It looks like you are using a scalar pandas_udf type, which doesn't support returning structs currently. I believe the return type you want is an array of strings, which is supported, so this should work. Try this: @pandas_udf("array<string>") def stringClassifier(x,y,z): # return a pandas series of a list of strings, that is same length as input - for example s = pd.Series([[u"a", u"b"]] * len(x)) return s If you are using Python 2, make sure your strings are in unicode otherwise they might get interpreted as bytes. Hope that helps! ... View more