import base64

class Base64Converter:

    regex = r"[a-zA-Z0-9+/]*={0,2}"

    def to_python(self, value):

        return base64.b64decode(value)

    def to_url(self, value):

        return base64.b64encode(value).decode("ascii")

class DynamicConverter:

    _dynamic_to_python = None

    _dynamic_to_url = None

    @property

    def regex(self):

        return r"[0-9a-zA-Z]+"

    @regex.setter

    def regex(self):

        raise Exception("You can't modify the regular expression.")

    def to_python(self, value):

        return type(self)._dynamic_to_python(value)

    def to_url(self, value):

        return type(self)._dynamic_to_url(value)

    @classmethod

    def register_to_python(cls, value):

        cls._dynamic_to_python = value

    @classmethod

    def register_to_url(cls, value):

        cls._dynamic_to_url = value