I was expecting that the API function PyArg_ParseTuple(args, "y#:foo",
&cp, &size) would accept a bytearray and implicitly convert it to bytes.
Currently it throws the error:
TypeError: foo() argument 1 must be bytes or read-only buffer, not bytearray

Yes, you must use y* instead: see
http://docs.python.org/dev/3.0/c-api/arg.html

y# would not be safe to use with bytearray since another thread could
mutate the bytearray in-between, possibly reallocating the internal
buffer (to shrink or grow it), and lead to a segfault when your thread
uses the obsolete pointer.

IMO, the documentation should mention that the '*' codes (y*, s*, etc.)
must be used in preference to the '#' codes, which are there for
backwards compatibility.

Documented in r66113.