Combining two lists into a list of lists

Question

I have two lists:

a = ['1', '2']
b = ['11', '22', '33', '44']

And I to combine them to create a list like the one below:

op = [('1', '11'), ('2', '22'), ('', '33'), ('', '44')]

How could I achieve this?

Answer 1

You want itertools.zip_longest with a fillvalue of an empty string:

a = ['1', '2']
b = ['11', '22', '33', '44']

from itertools import zip_longest # izip_longest for python2

print(list(zip_longest(a,b, fillvalue="")))
[('1', '11'), ('2', '22'), ('', '33'), ('', '44')]

For python2 it is izip_longest:

from itertools import izip_longest 

print(list(izip_longest(a,b, fillvalue="")))
[('1', '11'), ('2', '22'), ('', '33'), ('', '44')]

If you just want to use the values you can iterate over the the izip object:

for i,j  in izip_longest(a,b, fillvalue=""):
   # do whatever

Some timings vs using map:

In [51]: a = a * 10000

In [52]: b = b * 9000

In [53]: timeit list(izip_longest(a,b,fillvalue=""))
100 loops, best of 3: 1.91 ms per loop

In [54]: timeit [('', i[1]) if i[0] == None else i  for i in map(None, a, b)]
100 loops, best of 3: 6.98 ms per loop

map also creates another list using python2 so for large lists or if you have memory restrictions it is best avoided.