Count lines in large csv files

gistfile1.py

#!/usr/bin/env python

def bufcount(filename):
    f = open(filename)                  
    lines = 0
    buf_size = 1024 * 1024
    read_f = f.read # loop optimization

    buf = read_f(buf_size)
    while buf:
        lines += buf.count('\n')
        buf = read_f(buf_size)
    print str(lines) + ' lines'

bufcount('file.txt')