A python solution for http://rosalind.info/problems/gc/.

gc-solution.py

#!/usr/bin/env python
from fileinput import input as data
from itertools import imap

def ifasta(lines):
    name, dna = None, ''

    for line in imap(str.rstrip, lines):
        if line.startswith('>'):
            if name:
                yield name, dna
            name, dna = line[1:], ''
        else:
            dna += line

    if name:
        yield name, dna

def gc(dna):
    return sum(c=='G' or c=='C' for c in dna) * 100.0 / len(dna)

igcname = ((gc(dna), name) for name, dna in ifasta(data()))
maxgc, maxname = max(igcname)

print maxname
print maxgc