如何从python中的日志文件中跳过每一行？

Question

The file for the code to use contains this bit of data:

<188> 2005 Sep 22 11:07:38 (FR114W-52-8f-a8) 66.190.168.225 UDP packet - Source:38.113.146.178,20841,WAN - Destination:66.190.168.225,1026,LAN [Drop] - [Inbound Default rule match]
#!^
<189> 2005 Sep 22 11:07:38 (FR114W-52-8f-a8) 66.190.168.225 Device Receive ICMP Packet - Source:192.168.1.201,[Echo Request],LAN - Destination:192.168.1.1,LAN [Receive]
#!^
<189> 2005 Sep 22 11:07:43 (FR114W-52-8f-a8) 66.190.168.225 Device Receive UDP Packet - Source:10.135.48.1,67,WAN - [Drop]

The code I have so far is:

import re
import string

with open('RouterLogger.log', 'r') as file:
     for line in file:
         words = line.split()
         print words
         print ("IP ", words[6], 'Time ', words[4])

The output for this code is this:

['#!^<188>', '2005', 'Sep', '22', '11:07:38', '(FR114W-52-8f-a8)', '66.190.168.225', 'UDP', 'packet', '-', 'Source:38.113.146.178,20841,WAN', '-', 'Destination:66.190.168.225,1026,LAN', '[Drop]', '-', '[Inbound', 'Default', 'rule', 'match]']   

('IP ', '66.190.168.225', 'Time ', '11:07:38')

['\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00#!^']   
Traceback (most recent call last):   
  File "/Users/PythonTutorials/print_line_with_match.py", line 10, in <module>   
    print ("IP ", words[6], 'Time ', words[4])   
IndexError: list index out of range   

Process finished with exit code 1

I would like to know how can I skip every second line to avoid this error. I know every other line causes an error because I get a Traceback error as soon as it hits the second line.

Answer 1

You can skip every second line explicitly:

evenline = False
with open('RouterLogger.log', 'r') as file:
     for line in file:
         if not evenline:
             words = line.split()
             print words
             print ("IP ", words[6], 'Time ', words[4])
         evenline = not evenline

Or you can (lazily) slice it with islice :

with open('RouterLogger.log', 'r') as file:
     for line in itertools.islice(file, 0, None, 2):
         words = line.split()
         print words
         print ("IP ", words[6], 'Time ', words[4])

Or you can iterate over pairs of lines instead of lines, using the pairwise function in the itertools recipes :

with open('RouterLogger.log', 'r') as file:
     for first, second in pairwise(file):
         words = first.split()
         print words
         print ("IP ", words[6], 'Time ', words[4])

However, are you absolutely sure that your format is "every second line"? If not, maybe you want to skip lines that start with # :

with open('RouterLogger.log', 'r') as file:
     for line in file:
         if not line.startswith('#'):
             words = line.split()
             print words
             print ("IP ", words[6], 'Time ', words[4])

… or try every line and skip the ones without enough words:

with open('RouterLogger.log', 'r') as file:
     for line in file:
         try:
             words = line.split()
             print words
             print ("IP ", words[6], 'Time ', words[4])
         except IndexError:
             pass

Answer 2

Rather than skipping a line during your for loop you can handle the exception by modifying your code to:

import re
import string

with open('RouterLogger.log', 'r') as file:
     for line in file:
         words = line.split()
         print words
         try:
            print ("IP ", words[6], 'Time ', words[4])
         except IndexError:
            continue