crates/vm/src/stdlib/io.rs (1)

1076-1093: Use of new_os_subtype_error for non‑blocking write looks correct; consider passing errno/written later.

Switching to vm.new_os_subtype_error(vm.ctx.exceptions.blocking_io_error.to_owned(), None, "write could not complete without blocking".to_owned()) keeps this in the OSError hierarchy and aligns with the new OSErrorBuilder/PyOSError::__str__ flow; behavior for now is equivalent to the old “message‑only” BlockingIOError. When you get to the TODO, it would be good to thread through the underlying errno and the number of bytes written so you can construct the full BlockingIOError(errno, msg, written) tuple for CPython parity, but that can be a separate change.

crates/vm/src/exceptions.rs (1)

1721-1810: New PyOSError::__str__ formatting is consistent and field‑driven; edge cases mostly handled well.

The new implementation:

• Reads errno, strerror, filename, filename2, and (on Windows) winerror via attributes and ignores values that are None, which is robust to partially‑initialized instances.
• On Windows, prefers winerror and produces:
  • [WinError N] msg: filename -> filename2 when both filenames are present,
  • [WinError N] msg: filename when only one is present,
  • [WinError N] msg when only winerror/strerror are present.
• Otherwise (or when no winerror), uses errno‑driven formats:
  • [Errno N] msg: filename -> filename2,
  • [Errno N] msg: filename,
  • [Errno N] msg (no filename),
  • and finally falls back to BaseException.__str__ when neither errno nor strerror is available.

This lines up well with CPython’s user‑facing strings and works correctly with the updated OSErrorBuilder/slot_init behavior (e.g., your new BlockingIOError path in io.rs still falls back to the single‑argument BaseException formatting when no errno/filename are set). No functional issues stand out here.

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Review profile: CHILL

Plan: Pro

Learnt from: CR
Repo: RustPython/RustPython PR: 0
File: .github/copilot-instructions.md:0-0
Timestamp: 2025-11-29T12:17:28.606Z
Learning: Applies to **/*.rs : Follow Rust best practices for error handling and memory management

Learnt from: youknowone
Repo: RustPython/RustPython PR: 6243
File: vm/src/function/buffer.rs:123-129
Timestamp: 2025-11-10T06:27:41.954Z
Learning: In vm/src/function/buffer.rs, line 36 in the try_rw_bytes_like method intentionally uses TypeError (not BufferError) for the "buffer is not a read-write bytes-like object" error case, even though a similar error message in ArgMemoryBuffer::try_from_borrowed_object uses BufferError. The TypeError is the intended exception type for this specific code path.

Learnt from: youknowone
Repo: RustPython/RustPython PR: 6358
File: crates/vm/src/exception_group.rs:173-185
Timestamp: 2025-12-09T08:46:58.660Z
Learning: In crates/vm/src/exception_group.rs, the derive() method intentionally always creates a BaseExceptionGroup instance rather than preserving the original exception class type. This is a deliberate design decision that differs from CPython's behavior.