python - 使用 ipywidgets 中的小部件的观察者模式

Question

尝试重现 http://traitlets.readthedocs.io/en/stable/using_traitlets.html#observe 中的示例，但是使用小部件作为类属性，行为会发生变化并且不会观察到变化

from ipywidgets import Label
from traitlets import HasTraits, Unicode, observe
class Foo(HasTraits):
    bar = Label()
    baz = Unicode()

    @observe('bar')
    def _observe_bar(self, change):
        print(change['old'])
        print(change['new'])

f = Foo()
f.bar = Label('hello')  # No printing

Answer 1

我从 Sylvain Corlay ( https://github.com/jupyter-widgets/ipywidgets/issues/2078 ) 那里得到了 ipywidgets repo 问题跟踪器的答案

You need to use the Instance trait type. (Instance(Label))

所以

from ipywidgets import Label
from traitlets import HasTraits, Unicode, observe, Instance
class Foo(HasTraits):
    bar = Instance(Label)
    baz = Unicode()

    @observe('bar')
    def _observe_bar(self, change):
        print(change['old'])
        print(change['new'])

f = Foo()
f.bar = Label('hello')  # No printing