Splitting a CSV file into multiple csv by target columns values

What you can do is get the value of the last column in each row. If the value is the same as the value in previous row, add that row to the same list, and if it's not just create a new list and add that row to that empty list. For data structure use list of lists.

Perhaps you want something like this:

from itertools import groupby
from operator import itemgetter

sep = '   '

with open('data.csv') as f:
    data = f.read()

split_data = [row.split(sep) for row in data.split('\n')]
gb = groupby(split_data, key=itemgetter(4))

for index, (key, group) in enumerate(gb):
    with open('sample{}.csv'.format(index), 'w') as f:
        write_data = '\n'.join(sep.join(cell) for cell in group)
        f.write(write_data)

Unlike pd.groupby, itertools.groupby doesn't sort the source beforehand. This parses the input CSV into a list of lists and performs a groupby on the outer list based on the 5th column, which contains the target. The groupby object is an iterator over the groups; by writing each group to a different file, the result you want can be achieved.

I propose to use a function to do what was asked for.

There is the possibility of leaving unreferenced the file objects that we have opened for writing, so that they are automatically closed when garbage collected but here I prefer to explicitly close every output file before opening another one.

The script is heavily commented, so no further explanations:

def split_data(data_fname, key_len=1, basename='file%03d.txt')

    data = open(data_fname)

    current_output = None # because we have yet not opened an output file
    prev_key = int(1)     # because a string is always different from an int
    count = 0             # because we want to count the output files

    for line in data:

        # line has a trailing newline so that to extract the key
        # we have to take into account that
        key = line[-key_len-1:-1]

        if key !=  prev_key     # key has changed!

           count += 1           # a new file is going to be opened
           prev_key = key       # remember the new key
           if current_output:   # if a file was opened, close it
               current_output.close()
           # open a new output file, its name derived from the variable count
           current_output = open(basename%count, 'w')

        # now we can write to the output file
        current_output.write(line)
        # note that line is already newline terminated

    # clean up what is still going
    current_output.close()

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