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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"`f(**d)` rejects str keys containing lone surrogates: TypeError: keywords must be strings","articleBody":"# Summary\n\nUnpacking a dict whose keys contain lone surrogates via `**` raises `TypeError: keywords must be strings`, but the keys *are* `str` instances. CPython accepts any `str` key through the `**` call path.\n\n```python\ndef collect(**kw):\n    return kw\n\ncollect(**{'\\udc81': 2})     # TypeError: keywords must be strings\ndict(**{'\\udc81': 1})        # TypeError: keywords must be strings\n```\n\nCPython 3.14.2:\n\n```\n\u003e\u003e\u003e collect(**{'\\udc81': 2})\n{'\\udc81': 2}\n```\n\n`'\\udc81'` is a real `str` (e.g. produced by `os.fsdecode` of undecodable bytes, `surrogateescape`-decoded data, etc.), so this rejects valid programs. Verified on `main` @ 7044fdcc8.\n\n# Cause\n\n`ExecutingFrame::collect_ex_args` (`crates/vm/src/frame.rs:6539`) validates kwargs keys with\n\n```rust\nkey.downcast_ref::\u003cPyUtf8Str\u003e()\n    .ok_or_else(|| vm.new_type_error(\"keywords must be strings\"))?;\n```\n\nA str containing a lone surrogate is a `PyStr` whose payload is Wtf8, not `PyUtf8Str`, so the downcast fails even though the key is a `str`. The same pattern exists at `frame.rs:6521`, `crates/vm/src/stdlib/_functools.rs` (partial), and `crates/capi/src/abstract_.rs`.\n\nThe deeper constraint is that `KwArgs`/`FuncArgs` keyword names are Rust `String` (UTF-8), so a surrogate-carrying key currently has no representation in the calling convention; fixing this properly means keyword names need to be `Wtf8Buf` (or `PyStrRef`) end-to-end. The error message check should be `downcast_ref::\u003cPyStr\u003e()` (matching CPython, which only rejects non-`str` keys) once the storage can hold it.\n\n---\n*Found while running synthetic benchmark/compat suites; researched and reported by Claude on behalf of @youknowone.*\n","author":{"url":"https://github.com/youknowone","@type":"Person","name":"youknowone"},"datePublished":"2026-07-06T15:34:33.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":1},"url":"https://github.com/8228/RustPython/issues/8228"}