object instance confusion in python

Question

Hi I am trying to manipulate csv file as below

import csv
data=csv.reader(open('data.csv', 'rb'))
a= [row for row in data]
print a

The output of the code is (comma spearated rows--> in list format):

 [['aaaaa', '1234234'], ['bbbb', '2343536'], ['cccc', '5675675'], ['dddd', '2344234'],    ['eeee', '5435324'], ['fffffff', '4353442']]

I will get null value when I use

print [row for row in data] 

I will be helped to have views to this confusion. Thank You

Answer 1

csv.reader returns an iterator which is good for only one pass through the data. You are probably trying to iterate through data twice; the second time, there is no content left in the interator, so

print [row for row in data] 

prints an empty list.

If you want to iterate through data more than once, and have enough memory, you could convert data to a list:

data = list(csv.reader(open('data.csv', 'rb')))

If you don't have enough memory, you would have to parse the CSV again.