Python list comprehension with two lists: evaluate lists

I have two lists:

input_list = ['a','b']
existing_list = ['q','w','r']

I want to create a new list that will be equal to input_list if it's not empty else it should be equal to existing_list.

The easiest way to do this is to use the plain if-else:

if input_list:
  list_to_use = input_list
else:
  list_to_use = existing_list

Is it possible to do this in a list comprehension or in any other manner that is more concise?

list_to_use = [x if input_list else y for y in existing_list for x in input_list]

The closest I could get is with this one that produces ['a', 'b', 'a', 'b', 'a', 'b'], a wrong result.