Object property with extra functions in Python

Question

I am developing a descriptor basing myself in the ideas of @property. What I would like is to understand if there is any way of easily extending the behavior of property beyond setting/getting.

The concrete example: I am communicating with a device, and would like to have a property that can set/get/update. In this way, when I set, I pass the new value to the device, and store it in the cache. When I get, I retrieve the cached value. When I update, the device is re-asked for the value. In this way, can avoid unnecessary communication with the device, unless explicitly triggered.

I don't know if there is a pattern that I am missing that may be a straightforward solution. An option would be to invalidate the cache when using a special value in set, but I don't think it is a good idea to assume a specific value for triggering the update.

I mean, you'r get function would essentially contain that sort of logic. Here's an example: github.com/pyglet/pyglet/blob/… which rely quite hevily on this. Your set could check if the value/key already exists and take action accordingly. — Torxed
– Torxed, Commented May 8, 2020 at 14:28
Does not look like what I am looking for. I don't see how they implement the update while getting, they are updating while setting. I'll extend my question. — Aquiles Carattino
– Aquiles Carattino, Commented May 8, 2020 at 14:33
Even if there is a way to extend the property function like you described, what would the syntax look like to make use of that additional functionality? Assume a is a property of class A, then b = A.a calls a.getter , while A.a = b calls a.setter. What syntax would you suggest that calls the 'updater'? — mapf
– mapf, Commented May 8, 2020 at 14:37
That is what I am asking, is there a way? For example, del triggers a special method. Perhaps there is an upd. Or perhaps descriptors are not the way to go and there is another approach I am not aware of. Or perhaps there is a way of knowing whether get is being accessed with an extra .update? — Aquiles Carattino
– Aquiles Carattino, Commented May 8, 2020 at 14:48
I'm sorry, I don't understand the difference between your getter and updater. If you want your updater to have its own syntax, you would need to define a new operator. — mapf
– mapf, Commented May 8, 2020 at 14:56

mapf · Accepted Answer · 2022-01-05 08:24:40Z

I still don't understand the difference between your getter and updater, but here is some sample code which implements a cache that stores previous variable values. Maybe this helps you to find what you are looking for.

class A:
    def __init__(self):
        self._a_cache = [1]

    @property  # <- this defines a.getter
    def a(self):
        return self._a_cache[-1]

    @a.setter
    def a(self, value):
        self._a_cache.append(value)

    def undo(self):
        if len(self._a_cache) > 1:
            self._a_cache.pop()
        else:
            print('No previous values')

    def update(self):
        # what is supposed to happen here as opposed to the getter?
        pass
        
        
var = A()
print(var.a)

var.a = 10
print(var.a)

var.undo()
print(var.a)

var.update()
# ?

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