Class variable V3 dependent on V1 and V2. How to define V3 in children classes

Question

Parent class is inherited by multiple other classes.

class Parent(object):

    V_1 = set()
    V_2 = set()

    ALL_V_ELEMENTS = V_1 | V_2

class Child1(Parent):
    V_1 = {1, }
    V_2 = {4, 7, 10}

class Child2(Parent):
    V_1 = {'a', 'b'}
    V_2 = {'a', 'c'}

V_1 and V_2 are different in each child (also they don't change once the class is created).

Using the code below I get the same value for ALL_V_ELEMENTS:

print(Parent.ALL_V_ELEMENTS)  # prints: set()
print(Child1.ALL_V_ELEMENTS)  # prints: set()
print(Child2.ALL_V_ELEMENTS)  # prints: set()

Which is something I don't want. What I need is this:

print(Parent.ALL_V_ELEMENTS)  # prints: set()
print(Child1.ALL_V_ELEMENTS)  # prints: {1, 10, 4, 7}
print(Child2.ALL_V_ELEMENTS)  # prints: {'a', 'c', 'b'}

To achieve my goal I can define the classes as follows:

class Child1(Parent):
    V_1 = {1, }
    V_2 = {4, 7, 10}
    ALL_V_ELEMENTS = V_1 | V_2

class Child2(Parent):
    V_1 = {'a', 'b'}
    V_2 = {'a', 'c'}
    ALL_V_ELEMENTS = V_1 | V_2

However, copy-pasting ALL_V_ELEMENTS = V_1 | V_2 on every child of Parent doesn't seem like a good idea.

Another alternative would be defining Parent in a different way:

class Parent(object):

    V_1 = set()
    V_2 = set()

    def __init__(self):
        self.ALL_V_ELEMENTS = self.V_1 | self.V_2

This would do the | operation on every instance which is redundant.

---

Is there a better way to achieve my goal?

Answer 1

You could define it as a property:

class Parent(object):

    V_1 = set()
    V_2 = set()

    @property
    def ALL_V_ELEMENTS(self):
       return V_1 | V_2

This would, however, re-calculate the set each time. Having the __init__ create the set means it'll be created for every instance.

You could calculate the set in a metaclass, so it is only produced when the class object is produced:

class AllVMeta(type):
    def __new__(typ, name, bases, attrs):
        cls = super(AllVMeta, typ).__new__(typ, name, bases, attrs)
        cls.ALL_V_ELEMENTS = cls.V_1 | cls.V_2
        return cls

This metaclass adds a ALL_V_ELEMENTS union to any subclass; use it like this:

class Parent(object, metaclass=AllVMeta):
    V_1 = set()
    V_2 = set()

class Child1(Parent):
    V_1 = {1, }
    V_2 = {4, 7, 10}

class Child2(Parent):
    V_1 = {'a', 'b'}
    V_2 = {'a', 'c'}

Demo:

>>> class AllVMeta(type):
...     def __new__(typ, name, bases, attrs):
...         cls = super(AllVMeta, typ).__new__(typ, name, bases, attrs)
...         cls.ALL_V_ELEMENTS = cls.V_1 | cls.V_2
...         return cls
...
>>> class Parent(object, metaclass=AllVMeta):
...     V_1 = set()
...     V_2 = set()
...
>>> class Child1(Parent):
...     V_1 = {1, }
...     V_2 = {4, 7, 10}
...
>>> class Child2(Parent):
...     V_1 = {'a', 'b'}
...     V_2 = {'a', 'c'}
...
>>> Child1.ALL_V_ELEMENTS
{1, 10, 4, 7}
>>> Child2.ALL_V_ELEMENTS
{'a', 'c', 'b'}