[Tutor] Comparing two different text file

Steven D'Aprano steve at pearwood.info
Wed Jul 30 02:34:03 CEST 2014

On Wed, Jul 30, 2014 at 08:13:37AM +0900, 이명교 wrote:

> #!/usr/bin/env python
> list1 = []
> list2 = []
> list3 = []
> list4 = []
> #imports list from first text file
> inf1 = open('first.txt')
> for line in inf1.readlines(): 
>     list1.append(line)
>     list1 = line[:-1].split('\n')

Try this:

infile = open('first.txt')
list1 = infile.readlines()

The readlines() method already returns a list of all the lines in the 
file, so there's no need to iterate over the lines a second time just to 
put them into a list.

Now do the same for 'second.txt' to get the lines from it. No need to 
have separate "infile1" and "infile2" variables, you can re-use the 
variable. (But don't forget to close the files after you've read them.)

So now you have two lists, list1 and list2, containing the lines from 
the two files.

> #append items of list1
> for a in list1:
>     if a not in list1:
>         list3.append(a)

That cannot possibly do anything. You are testing each line of list1, to 
see if it *isn't* from list1. Of course it will be from list1.

I *think* that what you want is to make a copy of list1 and put it in 

list3 = []
for line in list1:

But wait! There's an easier way: list slicing.


So you ought to be able to make a copy of list1 in a single line.

Then you can walk over each line of list2, check whether or not that 
line is in list1 (NOT list2!!!), and append to list3:

for line in list2:
    if line not in list1:

P.S. no need to show us dozens and dozens of lines of data. Just show us 
a few examples, say three or five, not fifty or a hundred.


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