Create a set of class objects which contain that very set as an attribute?

Question

class Something:
    def __init__(self,x,y):
        self.x = x
        self.y = y
        self.z = set()

    def __hash__(self):
        return hash((self.x, self.y, self.z))

    def __eq__(self,other):
        if not isinstance(other, Something):
            return NotImplemented
        return self.x == other.x and self.y == other.y

The above is the class definition. Inside main:

blah = []

for i in range(5):
    blah.append(Something(i,i+1))

blah[0].z.add(blah[1])

For this, I get the TypeError: unhashable type: 'set' error. One solution I found here was to make z a frozenset() but that gives AttributeError: 'frozenset' object has no attribute 'add'.

Any suggestions?

Answer 1

You should omit self.z from your __hash__ method.

Since your __eq__ method ignores self.z, it would be incorrect for __hash__ to take account of it anyway. And it solves the issue of self.z being an unhashable (and mutable) type.