GC-content of a nucleotide sequence

(a) For example,

>>> seq = 'GATTACCTGAGCCTAC'
>>> (seq.count('G') + seq.count('C')) / len(seq)
0.5

(b) One solution is to create the complementary sequence using replace:

>>> seq = 'ACCTAGGT'
>>> seqc = seq.replace('A', 't').replace('C', 'g').replace('G', 'c')
              .replace('T', 'a').upper()
>>> seqc
'TGGATCCA'
>>> seqc == seq[::-1]
True

Note that this would not be a suitable method for long sequences because each replace operation generates an entirely new string in memory (recall that str is an immutable type).