如何从python中的列表列表中获取n个最大列表

我正在使用heapq从列表列表中获取最大的元素.我写的程序如下.

import csv
import heapq
f = open("E:/output.csv","r")
read = csv.reader(f)

allrows = [row for row in read]

for i in xrange(0,2):
     print allrows[i]

allrows.sort(key=lambda x: x[2]) #this is working properly
it=heapq.nlargest(20,enumerate(allrows),key=lambda x:x[2]) #error

我只想要前20名的元素.因此,我没有考虑使用堆.我得到的错误是,

  Traceback (most recent call last):
File "D:\eclipse_progs\DaDv\IMDB\Assignment1.py", line 42, in 
  it=heapq.nlargest(2,enumerate(allrows),key=lambda x:x[2])
File "C:\Python27\lib\heapq.py", line 470, in nlargest
  result = _nlargest(n, it)
File "D:\eclipse_progs\DaDv\IMDB\Assignment1.py", line 42, in 
  it=heapq.nlargest(2,enumerate(allrows),key=lambda x:x[2])
IndexError: tuple index out of range

我能知道为什么我会收到错误以及如何解决它.有没有使用heapq的属性我缺少.