Python class attribute error despite existing?

Question

I found a solution for this on here, but then I realized it was Python 2.7. I'm using Python 3.3.7, and I've recently been experimenting with classes, and I believe I may have ended up confusing myself. I have two files; my class file (Dice.py):

class Dice:
    def __init__(self, sides):
        self.sides = sides

    @classmethod
    def roll(self):
        return self.sides + 2

and my main file.

from Dice import Dice

Dice(int(input('How many sides are on the dice? ')))
print(Dice.roll())

Very simple for now, theoretically. However, I've been running into a strange attribute error that's had me stuck on these ten or so lines of codes for far longer than I'd care to admit. I keep receiving a "AttributeError: type object 'Dice' has no attribute 'sides' " each time I attempt to run the main file, which should theoretically give me an answer of the input amount + 2.

I've gone through and initialized it multiple ways, added @classmethod, renamed the attribute and all of that; I just can't same to shake this error. I was wondering exactly what the error meant if it wasn't referring to the init function at the start, and how I could fix it so it can actually run. Thank you!

Classmethods don't work that way. Are you certain you want roll() as a classmethod? — ewokx
– ewokx, Commented Mar 29, 2022 at 0:02
@wim Honestly I'm not quite sure, I was perusing here and I think I saw another solution used that to solve and implemented obviously unsuccessfully. Without it I receive another error I just couldn't figure out either; Dice.roll() missing 1 required positional argument: 'self' . I'm not sure how I'm supposed to include self other than it being... self? — deuil
– deuil, Commented Mar 29, 2022 at 0:09

Green Cloak Guy · Accepted Answer · 2022-03-29 00:15:04Z

The more idiomatic method to write a class method is

class Dice:
    ...

    @classmethod
    def roll(cls):
        ...

Note the parameter is usually called cls, not self. What the @classmethod method does, is make it so that when you call Dice.roll(), the 'instance' that's passed to the method is the object Dice - literally, the class that owns the method (not any particular instance of the class). This is usually used in conjunction with inheritance and polymorphism.

Now, since __init__() has never been called on the Dice class itself (it would only ever be called on an instance of the Dice class), the Dice class does not ever have an attribute called sides attached to it.

In your case, you probably want roll() to be an instance method, and to save the new instance to a variable:

class Dice:
    def __init__(self, sides):
        self.sides = sides

    def roll(self):
        return self.sides + 2

die = Dice(int(input('How many sides are on the dice? ')))
print(die.roll())

# alternatively

sides = int(input('How many sides are on the dice? '))
print(Dice(sides).roll())

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