[FIXED] How to observe Instance member variables using traitlets library?

Issue

I am trying to use the traitlets library provided by ipython in my code. Suppose if a trait is of instance of a particular class how do i observe change in value for a member of the class..

example:

class A:
    def init(self,val):
    self.value = val

class myApp(HasTraits):
    myA = Instance(A,kw={'val':2})

I want to have an observe method to be called if 'value' member variable of object myA is changed.. SOmething like below:

@observe('myA.value')
def onValueChange(self,change):
    return

is it possible with the trailets implementation?

Solution

In order to observe changes to the value of an instance trait, the class for the instance trait should subclass Hastraits.

traitlets.observe(*names, **kwargs)

A decorator which can be used to observe Traits on a class.

from traitlets import HasTraits, observe, Instance, Int
​
class A(HasTraits):
    value = Int()
​
    def __init__(self, val):
        self.value = val
    
    @observe('value')
    def func(self, change):
        print(change)
​
class App(HasTraits):
    myA = Instance(klass=A, args=(2,))
​

app = App()
app.myA.value = 1

{'name': 'value', 'old': 0, 'new': 2, 'owner': <__main__.A object at 0x10b0698d0>, 'type': 'change'}
{'name': 'value', 'old': 2, 'new': 1, 'owner': <__main__.A object at 0x10b0698d0>, 'type': 'change'}

Edit

To keep the change handler in the composed class, you can dynamically set an observer.

Note that, the attribute observed must be property of a trait. In case you don't have access to modify class A to subclass HasTraits, you may be able to compose a class that subclasses HasTraits on the fly using some type magic.

from traitlets import HasTraits, observe, Instance, Int

class A(HasTraits):
    value = Int()

    def __init__(self, val):
        self.value = val

class App(HasTraits):
    myA = Instance(klass=A, args=(2,))
    
    def __init__(self):
        super().__init__()
        self.set_notifiers()
        
    def set_notifiers(self):
        HasTraits.observe(self.myA, func, 'value')
        
    
    def func(self, change):
        print(change)
        
app = App()

app.myA.value = 1

Answered By - Oluwafemi Sule