Py new by youknowone · Pull Request #6402 · RustPython/RustPython

youknowone

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# Find the PyBytesInner::try_from_borrowed_object implementation
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Repository: RustPython/RustPython

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🌐 Web query:

CPython bytes constructor __bytes__ return type error message TypeError

💡 Result:

When you call bytes(x), CPython will call x.bytes() (if present) and that method must return a bytes object. If it returns some other type, CPython raises a TypeError such as:

"TypeError: bytes returned non-bytes (type X)" (e.g. X == int or str). [1][2]

Fix: make bytes return a bytes object (e.g. return b"..." or return some_str.encode('utf-8')). [3]

Sources:

object.bytes doc / pydoc requirement that bytes should return bytes. [1]
Python tracker / example error message showing "bytes returned non-bytes (type ...)". [2]
Explanation and example usage showing the TypeError and correct return type. [3]

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The downcast usage on line 129 is correct; consider adding a specific error for __bytes__ returning non-bytes.

Line 129 correctly uses downcast::<PyBytes>() (which accepts subclasses) rather than downcast_exact. This matches CPython semantics: bytes() returns the __bytes__ result directly if it's any bytes instance, including subclasses.

However, when __bytes__ returns a non-bytes type, the current code delegates to PyBytesInner::try_from_borrowed_object, which raises a generic error: "can assign only bytes, buffers, or iterables of ints in range(0, 256)". CPython raises a more specific error: "TypeError: bytes returned non-bytes (type X)". Consider adding explicit validation to match CPython's error message.