode_in_error_message, and test_specifier_z_error tests without changing their assertions or test data.

Tests

cargo run --release -- -m test test_format
cargo test --workspace --exclude rustpython_wasm --exclude rustpython-venvlauncher --exclude rustpython-capi
(cd crates/capi && cargo test) — 102 passed
cargo clippy --workspace --exclude rustpython_wasm --exclude rustpython-venvlauncher --all-targets
repository pre-commit and pre-push checks

AI assistance

Codex (GPT-5.6) assisted with comparing CPython and RustPython error behavior, implementing the shared parsing helper, enabling the existing regression tests, running validation, and drafting this PR description. I reviewed the final diff and verified the behavior locally on macOS.

Summary by CodeRabbit

Bug Fixes
- Improved format-specification handling for booleans, integers, floating-point numbers, complex numbers, and strings.
- Invalid format specifications now produce consistent errors that identify the specification and affected value type.
- Formatting errors retain their appropriate error types and messages.
- Existing locale-aware, standard, and empty-spec formatting behavior remains unchanged.

coderabbitai · 2026-08-09T21:21:49Z

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Reviewing files that changed from the base of the PR and between dd16a10 and ebde064.

⛔ Files ignored due to path filters (1)

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

📒 Files selected for processing (6)

crates/vm/src/builtins/bool.rs
crates/vm/src/builtins/complex.rs
crates/vm/src/builtins/float.rs
crates/vm/src/builtins/int.rs
crates/vm/src/builtins/str.rs
crates/vm/src/format.rs

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📝 Walkthrough

Walkthrough

Built-in boolean, complex, float, integer, and string formatting methods now use shared helpers to parse format specifications and convert formatting errors. Invalid-specifier errors include the specification and object type.

Changes

Format-spec parsing

Layer / File(s)	Summary
Shared format-spec parser `crates/vm/src/format.rs`	Adds `parse_format_spec` and `format_spec_error` to parse specifications and map errors to Python exceptions. Invalid-specifier errors include the specification and object type.
Built-in formatting integration `crates/vm/src/builtins/bool.rs`, `crates/vm/src/builtins/complex.rs`, `crates/vm/src/builtins/float.rs`, `crates/vm/src/builtins/int.rs`, `crates/vm/src/builtins/str.rs`	Built-in `__format__` methods use the shared helpers instead of direct `FormatSpec` parsing and prior exception conversion. Existing formatting paths remain in place.

Priority: ➖ Normal

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

Change: Bug fix

Suggested reviewers: leehanjeong

Merge Risk: ⚪ Minimal · up to ebde0

This change makes invalid format specifier errors more detailed and consistent with CPython. No concrete merge-blocking risk was identified.

Security Architecture Review

Security architecture risk: ⚪ Minimal · up to ebde0

The change improves invalid-format diagnostics without expanding access or weakening the inspected formatting safeguards. No material security issue was identified in the change.

Retained concerns
No architecture-level concerns identified.

Security review details

Security Blast Radius

inferred — The confirmed exposure is the existing formatting operations within an executing interpreter and their returned exceptions. Their new shared adapter introduces neither an additional public invocation path nor a privileged operation.

Trust Boundaries and Controls

observed — Caller-controlled specifier text and potentially caller-defined type names are interpolated into the returned ValueError. The new class-name lookup reads stored VM metadata rather than invoking Python attribute access or methods; these values do not select authority or a sensitive operation in the helper.

Resilience and Maintainability Implications

inferred — The new helpers borrow their inputs and return a parsed specification or a newly constructed exception. They introduce no shared mutable state or durable multi-step transition requiring recovery, rollback, or repeated-call coordination.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 75.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 8 functions across 6 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 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: matching CPython-style error messages for invalid format specifiers.
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 · 2026-08-09T21:22:42Z

📦 Library Dependencies

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

[x] test: cpython/Lib/test/test_format.py

dependencies:

dependent tests: (no tests depend on format)

Legend:

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

let me think once again if this is the best way

youknowone

Hi! I left a change request not to make confusion to old path.
Sorry for late review!

youknowone · 2026-08-18T03:51:48Z

    }
 }

+pub(crate) fn parse_format_spec(


due to this helper, FormatSpecError::InvalidFormatSpecifier arm in impl IntoPyException for FormatSpecError is now a trap. The error message will be diverged by its calling path.
if we go this direction, I prefer to remove impl IntoPyException for FormatSpecError and bring the implementation into this helper.

Assisted-by: Codex:gpt-5.6-sol

InvalidFormatSpecifier needs the spec and the object type. Drop IntoPyException so that path cannot produce a generic message. Assisted-by: Grok:4.6

codspeed · 2026-10-05T15:59:34Z

Merging this PR will not alter performance

✅ 62 untouched benchmarks
⏩ 4 skipped benchmarks¹

_{Comparing shAn-kor:fix-invalid-format-specifier-error-message (ebde064) with main (524f1a2)}

4 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

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

shAn-kor marked this pull request as ready for review August 9, 2026 23:11

youknowone previously approved these changes Aug 11, 2026

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youknowone requested changes Aug 18, 2026

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luantaraschi mentioned this pull request Aug 21, 2026

Reject the string format flags CPython refuses #8571

Merged

1 task done

youknowone force-pushed the main branch from d1c4925 to 3191eca Compare September 16, 2026 17:13

youknowone force-pushed the fix-invalid-format-specifier-error-message branch from 6e4ba86 to dd16a10 Compare September 16, 2026 17:17

shAn-kor and others added 2 commits October 5, 2026 23:10

Match invalid format specifier error details

a172d0b

Assisted-by: Codex:gpt-5.6-sol

format: convert FormatSpecError in one helper

ebde064

InvalidFormatSpecifier needs the spec and the object type. Drop IntoPyException so that path cannot produce a generic message. Assisted-by: Grok:4.6

youknowone force-pushed the fix-invalid-format-specifier-error-message branch from dd16a10 to ebde064 Compare October 5, 2026 14:37

youknowone approved these changes Oct 5, 2026

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youknowone enabled auto-merge (squash) October 5, 2026 15:25

youknowone merged commit 3d1c174 into RustPython:main Oct 5, 2026
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Match invalid format specifier error messages - #8477

Match invalid format specifier error messages#8477
youknowone merged 2 commits into
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shAn-kor:fix-invalid-format-specifier-error-message

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codspeed Bot commented Oct 5, 2026

Merging this PR will not alter performance

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shAn-kor commented Aug 9, 2026 •

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