"Take 2 strings s1 and s2 including only letters from a to z."

the input is a string

ignore the unhelpful comment above

a recursive function is a function in which the function itself is called within the function definition

def rec_func:
  #do some stuff
  rec_func

However this solution is a list comprehension, with syntax [value if condition(s) else value for value in iterable]

I'm sure you don't need this information anymore, but posting this just in case anybody else comes across this with the same question