python统计日志ip访问数的方法

import re
f=open("/tmp/a.log","r")
arr={} 
lines = f.readlines()
for line in lines: 
  ipaddress=re.compile(r'^#(((2[0-4]\d|25[0-5]|[01]"->"+str(arr[key])

#111.172.249.84 - - [12/Dec/2011:05:33:36 +0800] "GET /images/i/goTop.png HTTP/1.0" 200 486 "http://wh.xxxx.com/" "Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.1; Trident/5.0; SLCC2; .NET CLR 2.0.50727; .NET CLR 3.5.30729; .NET CLR 3.0.30729; Media Center PC 6.0; .NET4.0C; .NET4.0E)" 
#111.172.249.84 - - [12/Dec/2011:05:33:36 +0800] "GET /images/i/goTop.png HTTP/1.0" 200 486 "http://wh.xxxx.com/" "Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.1; Trident/5.0; SLCC2; .NET CLR 2.0.50727; .NET CLR 3.5.30729; .NET CLR 3.0.30729; Media Center PC 6.0; .NET4.0C; .NET4.0E)" 
#111.172.249.85 - - [12/Dec/2011:05:33:36 +0800] "GET /images/i/goTop.png HTTP/1.0" 200 486 "http://wh.xxxx.com/" "Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.1; Trident/5.0; SLCC2; .NET CLR 2.0.50727; .NET CLR 3.5.30729; .NET CLR 3.0.30729; Media Center PC 6.0; .NET4.0C; .NET4.0E)" 
#111.172.249.86 - - [12/Dec/2011:05:33:36 +0800] "GET /images/i/goTop.png HTTP/1.0" 200 486 "http://wh.xxxx.com/" "Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.1; Trident/5.0; SLCC2; .NET CLR 2.0.50727; .NET CLR 3.5.30729; .NET CLR 3.0.30729; Media Center PC 6.0; .NET4.0C; .NET4.0E)" 

111.172.249.86->1
111.172.249.84->2
111.172.249.85->1

> a={} 
> a['key']='123' 
> print (a) 
{'key': '123'} 
> print (a.setdefault('key','456'))
#显示a这个字典的'key'值的内容,因为字典有，所以不会去设置它 
123 
> print (a.setdefault('key1','456'))
#显示a这个字典的'key1'值的内容，因为字典没有，所以设置为456了 
456 
> a 
{'key1': '456', 'key': '123'}