Filter
        Performs value filtering and returns filtered result.

        :param value:           input value
        :param model:           parent model being validated
        :param context:         object, filtering context
        :return:                filtered value

def filter(self, value, model=None, context=None):
        """
        Filter
        Performs value filtering and returns filtered result.

        :param value:           input value
        :param model:           parent model being validated
        :param context:         object, filtering context
        :return:                filtered value
        """
        http = ['http://', 'https://']
        if all(not str(value).startswith(s) for s in http):
            value = 'http://{}'.format(value)

        return value