Reading and Writing CSV Files with Python DictReader and DictWriter

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Python ScriptingPython can be an extremely powerful tool for reading and writing csv files. Here I’ll demonstrate an example of each using the csv DictWriter function.

Sample File: (test.csv)
firstname,lastname,age,favoritecolor
john,smith,24,red
jane,doe,26,blue
sophe,miller,33,yellow
jason,doe,21,red

From here, I will import the csv file into a dictionary array using the csv DictReader function in the python console.

import csv
test_file = 'test.csv'
csv_file = csv.DictReader(open(test_file, 'rb'), delimiter=',', quotechar='"')

You can now parse through the data as a normal array.

for line in csv_file:
    print line['age']

24
26
33
21

Writing an array to a csv file with DictWriter

Here is an example of using DictWriter to write a csv file. The main difference is that the fieldnames have to be entered as a separate array. The fieldnames have to match all of the keys in the dictionary array but can be ordered differently.

test_array = []
test_array.append({'fruit': 'apple', 'quantity': 5, 'color': 'red'});
test_array.append({'fruit': 'pear', 'quantity': 8, 'color': 'green'});
test_array.append({'fruit': 'banana', 'quantity': 3, 'color': 'yellow'});
test_array.append({'fruit': 'orange', 'quantity': 11, 'color': 'orange'});
fieldnames = ['fruit', 'quantity', 'color']
test_file = open('test2.csv','wb')
csvwriter = csv.DictWriter(test_file, delimiter=',', fieldnames=fieldnames)
csvwriter.writerow(dict((fn,fn) for fn in fieldnames))
for row in test_array:
     csvwriter.writerow(row)
test_file.close()

The writeheader() function was added to DictWriter in 2.7 and gives a cleaner way to write the first row of the csv file. You could replace line 9 with it, but I would advise against it to maintain backward compatibility.

Results of test2.csv after using the csv DictWriter:
fruit,quantity,color
apple,5,red
pear,8,green
banana,3,yellow
orange,11,orange

Written by Jim Biggs

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