This works in py3k but not in 2.x. I don't know if it is deliberate:

Python 3.0b2+ (py3k, Jul 27 2008, 12:52:40) 
[GCC 4.3.1 20080626 (prerelease)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> b"".join([bytearray(b"")])
b''

Python 2.6b2+ (trunk, Aug  1 2008, 01:47:52) 
[GCC 4.3.1 20080626 (prerelease)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> b"".join([bytearray(b"")])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: sequence item 0: expected string, bytearray found