In Python 2, the following works:

s = pd.Series(np.array(list('abc'), 'S1').astype(object)) t = pd.Series(np.array(list('def'), 'S1').astype(object)) s.str.cat(t) # 0 b'ad' # 1 b'be' # 2 b'cf' # dtype: object 

whereas in Python 3, s.str.cat(t) yields:

TypeError: sequence item 0: expected str instance, bytes found 

Clearly, Python 3 fundamentally changed the separation between bytes and strings, but in principle, there is no reason (other than implementation) why this should not have the same result in Python 3 as in Python 2.