How to interleave two lists of different length?

Question

I'm quite new to Python and I am still having a hard time actually using the language itself into my program. Here's what I have so far:

# Purpose: 'twolists' = takes 2 lists, & returns a new list containing
# alternating elements of lists. 
# Return = final_list
# Parameter = list1, list2

def twolists(list1, list2): # don't forget to return final_list
    alt_list = []
    a1 = len(list1)
    a2 = len(list2)

    for i in range(# ? ):
        # append one thing from list1 to alt_list - How?
        # append one thing from list2 to alt_list - How?

Now the program is supposed to yield outputs like these:

outcome = twolists([ ], ['w', 'x', 'y', 'z'])
print(outcome)
['w', 'x', 'y', 'z']

outcome = twolists([0, 1], ['w', 'x'])
print(outcome)
[0, 'w', 1, 'x']

outcome = twolists([0, 1], ['w', 'x', 'y', 'z'])
print(outcome)
[0, 'w', 1, 'x', 'y', 'z']

outcome = twolists([0, 1, 2, 3], ['w', 'x'])
print(outcome)
[0, 'w', 1, 'x', 2, 3]

Answer 1

This composes a list comprehension using zip_longest from itertools (which is part of the standard library) to interleave items from both lists into a tuple, which by default uses None as the fillvalue.

This also uses chain also from itertools to flatten the list.

Finally it filters the None items from the list:

from itertools import chain, zip_longest
def twolists(l1, l2):
    return [x for x in chain(*zip_longest(l1, l2)) if x is not None]

Or as recommended from @EliKorvigo, use itertools.chain.from_iterable for iterating lazily:

def twolists(l1, l2):
    return [x for x in chain.from_iterable(zip_longest(l1, l2)) if x is not None]

Testing

In [56]: twolists([0, 1], ['w', 'x'])
Out[56]: [0, 'w', 1, 'x']

In [57]: twolists([0, 1], ['w', 'x', 'y', 'z'])
Out[57]: [0, 'w', 1, 'x', 'y', 'z']

In [74]: twolists([0, 1, 2, 3], ['w', 'x'])
Out[74]: [0, 'w', 1, 'x', 2, 3]

Answer 2

def twolists(list1, list2):
    newlist = []
    a1 = len(list1)
    a2 = len(list2)

    for i in range(max(a1, a2)):
        if i < a1:
            newlist.append(list1[i])
        if i < a2:
            newlist.append(list2[i])

    return newlist

Answer 3

def CombineLists(lst1, lst2):
   return [item for x in zip(lst1,lst2) for item in x] + /
         (lst2[len(lst1):] if len(lst2)>len(lst1) else lst1[len(lst2):])