exceptions.py

class SchemaUpdateConflict:
    CONFLICT_TYPES = {
        'required_no_default':  ("Required field '{}' added "
                                 "without supplying a default value"),
        'incompatible_type':    "Incompatible change in type of field '{}'",
        'incompatible_choices': ("Choices list for field '{}' doesn't "
                                 "include current field value")
    }

    def __init__(self, field, typ, **kwargs):
        if typ not in self.CONFLICT_TYPES:
            raise ValueError(
                "Invalid type '{}' for SchemaUpdateConflict".format(typ)
            )
        self.field = field
        self.typ = typ
        self.args = kwargs

    def message(self):
        return self.CONFLICT_TYPES[self.typ].format(self.field)

    def __eq__(self, other):
        return self.field == other.field and self.typ == other.typ


class SchemaUpdateException(Exception):
    def __init__(self, *args, **kwargs):
        self.conflicts = kwargs.pop('conflicts', [])
        super().__init__(*args, **kwargs)

    @property
    def messages(self):
        if not hasattr(self, '_messages'):
            self._messages = list(map(lambda c: c.message(), self.conflicts))
        return self._messages

    def __str__(self):
        return 'SchemaUpdateException: ' + str(self.messages)

    def __repr__(self):
        return str(self)