gistfile1.py

#!/usr/bin/env python

import csv
import tempfile

def generator(reader):
    """Generates dictionaries from a DictReader that contain only 'a' and 'b' keys."""
    cols = ['a', 'b']
    for row in reader:
        yield dict((key,val) for key,val in row.iteritems() if key in cols)        

if __name__ == '__main__':
    data = ('a,b,c', '1,2,3', '4,5,6')
    dr = csv.DictReader(data)
    
    # Prints:
    # {'a': '1', 'b': '2'}
    # {'a': '4', 'b': '5'}
    for line in generator(dr):
        print line