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

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shAn-kor:fix-invalid-format-specifier-error-message
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Match invalid format specifier error messages#8477
shAn-kor wants to merge 1 commit into
RustPython:mainfrom
shAn-kor:fix-invalid-format-specifier-error-message

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@shAn-kor

@shAn-kor shAn-kor commented Aug 9, 2026 •
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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 formatting support for booleans, integers, floating-point numbers, complex numbers, and strings.
    • Invalid format specifications now produce clearer, more consistent error messages that identify the affected value type and specification.
    • Preserved existing locale-aware and standard formatting behavior.

@shAn-kor
Match invalid format specifier error details
6e4ba86 
Assisted-by: Codex:gpt-5.6-sol
@coderabbitai

coderabbitai Bot commented Aug 9, 2026 •
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github-actions Bot commented Aug 9, 2026

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📦 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)

[x] lib: cpython/Lib/ssl.py
[ ] test: cpython/Lib/test/test_ssl.py (TODO: 10)

dependencies:

  • ssl

dependent tests: (53 tests)

  • ssl: test_asyncio test_ftplib test_httplib test_httpservers test_imaplib test_logging test_poplib test_ssl test_urllib test_urllib2_localnet test_venv test_xmlrpc
    • asyncio.selector_events: test_asyncio
    • ftplib: test_urllib2
      • urllib.request: test_http_cookiejar test_pathlib test_pydoc test_sax test_site test_urllib2net test_urllibnet
    • http.client: test_docxmlrpc test_hashlib test_ucn test_unicodedata test_wsgiref
      • logging.handlers: test_concurrent_futures test_pkgutil
    • http.server: test_robotparser
      • pydoc: test_enum
    • smtplib: test_smtplib test_smtpnet
    • urllib.request:
      • pathlib: test_ast test_dbm_sqlite3 test_ensurepip test_importlib test_json test_launcher test_pathlib test_peg_generator test_pyrepl test_runpy test_tarfile test_tempfile test_tomllib test_tools test_traceback test_unparse test_winapi test_zipapp test_zipfile test_zoneinfo test_zstd

Legend:

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

@moreal moreal added the z-ca-2026 Tag to track Contribution Academy 2026 label Aug 9, 2026
@shAn-kor
shAn-kor marked this pull request as ready for review August 9, 2026 23:11
youknowone
youknowone previously approved these changes Aug 11, 2026
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@youknowone
youknowone dismissed their stale review August 11, 2026 12:22

let me think once again if this is the best way

youknowone
youknowone requested changes Aug 18, 2026
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Hi! I left a change request not to make confusion to old path.
Sorry for late review!

Comment thread crates/vm/src/format.rs

@youknowone youknowone Aug 18, 2026 •
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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.

@luantaraschi luantaraschi mentioned this pull request Aug 21, 2026
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@shAn-kor @youknowone @moreal