python - How to inform class of modifications of its member variables?

Question

Suppose I have a class Foo that stores two dependent attributes (both NumPy arrays) x and y. They should always have the same shape and satisfy x=1/y. On possible implementation of Foo is as follows:

class Foo:
    def __init__(self, x):
        self._x = np.asanyarray(x)
        self._y = 1 / self.x

    @property
    def x(self):
        ret = self._x[:]

        ret.flags.writeable = False

        return ret

    @x.setter
    def x(self, val):

        val = np.asanyarray(val)

        self._x = val
        self._y = 1 / self.x

    @property
    def y(self):
        ret = self._y[:]

        ret.flags.writeable = False

        return ret

    @y.setter
    def y(self, val):

        val = np.asanyarray(val)

        self._y = val
        self._x = 1 / self.y

This guarantees that (unless the client attempts protected access), x and y always have the same shape and satisfy x = 1/y. In particular, the fact that Foo.x (similarly Foo.y) return a read-only view of the arrays, means the client cannot modify a single element of x (or y), nor can (s)he performs compound assignment operators such as foo.x += 1.5.

My question is if there is a (better) mechanism of informing Foo of modifications in x (or y) so that it updates the other member variable accordingly. Specifically, I think it would be convenient from a client's perspective to be able to modify elements of x (or y) instead of re-assigning the entire array.

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UPDATE: What is interesting is that if the attributes x (and y) are Python float scalars, foo.x += 1.5 automatically calls the getter, and then the setter. (Hence the relationship between x and y is preserved, through the setter.) NumPy arrays (or maybe any other object that overrides __iadd__) seem to be special.

question from:https://stackoverflow.com/questions/65940538/how-to-inform-class-of-modifications-of-its-member-variables

Answer 1

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