input: Write a function to return the length of a given string.
  
  def string_length(s):
      return len(s)
  
  assert string_length("Python")==6
  assert string_length("Java")==4
  assert string_length("Programming")==11
  assert string_length("")==0
  assert string_length("Hello, World!")==13
output: def string_length(s):
      return len(s)