loop over 2 lists, repeating the shortest until end of longest

Question

I am sure there is an easy and obvious way to do this, but I have been googling and reading the docs and I just cannot find anything.

This is what I want to achieve:

la = ['a1','a2','a3','a4']
lb = ['b1','b2']
result = ['a1_b1','a2_b2','a3_b1','a4_b2']

I have a list of dates and some of them has something marked on them. I then have a much larger list of dates and I want to put the smallest list inside the bigger list as many times as possible. It will probably require some sort of loop as I need access to the dates in the larger list for the end result.

For some reason I just cannot see a good way to do this.

Answer 1

Try

result = ["_".join((i, j)) for i, j in itertools.izip(la, itertools.cycle(lb))]

Answer 2

Assuming la is longer than lb:

>>> import itertools
>>> [x+'_'+y for x,y in zip(la, itertools.cycle(lb))]
['a1_b1', 'a2_b2', 'a3_b1', 'a4_b2']

• itertools.cycle(lb) returns a cyclic iterator for the elements in lb.

• zip(...) returns a list of tuples in which each element corresponds to an element in la coupled with the matching element in the iterator.