Seq = "AA" #This the sequence of characters

def complement (Seq):
    for nuc in Seq: # converting the sequence of characters into the desirable character
        if nuc == 'A':
           comp = 'T'
    return comp

print "The complement of the Sequence AA is", complement(Seq)

if nuc == 'A':
       comp = 'T'
return comp

>>> Seq = "AA"
>>> Seq.replace('A','T')
'TT'

>>> Seq = "AA"
>>> ''.join(['T' for i in Seq if i=='A'])
'TT'

complement = {"A":"T", "C":"G", "G":"C", "T":"A"}
seq = "ACGT"
complement_seq = "".join([complement[b] for b in seq])
complement_seq

TGCA

from Bio.Seq import Seq
my_seq = Seq("ACGT")
my_seq.complement()

Seq('TGCA', Alphabet())