input: Write a function to check whether a string contains only digits. Return True if the string contains only digits, else return False.
  def is_digit_only(string):
      for char in string:
          if not char.isdigit():
              return False
      return True
  
  assert is_digit_only("0123")==True
  assert is_digit_only("012a3")==False
  assert is_digit_only("")==True
  assert is_digit_only("12345")==True
  assert is_digit_only("12.34")==False
output: def is_digit_only(string):
      for char in string:
          if not char.isdigit():
              return False
      return True