class Serializer

    def _read_only_defaults(self):
        fields = [
            field for field in self.fields.values()
            if (field.read_only) and (field.default != empty) and (field.source != '*') and ('.' not in field.source)
        ]

        defaults = OrderedDict()
        for field in fields:
            try:
                default = field.get_default()
            except SkipField:
                continue
            defaults[field.field_name] = default

        return defaults

    def bind(self, field_name, parent):
        """
        Initializes the field name and parent for the field instance.
        Called when a field is added to the parent serializer instance.
        """

        # In order to enforce a consistent style, we error if a redundant
        # 'source' argument has been used. For example:
        # my_field = serializer.CharField(source='my_field')
        assert self.source != field_name, (
            "It is redundant to specify `source='%s'` on field '%s' in "
            "serializer '%s', because it is the same as the field name. "
            "Remove the `source` keyword argument." %
            (field_name, self.__class__.__name__, parent.__class__.__name__)
        )

        self.field_name = field_name
        self.parent = parent

        # `self.label` should default to being based on the field name.
        if self.label is None:
            self.label = field_name.replace('_', ' ').capitalize()

        # self.source should default to being the same as the field name.
        if self.source is None:
            self.source = field_name

        # self.source_attrs is a list of attributes that need to be looked up
        # when serializing the instance, or populating the validated data.
        if self.source == '*':
            self.source_attrs = []
        else:
            self.source_attrs = self.source.split('.')

    def create(self, validated_data):
        raise NotImplementedError('`create()` must be implemented.')

    def fail(self, key, **kwargs):
        """
        A helper method that simply raises a validation error.
        """
        try:
            msg = self.error_messages[key]
        except KeyError:
            class_name = self.__class__.__name__
            msg = MISSING_ERROR_MESSAGE.format(class_name=class_name, key=key)
            raise AssertionError(msg)
        message_string = msg.format(**kwargs)
        raise ValidationError(message_string, code=key)

    def get_attribute(self, instance):
        """
        Given the *outgoing* object instance, return the primitive value
        that should be used for this field.
        """
        try:
            return get_attribute(instance, self.source_attrs)
        except (KeyError, AttributeError) as exc:
            if self.default is not empty:
                return self.get_default()
            if self.allow_null:
                return None
            if not self.required:
                raise SkipField()
            msg = (
                'Got {exc_type} when attempting to get a value for field '
                '`{field}` on serializer `{serializer}`.\nThe serializer '
                'field might be named incorrectly and not match '
                'any attribute or key on the `{instance}` instance.\n'
                'Original exception text was: {exc}.'.format(
                    exc_type=type(exc).__name__,
                    field=self.field_name,
                    serializer=self.parent.__class__.__name__,
                    instance=instance.__class__.__name__,
                    exc=exc
                )
            )
            raise type(exc)(msg)

    def get_default(self):
        """
        Return the default value to use when validating data if no input
        is provided for this field.

        If a default has not been set for this field then this will simply
        raise `SkipField`, indicating that no value should be set in the
        validated data for this field.
        """
        if self.default is empty or getattr(self.root, 'partial', False):
            # No default, or this is a partial update.
            raise SkipField()
        if callable(self.default):
            if hasattr(self.default, 'set_context'):
                self.default.set_context(self)
            return self.default()
        return self.default

    def get_fields(self):
        """
        Returns a dictionary of {field_name: field_instance}.
        """
        # Every new serializer is created with a clone of the field instances.
        # This allows users to dynamically modify the fields on a serializer
        # instance without affecting every other serializer instance.
        return copy.deepcopy(self._declared_fields)

    def is_valid(self, raise_exception=False):
        assert not hasattr(self, 'restore_object'), (
            'Serializer `%s.%s` has old-style version 2 `.restore_object()` '
            'that is no longer compatible with REST framework 3. '
            'Use the new-style `.create()` and `.update()` methods instead.' %
            (self.__class__.__module__, self.__class__.__name__)
        )

        assert hasattr(self, 'initial_data'), (
            'Cannot call `.is_valid()` as no `data=` keyword argument was '
            'passed when instantiating the serializer instance.'
        )

        if not hasattr(self, '_validated_data'):
            try:
                self._validated_data = self.run_validation(self.initial_data)
            except ValidationError as exc:
                self._validated_data = {}
                self._errors = exc.detail
            else:
                self._errors = {}

        if self._errors and raise_exception:
            raise ValidationError(self.errors)

        return not bool(self._errors)

