input: Write a function to check whether a string contains only digits. If the string contains only digits 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("012a3")==False
  assert is_digit_only("")==True
  assert is_digit_only("1234567890")==True
  assert is_digit_only("abc123")==False
output: def is_digit_only(string):
      for char in string:
          if not char.isdigit():
              return False
      return True