Return the sum of values returned by another method of all the instances of that class?

Question

class Square():
    def __init__(self, side):
        self.side = side

    def getArea(self):
        return side*side

    def getAreaOfAllInstances(self):
       {need to write this method}

s1 = Square(2)
s2 = Square(3)

print(s1.getAreaOfAllInstances()) ===> This should print 13 (2*2 + 3*3)
print(s2.getAreaOfAllInstances()) ===> This should print 13 (2*2 + 3*3)`

You need to keep track of all instances yourself somewhere, be it an auxiliary data structure or as an attribute on the class. — juanpa.arrivillaga, Jul 24 '18 at 09:01
@AzatIbrakov, just wanted to know if this is possible to do it? — Ashok, Jul 24 '18 at 09:10
Reference https://stackoverflow.com/questions/328851/printing-all-instances-of-a-class — MaNKuR, Jul 24 '18 at 09:10
how would you know that you don't need specific `Square` anymore and can remove it so gc can grab it? or it is ok to store all (potentially infinite) instances? — Azat Ibrakov, Jul 24 '18 at 09:22

Kiro · Answer 1 · 2018-07-24T09:18:39.580

You can use class variable:

class Square():
    squares = []
    def __init__(self, side):
        self.side = side
        self.squares.append(self)

    def getArea(self):
        return self.side * self.side

    def getAreaOfAllInstances(self):
        return sum(s.getArea() for s in self.squares)

s1 = Square(2)
s2 = Square(3)

print(s1.getAreaOfAllInstances()) #===> This should print 13 (2*2 + 3*3)
print(s2.getAreaOfAllInstances()) # ===> This should print 13 (2*2 + 3*3)

But that's not a clean solution. I'd rather create another class, to keep track of Squares. Like this:

class Square:
    def __init__(self, side):
        self.side = side

    def getArea(self):
        return self.side * self.side


class SquareContainer:
    def __init__(self):
        self.squares = []

    def create_square(self, side):
        square = Square(side)
        self.squares.append(square)
        return square

    def getAreaOfAllInstances(self):
        return sum(s.getArea() for s in self.squares)

sc = SquareContainer()
s1 = sc.create_square(2)
s2 = sc.create_square(3)

print(sc.getAreaOfAllInstances())

score 0 · Answer 2 · answered Jul 24 '18 at 09:11

Sure you can:

class Square():

    _all_instances = []

    def __init__(self, side):
        self.side = side
        self._all_instances.append(self)

    def getArea(self):
        return side*side

    @classmethod
    def getAreaOfAllInstances(cls):
        return sum(inst.getArea() for inst in cls._all_instances)

Note however that this means that Squares will never be gargabe collected, except if you empty the Square._all_instances attribute.

madjaoue · Answer 3 · 2018-07-24T09:32:38.287

You can use classmethods to achieve this.

class Square(object):
    instances_list = []  # Property of the class

    def __init__(self, side):
        self.trackInstances(self)
        self.side = side

    def getArea(self):
        return self.side**2

    @classmethod
    def trackInstances(cls, self):
        self.instances_list.append(self)

    @classmethod
    def getAreaOfAllInstances(cls):
        areaSum = sum(map(lambda i: i.getArea(), cls.instances_list))
        print('square of all instances: {}'.format(areaSum))

s1 = Square(2)
s2 = Square(3)

s2.getAreaOfAllInstances()

Output:

square of all instances: 13

Note that only s2.getAreaOfAllInstances is executed, and it gives the areas of all instances.

Return the sum of values returned by another method of all the instances of that class?

3 Answers3

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