input: Write a function to determine if a given string contains only digits. If it does, return True; otherwise, 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("abc123")==False
  assert is_digit_only("123.45")==False
  assert is_digit_only("9876543210")==True
  assert is_digit_only("")==True
output: def is_digit_only(string):
      for char in string:
          if not char.isdigit():
              return False
      return True