Question

I have a CSV file with multiple entries. Example csv:

user, phone, email
joe, 123, [e-mail]
mary, 456, [e-mail]
ed, 123, [e-mail]

I’m trying to remove the duplicates by a specific column in the CSV however with the code below I’m getting an “list index out of range”. I thought by comparing row[1] with newrows[1] I would find all duplicates and only rewrite the unique entries in file2.csv. This doesn’t work though and I can’t understand why.

f1 = csv.reader(open('file1.csv', 'rb'))
    newrows = []
    for row in f1:
        if row[1] not in newrows[1]:
            newrows.append(row)
    writer = csv.writer(open("file2.csv", "wb"))
    writer.writerows(newrows)

My end result is to have a list that maintains the sequence of the file (set won’t work…right?) which should look like this:

user, phone, email
joe, 123, [e-mail]
mary, 456, [e-mail]

Answer 1

row[1] refers to the second column in the current row (phone). That’s all well in good.

However, you newrows.append(row) add the entire row to the list.

When you check row[1] in newrows you are checking the individual phone number against a list of complete rows. But that’s not what you want to do. You need to check against a list or set of just phone numbers. For that, you probably want to keep track of the rows and a set of the observed phone numbers.

Something like:

f1 = csv.reader(open('file1.csv', 'rb'))
writer = csv.writer(open("file2.csv", "wb"))
phone_numbers = set()
for row in f1:
    if row[1] not in phone_numbers:
        writer.writerow(row)
        phone_numbers.add( row[1] )