Method observe

traitlets/traitlets.py:1649–1681

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

(
        self,
        handler: t.Callable[..., t.Any],
        names: Sentinel | str | t.Iterable[Sentinel | str] = All,
        type: Sentinel | str = "change",
    )

            self.observe(_callback_wrapper(handler), names=name)

    def observe(
        self,
        handler: t.Callable[..., t.Any],
        names: Sentinel | str | t.Iterable[Sentinel | str] = All,
        type: Sentinel | str = "change",
    ) -> None:
        """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.
        """
        for name in parse_notifier_name(names):
            self._add_notifiers(handler, name, type)

    def unobserve(
        self,