Create a dictionary by zipping together two lists of uneven length

You can also use a collections.deque() to create an circular FIFO queue:

from collections import deque

L1 = ['A', 'B', 'C', 'D', 'E']    
L2 = deque(['1', '2', '3'])

result = {}
for letter in L1:
    number = L2.popleft()
    result[letter] = number
    L2.append(number)

print(result)
# {'A': '1', 'B': '2', 'C': '3', 'D': '1', 'E': '2'}

Which pops the left most item currently in L2 and appends it to the end once the number is added to the dictionary.

Note: Both collections.deque.popleft() and collections.deque.append() are O(1) operations, so the above is still O(N), since you need to traverse all the elements in L1.

Use itertools.cycle:

from itertools import cycle

L1 = ['A', 'B', 'C', 'D', 'E']
L2 = ['1', '2', '3']

result = dict(zip(L1, cycle(L2)))

print(result)

Output

{'E': '2', 'B': '2', 'A': '1', 'D': '1', 'C': '3'}

As an alternative you could use enumerate and index L2 modulo the length of L2:

result = {v: L2[i % len(L2)] for i, v in enumerate(L1)}
print(result)

cycle is fine, but I shall add this modulo based approach:

{x: L2[i % len(L2)] for i, x in enumerate(L1)]}

Use itertools.cycle to cycle around to the beginning of L2:

from itertools import cycle
dict(zip(L1, cycle(L2)))
# {'A': '1', 'B': '2', 'C': '3', 'D': '1', 'E': '2'}

---

In your case, concatenating L2 with itself also works.

# dict(zip(L1, L2 * 2))
dict(zip(L1, L2 + L2))
# {'A': '1', 'B': '2', 'C': '3', 'D': '1', 'E': '2'}