>chr_1
atcgaatt
>chr_2
atcgc
gat
>chr_3
atcgCGcgat
>chr_4
CGCGCGcgcg

#!/usr/bin/python
import sys
def gc(seq):
	seq=seq.upper()
	return (float(seq.count("G")+seq.count("C"))/len(seq))

f=open(sys.argv[1],"r")

dic={}
for line in f:
	line=line.strip()
	if line.startswith('>'):
		head=line
		dic[head]=""
	else:
		dic[head]+=line
for head,seq in dic.items(): ##
	print head
	print gc(seq)