ipywidget 观察方法每次更改都会调用几次，为什么？

Question

我正在尝试了解 ipywidget 下拉菜单如何与观察方法一起工作，并遇到了这个非常有用的 SO 问题：

那里可以找到很好的解释。

稍微调整一下我创建这个小脚本的答案代码：

w = wd.Dropdown(
    options=['Addition', 'Multiplication', 'Subtraction'],
    value='Addition',
    description='Task:',
)

def on_change(change):
    print('method is called when printing this')
    if change['type'] == 'change' and change['name'] == 'value':
        print("changed to %s" % change.new)
    else:
        print('chage type is not change it is actually:', change['type'])
        print('chage name is not value it is actually:', change['name'])

w.observe(on_change)

display(w)

奇怪的是，当一次更改下拉菜单的值时，打印出来的是这样的：

method is called when printing this
chage type is not change it is actually: change
chage name is not value it is actually: _property_lock
method is called when printing this
chage type is not change it is actually: change
chage name is not value it is actually: label
method is called when printing this
changed to Multiplication
method is called when printing this
chage type is not change it is actually: change
chage name is not value it is actually: index
method is called when printing this
chage type is not change it is actually: change
chage name is not value it is actually: _property_lock

因此菜单下拉列表的一次更改调用了 4 次 observe 方法。

这是为什么？

其次，当 observe 写成这样时，这不会发生：

w.observe(on_change, names='value')

那么输出就是：

method is called when printing this
changed to Multiplication

所以在第二种情况下，该方法被调用一次。

谁能解释一下这是怎么回事？

Answer 1

在

w.observe(on_change)

你告诉小部件每次它的任何属性（我猜实际上是 Trait 属性）发生变化时调用 on_change 函数：这包括 value（这只是你大部分时间都需要）还有它的所有更多“内部属性”（比如 _property_lock，大多数时候你不需要）。此行为记录在 w.observe 文档中：

Signature: w.observe(handler, names=traitlets.All, type='change')
Docstring:
Setup a handler to be called when a trait changes.

This is used to setup dynamic notifications of trait changes.

Parameters
----------
handler : callable
    A callable that is called when a trait changes. Its
    signature should be ``handler(change)``, where ``change`` is a
    dictionary. The change dictionary at least holds a 'type' key.
    * ``type``: the type of notification.
    Other keys may be passed depending on the value of 'type'. In the
    case where type is 'change', we also have the following keys:
    * ``owner`` : the HasTraits instance
    * ``old`` : the old value of the modified trait attribute
    * ``new`` : the new value of the modified trait attribute
    * ``name`` : the name of the modified trait attribute.
names : list, str, All
    If names is All, the handler will apply to all traits.  If a list
    of str, handler will apply to all names in the list.  If a
    str, the handler will apply just to that name.
type : str, All (default: 'change')
    The type of notification to filter by. If equal to All, then all
    notifications are passed to the observe handler.

names 默认为 traitlets.All，在这种情况下：If names is All, the handler will apply to all traits.

因此在定义回调时 names=Value 很重要。

ipywidget 观察方法每次更改都会调用几次，为什么？

ipywidget observe method called several times for every change, why?

python

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