base_model_validation_errors.py

class ModelValidationError(object):

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

    @property
    def message(self):
        return None

    def __str__(self):
        return '%s: %s' % (self.key, self.message) if self.message else self.key

    def __eq__(self, other):
        if self.__class__ is other.__class__:
            return (self.key, self.message) == (other.key, other.message)
        return NotImplemented

    def __ne__(self, other):
        return not self.__eq__(other)

    def __hash__(self):
        return hash((self.__class__, self.key, self.message))


class TimeFieldModelValidationError(ValidationError):

    def __init__(self, model):
        self._message = (
            'Entity ID %s: The created_on field has a value %s which '
            'is greater than the value %s of last_updated field' % (
                model.id, model.created_on, model.last_updated))

    @property
    def message(self):
        return self._message


class ValidateModelTimeFields(beam.DoFn):

    def process(self, model):
        yield TimeFieldValidationError(model)