Dunder methods in Python

Let's add another dunder method to the class.

import random 

class Dice:
    def __init__(self, probs):
        self.probs = probs
        self.expected_value = sum(i * p for i, p in self.probs.items())
    
    @classmethod
    def from_sides(cls, n=6):
        return cls({i: 1/n for i in range(1, n + 1)})
    
    def roll(self, n=1):
        sides = list(self.probs.keys())
        probabilities = list(self.probs.values())
        return random.choices(sides, weights=probabilities, k=n)
    
    def __len__(self):
        return len(self.probs)

The __len__ dunder method returns the number of sides of the dice and it allows you to use the generic len() function on the object.

dice = Dice({1: 1/6, 2: 1/6, 3: 1/6, 4: 1/6, 5: 1/6, 6: 1/6})
len(dice)

It might feel like a modest feature, but there's lots to appreciate here. Because Python carries a len() function that is generic we can have one single way of getting a length on any Python object. This is really pragmatic!