How to detect changes of a list whithin a Python class, or why does "setter" not trigger

Question

I would like to have a list in a Python class. Whenever an element in the list is changed I need to run some logic. I'm pretty new to classes in python and my approach with the setter might be pretty naive. This is what makes intuitive sense to me:

class test():
    def __init__(self):
        self._R = [False]*16

    @property
    def R(self):
        return self._R

    @R.setter
    def R(self,a):
        print('Why do I not get reached?')
        self._R = a

W = test()
W.R[0] = True

But the setter never gets triggered. If you could give me a notch in the right direction, I would be very great full.

Answer 1

You can create a new List-like class that takes a callback function and executes it whenever the list is changed:

class CallbackList:  # PEP-8 style suggests UpperCase class names
    def __init__(self, callback=None):
        self._list = [False]
        self._callback = callback  # Python functions are first-class objects just like ints, strings, etc, so this is completely legal

    def __setitem__(self, index, value):
        self._list[index] = value
        if self._callback:
            self._callback()  # Executes the callback function whenever a value is set

    def __getitem__(self, index):
        return self._list[index]


class Test:
    def __init__(self):
        self.callback_list = CallbackList(callback=self.foo)

    def foo(self):
        print("You changed the list!")


W = Test()
W.callback_list[0] = True  # This prints "You changed the list!"

Note that this still won't catch every possible change. For example:

W = Test()
some_list = [1, 2, 3]

W.callback_list[0] = some_list  # This triggers the callback function
print(W.callback_list[0])  # [1, 2, 3]

some_list.append(4)  # This does NOT trigger the callback function!
print(W.callback_list[0])  # [1, 2, 3, 4] !