How classes work in terms of pens

pen.py

class Pen:
    color = None
    ink_level = None
    active = True

    def __init__(self, color="black", ink_level=1000):
        self.color = color
        self.ink_level = ink_level
        
    def __str__(self):
        return "A " + self.color + " pen."

    def write_string(self, string):
        if self.ink_level > len(string) and self.active:
            print(string + " in " + self.color)
            self.ink_level -= len(string)
        elif self.ink_level < len(string):
            raise (InkLevelError(self.ink_level, "You're out of ink!"))
        else:
            raise (ActiveError("Dude, not active!"))

    def activate(self):
        self.active = True

    def deactivate(self):
        self.active = False


class CappedPen(Pen):
    cap_on = None

    def __init__(self, cap_on=True, color="black", ink_level=1000):
        super().__init__(color, ink_level)
        self.active = not cap_on
        self.cap_on = True

    def put_cap_on(self):
        print("I'm putting the cap on")
        self.deactivate()

    def take_cap_off(self):
        print("I'm taking the cap off")
        self.activate()


class ActiveError(RuntimeError):
    """ This is just a dummy error so we can call it ActiveError. """
    
    def __init__(self, *args: object, **kwargs: object) -> None:
        super().__init__(*args, **kwargs)


class InkLevelError(RuntimeError):
    ink_level = None

    def __init__(self, ink_level, *args: object, **kwargs: object) -> None:
        super().__init__(*args, **kwargs)
        self.ink_level = ink_level

# This bit is just a script that implements the pen stuff.
p = CappedPen(color="red")
p.take_cap_off()
try:
    for i in range(1000000):
        p.write_string("A poem")
except InkLevelError as err:
    print("Dude, your ink level was " + str(err.ink_level))
p.put_cap_on()