How to use the cerberus.errors.DocumentErrorTree function in Cerberus

def __init_processing(self, document, schema=None):
        self._errors = errors.ErrorList()
        self.recent_error = None
        self.document_error_tree = errors.DocumentErrorTree()
        self.schema_error_tree = errors.SchemaErrorTree()
        self.document = copy(document)
        if not self.is_child:
            self._is_normalized = False

        self.__init_schema(schema)

        if self.schema is None:
            if isinstance(self.allow_unknown, Mapping):
                self.schema = {}
            else:
                raise SchemaError(errors.MISSING_SCHEMA)

        if document is None:
            raise DocumentError(errors.DOCUMENT_MISSING)
        if not isinstance(document, Mapping):

def __init_processing(self, document, schema=None):
        self._errors = errors.ErrorList()
        self.recent_error = None
        self.document_error_tree = errors.DocumentErrorTree()
        self.schema_error_tree = errors.SchemaErrorTree()
        self.document = copy(document)
        if not self.is_child:
            self._is_normalized = False

        if schema is not None:
            self.schema = DefinitionSchema(self, schema)
        elif self.schema is None:
            if isinstance(self.allow_unknown, Mapping):
                self._schema = {}
            else:
                raise SchemaError(errors.SCHEMA_ERROR_MISSING)
        if document is None:
            raise DocumentError(errors.DOCUMENT_MISSING)
        if not isinstance(document, Mapping):
            raise DocumentError(errors.DOCUMENT_FORMAT.format(document))

__init__(self, schema=None, ignore_none_values=False,
                 allow_unknown=False, purge_unknown=False,
                 error_handler=errors.BasicErrorHandler)
        """

        self.document = None
        """ The document that is or was recently processed.
            Type: any :term:`mapping` """
        self._errors = errors.ErrorList()
        """ The list of errors that were encountered since the last document
            processing was invoked.
            Type: :class:`~cerberus.errors.ErrorList` """
        self.recent_error = None
        """ The last individual error that was submitted.
            Type: :class:`~cerberus.errors.ValidationError` """
        self.document_error_tree = errors.DocumentErrorTree()
        """ A tree representiation of encountered errors following the
            structure of the document.
            Type: :class:`~cerberus.errors.DocumentErrorTree` """
        self.schema_error_tree = errors.SchemaErrorTree()
        """ A tree representiation of encountered errors following the
            structure of the schema.
            Type: :class:`~cerberus.errors.SchemaErrorTree` """
        self.document_path = ()
        """ The path within the document to the current sub-document.
            Type: :class:`tuple` """
        self.schema_path = ()
        """ The path within the schema to the current sub-schema.
            Type: :class:`tuple` """
        self.update = False
        self.error_handler = self.__init_error_handler(kwargs)
        """ The error handler used to format :attr:`~cerberus.Validator.errors`

def __init_processing(self, document, schema=None):
        self._errors = errors.ErrorList()
        self.recent_error = None
        self.document_error_tree = errors.DocumentErrorTree()
        self.schema_error_tree = errors.SchemaErrorTree()
        self.document = copy(document)
        if not self.is_child:
            self._is_normalized = False

        if schema is not None:
            self.schema = DefinitionSchema(self, schema)
        elif self.schema is None:
            if isinstance(self.allow_unknown, Mapping):
                self._schema = {}
            else:
                raise SchemaError(errors.SCHEMA_ERROR_MISSING)
        if document is None:
            raise DocumentError(errors.DOCUMENT_MISSING)
        if not isinstance(document, Mapping):
            raise DocumentError(errors.DOCUMENT_FORMAT.format(document))

allow_unknown=False, require_all=False,
                 purge_unknown=False, purge_readonly=False,
                 error_handler=errors.BasicErrorHandler)
        """

        self.document = None
        """ The document that is or was recently processed.
            Type: any :term:`mapping` """
        self._errors = errors.ErrorList()
        """ The list of errors that were encountered since the last document
            processing was invoked.
            Type: :class:`~cerberus.errors.ErrorList` """
        self.recent_error = None
        """ The last individual error that was submitted.
            Type: :class:`~cerberus.errors.ValidationError` """
        self.document_error_tree = errors.DocumentErrorTree()
        """ A tree representiation of encountered errors following the
            structure of the document.
            Type: :class:`~cerberus.errors.DocumentErrorTree` """
        self.schema_error_tree = errors.SchemaErrorTree()
        """ A tree representiation of encountered errors following the
            structure of the schema.
            Type: :class:`~cerberus.errors.SchemaErrorTree` """
        self.document_path = ()
        """ The path within the document to the current sub-document.
            Type: :class:`tuple` """
        self.schema_path = ()
        """ The path within the schema to the current sub-schema.
            Type: :class:`tuple` """
        self.update = False
        self.error_handler = self.__init_error_handler(kwargs)
        """ The error handler used to format :attr:`~cerberus.Validator.errors`