os.walk trouble

[The Prophet]

As my first Python script, I am trying to make a program that recurses
a directory searching for files whose names match a pattern. I have a
basic idea of what the regexp would look like (and I can get more
information about that), but I am stuck with incorrect understanding of
os.walk. I've tried:

root, dirs, files = os.walk(dirname)

but it fails for some reason. I have seen this done before in the
tutorial (I think it's called sequence unpacking). What is the proper
way to do this?

Thanks in advance.