input: Write a function to compare two strings and print the longer one.
  assert compare_strings('Inception', 'Interstellar')=='Interstellar'
  assert compare_strings('Hello', 'World')=='Hello'
  assert compare_strings('Python', 'Pythonista')=='Pythonista'
output: def compare_strings(str1, str2):
      if len(str1) >= len(str2):
          return str1
      else:
          return str2