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If you pass a valid PyUnicodeObject into PyUnicode_AsObject(), it incref's the original object and returns it.
If you pass a valid PyBytesObject into PyBytes_AsObject()... it fails.
I assert that in the PyBytes_AsObject() should behave like PyUnicode_AsObject() when faced with an identity transformation.
(Brett: I tagged you because I saw your name in the comments, so I figured you were a good candidate.)

The appropriate four line patch. (Six with whitespace.)

I suppose you are referring to PyUnicode_FromObject() and PyBytes_FromObject()...
And by "it fails" did you simply mean "it fails to return the same object"?

1) Yes, whoopsies. It's late.
2) It fails, as in, it returns NULL.

> 2) It fails, as in, it returns NULL.
It's not my experience. Do you have an example?

Well, the patch is nice but the PyObject_CheckBuffer(...) part should have succeeded, so it's a bit mysterious why it doesn't.

I also wonder how the buffer interface section can fail.
PyBuffer_ToContiguous() should translate to a simple memcpy()
for a bytes object. What is going on?

I can't reproduce this, and it was pretty late in my day when I saw it, so let's assume I was mistaken and PyBytes_CheckBuffer() works fine. Nevertheless I think the patch is a good idea--why create a new object when you don't have to? Unless I hear otherwise I'll check it in in the next day or so.
As for what I saw, I suspect it was a deliberate TypeError from the regression test suite, passing in an integer 0 as a filename to os.rename(). Naturally PyBytes_FromObject() fails to convert that to a bytes object.

Changing the name of the report to accurately reflect reality. If you passed in a bytes object, PyBytes_FromObject would create a new object, when all it really needed to do was incref and return. My checkin in a minute will add that shortcut.

New changeset f8cd75e8a1d6 by Larry Hastings in branch 'default':
Issue #14889: PyBytes_FromObject(bytes) now just increfs and returns.
http://hg.python.org/cpython/rev/f8cd75e8a1d6