How about this:
a = 'ATGTATTATTAA'
codons = (a[n:n+3] for n in xrange(0,len(a),3)) # creates generator
dict_codons = {}
for codon in codons:
if dict_codons.has_key(codon):
dict_codons[codon] += 1
else:
dict_codons[codon] = 1
print dict_codons
To put it short, this code generates a generator that yields codons in frame 0, and counts them to store data in dictionary.