Exercise 1 solution

data = """>alpha first
GGCCGGAT
TATTAAGC
>beta second
atatatat
atGCat
>gamma third
ACGTNNACGT
>delta fourth
GGGGCCCC
"""

def parse_fasta(text):
    records = {}
    name = None
    for line in text.splitlines():
        line = line.strip()
        if not line:
            continue
        if line.startswith(">"):
            name = line[1:].split()[0]
            records[name] = ""
        else:
            records[name] += line.upper()
    return records

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

best = None
for name, s in parse_fasta(data).items():
    bad = sorted(set(s) - set("ACGT"))
    if bad:
        print(name, len(s), "skipped, unexpected characters:", bad)
        continue
    print(name, len(s), round(gc(s), 3))
    if best is None or gc(s) > best[1]:
        best = (name, gc(s))
print("highest GC:", best[0], best[1])

Output:

alpha 16 0.5
beta 14 0.143
gamma 10 skipped, unexpected characters: ['N']
delta 8 1.0
highest GC: delta 1.0

Reading it: the parser joins the wrapped lines of each record and upper-cases them, so the lower-case record is read the same way as the others. A record with a character outside A, C, G and T (here N, an unknown base) is skipped with a message instead of giving a misleading GC value. The record made only of G and C scores 1.0 and wins.