"a" + 1 reports the generic binary-op error instead of CPython's concatenation error:

RustPython: TypeError: unsupported operand type(s) for +: 'str' and 'int'
CPython:    TypeError: can only concatenate str (not "int") to str

PyStr::__add__ delegates to the right operand's __radd__ whenever it has one and returns whatever comes back. Every numeric type has __radd__, and it returns NotImplemented for a str left operand, so the result reached the generic handler. CPython instead falls back to str's sq_concat once the reflected call declines, which is where its message comes from.

The correct message was already in the else branch, reachable only for operands with no __radd__ at all — so list, object, bytes and None were already right, while int, float, bool and any class whose __radd__ returns NotImplemented were not. The fix keeps the reflected call, and only falls through to the concat error when it declines. A __radd__ that returns a value still wins, unchanged.

Verification:

• The new snippet in extra_tests/snippets/builtin_str.py passes under both RustPython and CPython 3.14.0, so its expectations are CPython's behaviour rather than my reading of it.
• Reverting only the str.rs change fails its first assertion.
• cargo test --workspace --exclude rustpython_wasm --exclude rustpython-venvlauncher --exclude rustpython-capi — all 42 test binaries pass.
• cargo run --release -- -m test test_str test_operator test_descr test_exceptions — SUCCESS for each.
• cargo fmt --check and ruff format --check clean.

Found by diffing ~70 expressions across both interpreters. Three other divergences turned up and are deliberately not in this PR, since they are unrelated one-line message fixes in different files: '{'.format() and '{0.}'.format(1) produce different ValueError text, and bytes([256]) says byte must be in range(0, 256) where CPython says bytes. Happy to send those separately if wanted.

AI disclosure per the AI policy: found, written and verified with Claude Code (Claude Opus 5), which is also recorded in the commit's Assisted-by: trailer. @rawsun007 reviewed the change and authorised the push.

Summary by CodeRabbit

• Bug Fixes

  • Improved string concatenation errors to report the specific incompatible operand type.
  • Correctly falls back to the standard string error when a reflected operation declines handling.
  • Preserved reflected-operation results when they provide a valid value.

• Tests

  • Added coverage for concatenation with incompatible types and reflected operations.