ValueError: unsupported format character while forming strings

You could escape the % with another % so %%20

This is a similar relevant question Python string formatting when string contains "%s" without escaping

You could escape the % in %20 like so:

print "Hello%%20World%s" %"!"

or you could try using the string formatting routines instead, like:

print "Hello%20World{0}".format("!")

http://docs.python.org/library/string.html#formatstrings