统计列表中元素出现的频率

cnt ={}
ll = 'abcdefgadefz'
for i in ll:
   if not cnt.has_key(i):cnt[i]=0
   else: cnt[i]+=1

cnt={}

ll='abcdefgadefz'
for i in ll:
    cnt[i] = cnt.get(i,0)+1

cnt={}
ll='abcdefgadefz'
for i in ll:
    cnt[i] = cnt.setdefaut(i,0)+1