Use repeating iterator in python to insert item into list

Question

The solution below is working but I wanted to know if the code can be improved or if there is a more effective method of achieving the same results. I need to insert a "prefix" in the beginning of my list and I am using an iterator to do so. The prefix is 'a' for line 1, 'b' for line 2 and 'c' for line 3 and then restart at 'a' for line 4 etc..

test file:

this,is,line,one
this,is,line,two
this,is,line,three
this,is,line,four
this,is,line,five
this,is,line,six
this,is,line,seven
this,is,line,eight
this,is,line,nine

Code:

l = ['a','b','c']
it = iter(l)

with open('C:\\Users\\user\\Documents\\test_my_it.csv', 'rU') as c:
    rows = csv.reader(c)
    for row in rows:
        try:
            i = it.next()
            newrow = [i] + row
        except StopIteration:
            it = iter(l)
            i = it.next()
            newrow = [i] + row
        print(newrow)

Results are:

['a', 'this', 'is', 'line', 'one']
['b', 'this', 'is', 'line', 'two']
['c', 'this', 'is', 'line', 'three']
['a', 'this', 'is', 'line', 'four']
['b', 'this', 'is', 'line', 'five']
['c', 'this', 'is', 'line', 'six']
['a', 'this', 'is', 'line', 'seven']
['b', 'this', 'is', 'line', 'eight']
['c', 'this', 'is', 'line', 'nine']

"The solution below is working but I wanted to know if the code can be improved or if there is a more effective method of achieving the same results." - This is a good sign that you might be better off posting on CodeReview, read through that how to ask page and see if it might be relevant there. — SuperBiasedMan
– SuperBiasedMan, Commented Aug 11, 2015 at 14:33

jonrsharpe · Accepted Answer · 2015-08-11 14:34:16Z

6

This could be much simpler with itertools.cycle, which will handle endlessly repeating l for you:

from itertools import cycle, izip

l = ['a','b','c']

with open('C:\\Users\\user\\Documents\\test_my_it.csv', 'rU') as c:
    rows = csv.reader(c)
    for prefix, row in izip(cycle(l), rows):
        newrow = [prefix] + row

answered Aug 11, 2015 at 14:34

jonrsharpe

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3 Comments

jonrsharpe Over a year ago

@PadraicCunningham the people have spoken - they prefer puppies to booze!

Padraic Cunningham Over a year ago

It was non-alcoholic, unlike the very alcoholic wine I had last night!

jonrsharpe Over a year ago

@PadraicCunningham clearly you've convinced the OP, at least ;o)

Padraic Cunningham · Accepted Answer · 2015-08-11 14:38:37Z

4

Just cycle the list l using itertools.cycle, zipping the cycle object and your rows with itertools.izip:

from itertools import cycle, izip

l = ['a','b','c']
it = iter(l)
import csv
with open('in.csv', 'rU') as c:
    rows = csv.reader(c)
    for a, row in izip(cycle(l), rows):
        print([a]+ row)

Output:

['a', 'this', 'is', 'line', 'one']
['b', 'this', 'is', 'line', 'two']
['c', 'this', 'is', 'line', 'three']
['a', 'this', 'is', 'line', 'four']
['b', 'this', 'is', 'line', 'five']
['c', 'this', 'is', 'line', 'six']
['a', 'this', 'is', 'line', 'seven']
['b', 'this', 'is', 'line', 'eight']
['c', 'this', 'is', 'line', 'nine']

edited Aug 11, 2015 at 14:38

answered Aug 11, 2015 at 14:33

Padraic Cunningham

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