Traitlet extension draft

traitlet_extension.py

import traitlets

class _Undefined(object):
    obj = None
    def __new__(cls):
        if cls.obj is None:
            cls.obj = object.__new__(cls)
        return cls.obj

class _UseDefault(_Undefined):
    obj = None

Undefined = _Undefined()
UseDefault = _UseDefault()

class Disallow(traitlets.TraitType):
    def validate(self, obj, val):
        self.error(obj, val)

class UndefMixin(object):
    def __init__(self, default_value=Undefined, *args, **kwargs):
        return super(UndefMixin, self).__init__(default_value, *args, **kwargs)

    def _validate(self, obj, val):
        if isinstance(val, _Undefined):
            return val
        return super(UndefMixin, self)._validate(obj, val)

class Float(UndefMixin, traitlets.Float):
    pass