Question

Pep8 advises to always use cls as the first argument of a class method definition.
Now suppose I want to use a class variable(in this case: cls.cartridge_state) that can also be used in an instance method (in this case: __init__). So for that I need to make the variable global(see code below). But instantiating FountainPen generates the following runtime error:

self.cartridge_state = cls.cartridge_state
NameError: global name 'cls' is not defined

But then again when I change global cartridge_state into global cls.cartridge_state I get a syntaxError when i try to import the module.

class FountainPen(object):
    cartridge_ink = "water-based"
    @classmethod
    def toggle_default_cartridge_state(cls):
        i = 0
        cartridge_states = ['non-empty','empty']
        global cartridge_state
        cls.cartridge_state = cartridge_states[i]
        i += 1

    def __init__(self):
        self.cartridge_state = cls.cartridge_state
        global number_of_refills
        self.number_of_refills = 0

    def write(self):
        print Pen.write(self)
        self.cartridge_state = "empty"
        return self.cartridge_state

    def refill(self):
        self.cartridge_state = "non-empty"
        self.number_of_refills += 1

How can I let the class variable cartridge_state be pep8 compliant and make this code work without errors?

Answer 1

A class attribute can be read via self:

class FountainPen(object):
    cartridge_ink = "water-based"
    default_cartridge_state = "empty"

    @classmethod
    def toggle_default_cartridge_state(cls):
        if cls.default_cartridge_state == "empty":
            cls.default_cartridge_state = "non-empty"  
        else:
            cls.default_cartridge_state = "empty"

    def __init__(self):
        self.cartridge_state = self.default_cartridge_state

    def write(self):
        print Pen.write(self)
        self.cartridge_state = "empty"
        return self.cartridge_state

    def refill(self):
        self.cartridge_state = "non-empty"
        self.number_of_refills += 1