python csv文件和for循环

Question

First time caller here ...

For some reason, I am not getting the desired result from my "for" loops. Below is the snippet of code. The loops work perfectly first time through as LOOP 2 does in fact return back to LOOP 1. But second time through LOOP 2 does not execute. I haven't written code in 25 years, but loving Python. Just need to get through some initial trial and error. Thanks in advance to anyone who can advise on this topic.

import csv

filename='cookie_data.csv'
lookup_filename='ip_addresses.csv'

with open(filename) as file_object, open(lookup_filename) as lookup_file_object:

    reader=csv.DictReader(file_object)
    l_reader=csv.DictReader(lookup_file_object)

    for row in reader:
        user_ip=row['ip_address']
        print("LOOP 1 " + user_ip)

        for l_row in l_reader:
            lookup_ip=l_row['lookup_ip']
            print("LOOP 2 " + user_ip + lookup_ip)

The output looks like this: LOOP 1 99.246.228.140 LOOP 2 99.246.228.140 LOOP 2 137.104.238.184 LOOP 2 137.104.238.184 LOOP 2 99.246.228.140 LOOP 2 81.182.15.119 LOOP 1 137.104.238.184 LOOP 1 137.104.238.184 LOOP 1 74.110.238.103

I intend to test for a match of the string(ip address) each time through the loop. The first loop is fine, then second time through it does not execute loop 2.

Answer 1

The problem is that your readers are attached to streams. As such, you are only expected to iterate through them once. The next time you iterate through, it's already at end of file, so it doesn't find anything else (hence "the second loop doesn't execute").

You can fix this by seek ing the fileobject back to the start each time you finish with it:

import os

for row in reader:
    user_ip=row['ip_address']
    print("LOOP 1 " + user_ip)

    for l_row in l_reader:
        lookup_ip=l_row['lookup_ip']
        print("LOOP 2 " + user_ip + lookup_ip)

    # I prefer to explicitly say where we're seeking from, though I 
    # could have just left it as seek(0).
    lookup_file_object.seek(0, os.SEEK_SET)