怎么用python搜索文本并筛选出来？

txtfile=open(r'test.txt',"r")
newtxtfile=open(r'new.txt',"w")
linelist=[]
for line in txtfile:
    linelist.append(line)
    if len(linelist)==4:
        if not linelist[1].startswith(r'aaa'):
            newtxtfile.writelines(linelist)
        linelist=[]
if len(linelist)>1:
    if not linelist[1].startswith(r'aaa'):
            newtxtfile.writelines(linelist)
elif len(linelist)==1:
    newtxtfile.writelines(linelist)
txtfile.close()
newtxtfile.close()

读取文件test.txt，将每四行中第二行以aaa开始的去除，写入新文件new.txt中

a=file('a.txt','r').readlines()
b=[a[4*i:4*(i+1)] for i in range(len(a)/4) if a[4*i+1]=='aaa\n']
file('b.txt','w').write(''.join(b))