"".join(set(foo))
set() will create a set of unique letters in the string, and "".join() will join the letters back to a string in arbitrary order.
If order does matter, you can use collections.OrderedDict in Python 2.7:
from collections import OrderedDict
foo = "mppmt"
print "".join(OrderedDict.fromkeys(foo))
printing
mpt
原文：http://www.jb51.net/article/47336.htm