input: Write a function to verify if a string consists solely of digits. If the string only contains digits, return True; otherwise, return False. If the string is empty, return True.
  
  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("0123a")==False
  assert is_digit_only("")==True
  assert is_digit_only("123.45")==False
  assert is_digit_only("     ")==False
output: def is_digit_only(string):
      for char in string:
          if not char.isdigit():
              return False
      return True