Python: detect changes in a subclass

Question

Let's consider the following example:

class SubClass:
    def __init__(self):
        self._param = None

    @property
    def param(self):
        print('read param')
        return self._param

    @param.setter
    def param(self, value):
        print('set param')
        self._param = value


class MainClass:
    def __init__(self):
        self._var = SubClass()

    @property
    def var(self):
        print('read var')
        return self._var

    @var.setter
    def var(self, value):
        print('set var')
        self._var = value

If I do:

cls = MainClass()
cls.var.param = 3

I obtain:

'read var'
'set param'

How can I make MainClass aware that var.param has changed?

Useful additional info: consider that in my actual code param is not a scalar but an array with hundreds of elements, so I would like to avoid to create a copy and then just compare them. Moreover, param and var are not the only properties.

why do you need this in the first place? looks like an XY problem to me — Azat Ibrakov, Dec 01 '21 at 11:02
to make it simple, because I need to make some operations in ```MainClass``` but only if ```var.param``` has changed. — mauro, Dec 01 '21 at 11:07
You could consider using an observer class to track changes. See e.g. https://stackoverflow.com/questions/22878220/looking-for-a-solution-to-detect-value-change-in-class-attribute-with-a-list-and — match, Dec 01 '21 at 11:14

score 0 · Answer 1 · answered Dec 01 '21 at 11:23

One approach is to pass a method from MainClass as an argument when instantiating SubClass, and have SubClass call it whenever it changes a variable:

class SubClass:
    def __init__(self, changed):
        self._param = None
        self.changed = changed

    @property
    def param(self):
        print('read param')
        return self._param

    @param.setter
    def param(self, value):
        print('set param')
        self._param = value
        self.changed('param')


class MainClass:
    def __init__(self):
        self._var = SubClass(self.changed)
    
    def changed(self, name):
        print("Subclass changed: ", name, getattr(self._var, name))

    @property
    def var(self):
        print('read var')
        return self._var

    @var.setter
    def var(self, value):
        print('set var')
        self._var = value


m = MainClass()
m.var.param = 'test'

Python: detect changes in a subclass

1 Answers1