python - if statement and list index outside of range

Use len() or str.count()

for line in filename:
    fields=line.split(",")
    if len(fields) == 3:
        data_3_entries_list.append(line)

for line in filename:
    if fields.count(",") == 2:
        data_3_entries_list.append(line)

There is no implicit value for non-existent elements of a list; if fields only has 2 items, then fields[2] simply does not exist and will produce an error.

Check the length of the list explicitly:

if len(fields) == 3:
    break
data_3_entries_list.append(line)  # You may want append here, not extend

If i have a csv file like this one, then this can work out

 with open('datafile.csv', 'w') as p:
    p.write("Welcome,to,Python")
    p.write("Interpreted,Programming,Language")

 filename=open("datafile.csv","r")
 data_3_entries_list=[]

 for line in filename:
    fields=line.split(",")
    if fields[2] == 0:
        break
    else:
        data_3_entries_list.extend(line)

Otherwise you'd have to atleast show us or me how your csv file is formatted.

You're getting the error because you're trying to get the value of an index that may not exist. There's a better way to do this, though.

Instead of testing the length outright, you can also use a split to see if the index exists. A list that isn't len(a_list) >= 3 will simply return an empty list.

>>> mylist = range(3) #this list is [0, 1, 2]
>>> mylist[2:]
[2]

>>> mylist = range(2) #however THIS list is [0, 1]
>>> mylist[2:]
[]

In this way you can use the pythonic if a_list: do_work() to test if you even want to work with the data. This syntax is generally preferred to if len(a_list) == value: do_work().

filename=open("datafile.txt","r")
data_3_entries_list=[]

 for line in filename:
    fields=line.split(",")
    if fields[2:]:  ##use a split to verify that such a split yields a result
        data_3_entries_list.extend(line)