在PyGTK/GObject中使用枚举属性

This tutorial gobject.TYPE_FLOAT .

我已将其修改为使用枚举类型:

import pygtk
pygtk.require('2.0')
import gobject

FUEL_NONE = 0
FUEL_SOME = 1
FUEL_FULL = 2

class Car(gobject.GObject):
  __gproperties__ = {
       'fuel' : (gobject.TYPE_ENUM,                         # type
                 'fuel of the car',                         # nick name
                 'amount of fuel that remains in the tank', # description
                 FUEL_SOME,                                 # default value
                 gobject.PARAM_READWRITE)                   # flags
  }

# <<rest of demo code>>

…但是当我尝试运行它时,会出现以下错误:

/usr/lib/pymodules/python2.5/gtk-2.0/gobject/__init__.py:114: Warning: g_param_spec_enum: assertion `g_enum_get_value (enum_class, default_value) != NULL' failed
  type_register(cls, namespace.get('__gtype_name__'))
Traceback (most recent call last):
  File "gcar.py", line 9, in <module>
    class Car(gobject.GObject):
  File "/usr/lib/pymodules/python2.5/gtk-2.0/gobject/__init__.py", line 60, in __init__
    cls._type_register(cls.__dict__)
  File "/usr/lib/pymodules/python2.5/gtk-2.0/gobject/__init__.py", line 114, in _type_register
    type_register(cls, namespace.get('__gtype_name__'))
TypeError: Error when calling the metaclass bases
    could not create param spec for type GEnum (while registering property 'fuel' for GType '__main__+Car')

我错过了什么?