Unicode getting in the way...

[Dan Maas]

Python 2.0. All I want to do is construct and parse simple
binary packets. e.g.

foo = "abcd" + struct.pack("B", 254)
foo += "efg%s" % "lalala"

At some random point in my code, foo changes from being a
'string' object to a  'unicode' object. Subsequent attempts
to manipulate foo fail with a UnicodeError (ASCII decoding
error, ordinal not in range(128)).

How can I stop this from happening? I just need raw 8-bit
strings everywhere!

Thanks,
Dan