    @classmethod
    def many_init(cls, *args, **kwargs):
        """
        This method implements the creation of a `ListSerializer` parent
        class when `many=True` is used. You can customize it if you need to
        control which keyword arguments are passed to the parent, and
        which are passed to the child.

        Note that we're over-cautious in passing most arguments to both parent
        and child classes in order to try to cover the general case. If you're
        overriding this method you'll probably want something much simpler, eg:

        @classmethod
        def many_init(cls, *args, **kwargs):
            kwargs['child'] = cls()
            return CustomListSerializer(*args, **kwargs)
        """
        allow_empty = kwargs.pop('allow_empty', None)
        child_serializer = cls(*args, **kwargs)
        list_kwargs = {
            'child': child_serializer,
        }
        if allow_empty is not None:
            list_kwargs['allow_empty'] = allow_empty
        list_kwargs.update({
            key: value for key, value in kwargs.items()
            if key in LIST_SERIALIZER_KWARGS
        })
        meta = getattr(cls, 'Meta', None)
        list_serializer_class = getattr(meta, 'list_serializer_class', ListSerializer)
        return list_serializer_class(*args, **list_kwargs)

    def save(self, **kwargs):
        assert not hasattr(self, 'save_object'), (
            'Serializer `%s.%s` has old-style version 2 `.save_object()` '
            'that is no longer compatible with REST framework 3. '
            'Use the new-style `.create()` and `.update()` methods instead.' %
            (self.__class__.__module__, self.__class__.__name__)
        )

        assert hasattr(self, '_errors'), (
            'You must call `.is_valid()` before calling `.save()`.'
        )

        assert not self.errors, (
            'You cannot call `.save()` on a serializer with invalid data.'
        )

        # Guard against incorrect use of `serializer.save(commit=False)`
        assert 'commit' not in kwargs, (
            "'commit' is not a valid keyword argument to the 'save()' method. "
            "If you need to access data before committing to the database then "
            "inspect 'serializer.validated_data' instead. "
            "You can also pass additional keyword arguments to 'save()' if you "
            "need to set extra attributes on the saved model instance. "
            "For example: 'serializer.save(owner=request.user)'.'"
        )

        assert not hasattr(self, '_data'), (
            "You cannot call `.save()` after accessing `serializer.data`."
            "If you need to access data before committing to the database then "
            "inspect 'serializer.validated_data' instead. "
        )

        validated_data = dict(
            list(self.validated_data.items()) +
            list(kwargs.items())
        )

        if self.instance is not None:
            self.instance = self.update(self.instance, validated_data)
            assert self.instance is not None, (
                '`update()` did not return an object instance.'
            )
        else:
            self.instance = self.create(validated_data)
            assert self.instance is not None, (
                '`create()` did not return an object instance.'
            )

        return self.instance

    def update(self, instance, validated_data):
        raise NotImplementedError('`update()` must be implemented.')

    def validate(self, attrs):
        return attrs

    def validate_empty_values(self, data):
        """
        Validate empty values, and either:

        * Raise `ValidationError`, indicating invalid data.
        * Raise `SkipField`, indicating that the field should be ignored.
        * Return (True, data), indicating an empty value that should be
          returned without any further validation being applied.
        * Return (False, data), indicating a non-empty value, that should
          have validation applied as normal.
        """
        if self.read_only:
            return (True, self.get_default())

        if data is empty:
            if getattr(self.root, 'partial', False):
                raise SkipField()
            if self.required:
                self.fail('required')
            return (True, self.get_default())

        if data is None:
            if not self.allow_null:
                self.fail('null')
            return (True, None)

        return (False, data)