CSV Columns to Arrays in Python

Question

I'm having a hard time dealing with what seems to me to be a simple problem. I'm trying to import a csv and split its columns into arrays that I can run different operations on, then zip() back together.

import csv

data = csv.reader(open('test.csv', 'rb'), delimiter=",", quotechar='|')
column1, column2 = [], []

for row in data:
    column1.extend(row[0])
    column2.extend(row[1])

print column1
print column2

This code prints two arrays with elements that are individual chars rather than strings. When I try to do this with a single column, column1.extend(row) does what I want.

I'm interested in ways to solve this particular problem or to generalize this to n number of columns.

Answer 1

You need to change column1.extend(row[0]) to column1.append(row[0]) (and the same for column2, clearly). Extend is for adding the contents of one list to another, append is for adding a single element. Extend is telling python to treat the string as a list of its chars and append each char.

>>> lst = []
>>> lst.extend("foo")
>>> lst
['f', 'o', 'o']
>>> lst.append("foo")
>>> lst
['f', 'o', 'o', 'foo']