python提取关键字所在行的后边几行

cat 1.txt

a1 =

  (DESCRIPTION =

    (ADDRESS = (PROTOCOL = TCP)(HOST = oracle)(PORT = 1000))

    (CONNECT_DATA =

      (SERVER = aaa)

      (SERVICE_NAME = aaa)

a2 =

  (DESCRIPTION =

    (ADDRESS = (PROTOCOL = TCP)(HOST = bbb-vip)(PORT = 1000))

    (CONNECT_DATA =

      (SERVER = ccc)

      (SERVICE_NAME = bbb)

a3 =

  (DESCRIPTION =

    (ADDRESS = (PROTOCOL = TCP)(HOST = ccc-vip)(PORT = 1000))

    (CONNECT_DATA =

      (SERVER = ccc)

      (SID = ccc)

#!/usr/bin/env python
# -*- coding: utf-8 -*-
b = ['a1', 'a2']
with open('1.txt', 'r') as f:
    a = []
    lines = f.readlines()
    print(lines[0])
    for x in lines:
        if x.startswith('a'):
            a.extend([x.strip().split()[0], lines.index(x),])
    for i in b[:-1]:
        if i in a:
            c = a.index(i)
            print ''.join(lines[a[c+1]:a[c+3]])
    if b[-1] == a[-2]:
        print ''.join(lines[a[int(a.index(b[-1]) + 1)]:])
    else:
        print ''.join(lines[a[int(a.index(b[-1])) + 1]:a[int(a.index(b[-1])) + 3]])