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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?

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  • do you mean you want to split the six elements into two three element lines? Commented Feb 14, 2014 at 16:01
  • right, then I want to write it to a new csv Commented Feb 14, 2014 at 16:02
  • OK then, can you format your CSV lines as code? right now the two lines are on the same line Commented Feb 14, 2014 at 16:04
  • Have a look at this post on stackoverflow [here][1] [1]: stackoverflow.com/questions/9777409/… Commented Feb 14, 2014 at 16:12

3 Answers 3

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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.

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3 Comments

I am really unsure about the "wb" for writing csv. The documentation doesn't use this in the examples and I think you loose the encoding facility in this case. This supposes you manipulate bytes all the way (and only works because you read also with rb, or you get TypeError: 'str' does not support the buffer interface)
@Cilyan: Python 2 recommends you use rb so that the CSV reader and writer objects fully control line endings. Python 3 uses newline='' instead, to the same purpose. So the recommendation differs between the Python versions here.
@Cilyan: Since Python 2 is still used by a majority of people, I stick to that; the csv.writer documentation link goes to the Python 2 documentation, you may note.
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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
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I think there are several typos here (missing target for the second open, missing colon.)
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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:])
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