Simple Example: Step 1

new_simple_step1.py

class TemperatureConverter(metaclass=abc.ABCMeta):    
    """
    Base class of temperature converters which supports dynamic substitution
    of implementations.
    """
    symbol = 'K'

    def __new__(cls, convert_to='celsius'):
        if issubclass(cls, TemperatureConverter):
            if convert_to == 'celsius':
                cls = _CelsiusConverter
            elif convert_to == 'fahrenheit':
                cls = _FahrenheitConverter
            else:
                raise ValueError('unexpected converter: %s' % convert_to)
        return object.__new__(cls)

    def convert(self, value):        
        self.check_value(value)
        return self._convert(value)

    def format(self, value):
        return ('%.2f' % self.convert(value)) + self.symbol

    @staticmethod
    def check_value(value):
        if value < 0:
            raise ValueError('temperature should be provided in Kelvin degrees')

    @property
    def name(self):
        return self.__class__.__name__

    def _convert(self, value):
        raise NotImplementedError()