Reading and formatting CSV file (New Line Insertion)

Question

I have a CSV file that has numerous data points included in each row, despite belonging to the same column. Something similar to this:

A, B, C, X, Y, Z

Now, what I would like to do is to reformat the file such that the resulting CSV is:

A, B, C
X, Y, Z

I'm not too sure how to go about this / express it in a pythonic way. One idea (which I wasn't sure how to code) was to create a for loop similar to

For elements in file:
   new_csv = []
   counter = 0
   max_counter = 3

   ##I'm using english because I dont know how to express it in python syntax
   take first element in CSV and add 1 to counter
   append first element to new_csv
   is counter < max_counter?
       if yes: go to next element
       if no: append /n to new csv then go to next element

This probably is a terrible way to do it, but I'm struggling to find another way. How would I express this in the python syntax? Any ideas?

Answer 1

Just write two rows to an output csv.writer() object :

with open(inputfilename, 'rb') as infh, open(outputfilename, 'wb') as outfh:
    reader = csv.reader(infh)
    writer = csv.writer(outfh)

    for row in reader:
        writer.writerows([row[:3], row[3:]])

Each row the csv.reader() object produces is a list object, so all we have to do is using the Python slice notation to slice of a new list object of the first 3 elements, and another list object for the remaining 3 elements.

Answer 2

You could probably use python's CSV module

Example:

#!/usr/bin/env python
import csv

with open("input.csv", "r") as input_file, open("output.csv", "w+"):
    input_csv, output_csv = csv.reader(input_file), csv.writer(output_file);
    for row in input_csv:
        output_csv.writerows([row[:len(row)/2],row[len(row)/2]]);

Which given the input

a,b,c,d,e,f
A,B,C,D,E,F,G
1,2,3,4,5,6

will output

a,b,c
d,e,f
A,B,C
D,E,F,G
1,2,3
4,5,6

Answer 3

This solution is for Python 3. For Python 2, see Martijn Pieters's answer (differs in the way files are opened).

Python has a module to read and write csv files. There are some nice examples in the official documentation on how to use them. (Note the newline="" for output file, this lets the csv module take care of line return, which according to the specification should be \\r\\n , \\n being used for line feed in a cell.)

While reading, you then split the rows with slicing and feed the parts in the writer.

import csv

# with ensures files are closed when leaving block
with open("in.csv", "r") as fin, open("out.csv", "w", newline="") as fout:
    reader = csv.reader(fin, delimiter=",")
    writer =csv.writer(fout, delimiter=",", quotechar='"', quoting=csv.QUOTE_MINIMAL)
    # Rows are each line of the csv returned as a list
    for row in reader:
        # Separate the row:
        # from beginning to 3rd element, and from 3rd element to end
        writer.writerows((row[:3], row[3:])