Python check if column exists in list

Question

I'm a beginner programmer writing a Python script which ultimately needs to pull the contents of three columns and write them to a new text file. When testing the script, i keep getting the error: IndexError: list index out of range applying to the third column I need to pull.

My code:

for ip in open("list.txt"):
    with open(ip.strip()+".txt", "a") as ip_file:
        for line in open("data.txt"):
            new_line = line.split(" ")
            if "blocked" in new_line:
                if "src="+ip.strip() in new_line:
                    ip_file.write(", " + new_line[11])
                    ip_file.write(", " + new_line[12])
                    ip_file.write(", " + new_line[14] + "\n")

In a few of the lines in data.txt, there is no 15th column. I have tried a couple ways to avoid the error message, but I still get the error:

if "src="+ip.strip() in new_line:
    ip_file.write(", " + new_line[11])
    ip_file.write(", " + new_line[12])
    if new_line[14] is None:
        pass
    else:
        ip_file.write(", " + new_line[14] + "\n")

and

if "src="+ip.strip() in new_line:
    ip_file.write(", " + new_line[11])
    ip_file.write(", " + new_line[12])
    if not new_line[14]:
        pass
    else:
        ip_file.write(", " + new_line[14] + "\n")

Question: Is there another way to check if the column exists and ignore if does not?

Thanks!

Ashwini Chaudhary · Accepted Answer · 2013-08-22T13:51:11.973

2

Use try-except:

ip_file.write(", " + new_line[12])
try:
    ip_file.write(", " + new_line[14] + "\n")
except IndexError:
    pass

By doing if new_line[14] is None or if not new_line[14] you're trying to check the value that exists at 14th index, but in python if there's no such index then IndexError is raised.

_{(In languages like javascript undefined is returned for non-existent indexes but that's not the case with python)}

edited Aug 22 '13 at 13:51

answered Aug 22 '13 at 13:42

Ashwini Chaudhary

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It's a common practice in Python, to simply try something instead of checking first, if it is possible/existing (EAFP style ("it's Easier to Ask for Forgiveness than Permission")) – Robert Caspary Aug 22 '13 at 13:46

score 0 · Answer 2 · edited May 23 '17 at 12:21

0

The error is caused by the lookup itself (new_line[14]), so you have to check the length of the row like if len(new_line) > 12. The other approach is handling the IndexError exception -- both are good, and usually the programmer can decide which approach to use. See "Ask forgiveness not permission" - explain

edited May 23 '17 at 12:21

Community

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answered Aug 22 '13 at 13:42

dnet

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score 0 · Answer 3 · answered Aug 22 '13 at 13:44

Using try will avoid your exception.

try:
    ip_file.write(", " + new_line[11])
except IndexError:
    pass   
try:
    ip_file.write(", " + new_line[12])
except IndexError:
    pass
try:
    ip_file.write(", " + new_line[14])
except IndexError:
    ip_file.write("\n")

You were getting an index out of bounds exception when you were doing your comparison to see if it existed.

NOTE:
This is probably not what you want to do since each column is likely unique and contains specific information. If column 2 is missing, this code will not behave as you expect. We would need to know what is in the columns to help you with that.

Python check if column exists in list

3 Answers3

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