Exercise 2 solution

def gc(s):
    return (s.count("G") + s.count("C")) / len(s)

seqs = {"s1": "ATGCATGCAT", "s2": "GGCCGGCCAA", "s3": "ATATATATAT", "s4": "GCGCGCGCGC"}
for name, s in sorted(seqs.items(), key=lambda kv: gc(kv[1])):
    print(name, s, round(gc(s), 2))
print(max(seqs, key=lambda k: gc(seqs[k])))

Output:

s3 ATATATATAT 0.0
s1 ATGCATGCAT 0.4
s2 GGCCGGCCAA 0.8
s4 GCGCGCGCGC 1.0
s4

Reading it: sorted with a key function orders the items by their GC content, and max with the same idea picks the highest. The sequence with only G and C bases scores 1.0 and the one with only A and T scores 0.0.