如果在一個4G的大檔案test.log中提取裡面有error的log:

1. 大家第一想法就是通過開啟檔案然後每一行的查詢，耗時19s

   import  time
   
   start_time = time.time()
   with open('/Users/test/Downloads/test.log') as f:
       for line in f.readlines():
           if 'error' in line:
               print line
   end_time = time.time()
   print "cost time is: ", end_time-start_time
   
   #cost time is:  19.1660439968
    
   

    

2. 列表解析：大多數工作在python直譯器內部完成，比等價的語句要快很多，特別是大檔案,我們可以看見在相同的大檔案中使用列表解析時間為4s，大大提高了效率

   import  time
   
   start_time = time.time()
   lines = [line for line in open('/Users/test/Downloads/test.log') if 'error' in line]
   for line in lines:
       print line
   end_time = time.time()
   print "cost time is: ", end_time-start_time
   
   # cost time is:  4.45141601562