Multiple Inheritance in Python - TypeError "missing 1 required positional argument"

Question

I'm only really getting to grips with OOP and I have been looking at multiple inheritance. The problem I have, is when I instantiate the class Hybrid, which inherits from more than one class, I get the following error:

Traceback (most recent call last):
  File "multi_inherit.py", line 44, in <module>
    hawk = Hybrid('hawk', 200, 650)
  File "multi_inherit.py", line 35, in __init__
    Archer.__init__(self, name, arrows)
  File "multi_inherit.py", line 13, in __init__
    super().__init__(name)
TypeError: Wizard.__init__() missing 1 required positional argument: 'power'

The other classes work absolutely fine.

class User:
    def __init__(self, name):
        self.name = name

    def greet(self):
        return f"Welcome, {self.name.title()}."


class Archer(User):
    def __init__(self, name, arrows):
        super().__init__(name)
        self.arrows = arrows

    def shoot_arrow(self):
        self.arrows -= 1
        return "You shot an arrow!"

    def arrows_left(self):
        return f"You have {self.arrows} arrows remaining."


class Wizard(User):
    def __init__(self, name, power):
        super().__init__(name)
        self.power = power

    def cast_spell(self):
        return f"You cast a spell with a power of {self.power}"


class Hybrid(Archer, Wizard):
    def __init__(self, name, arrows, power):
        Archer.__init__(self, name, arrows)
        Wizard.__init__(self, name, power)

    def powerful(self):
        return "Hybrids are super powerful!"


merlin = Wizard('merlin', 1000)
robin = Archer('robin', 150)
hawk = Hybrid('hawk', 200, 650)

print(merlin.greet())
print(merlin.cast_spell())

print(robin.arrows_left())
print(robin.shoot_arrow())
print(robin.arrows_left())

You are trying to use super wiht multiple inheritance, then every class should be using super. Your classes have to be designed to support this from the ground up. see: rhettinger.wordpress.com/2011/05/26/super-considered-super — juanpa.arrivillaga
– juanpa.arrivillaga, Commented Apr 27, 2022 at 20:35
Multiple inheritance, use of super(), parameter lists that are different for each class - pick any two, all three simply do not get along. The specific problem is that super() doesn't necessarily invoke the class you expect when there's a diamond in the inheritance graph - so you don't know what set of parameters to pass to it. Explicitly naming the parent class in each inherited call may be the simplest fix here. — jasonharper
– jasonharper, Commented Apr 27, 2022 at 20:37
What's basically happening is that you call Archer.__init__(self, name, arrows) where self is a Hybrid object, then in Archer.__init__, you call super().__init__(name). So the MRO of Hybrid is [__main__.Hybrid, __main__.Archer, __main__.Wizard, __main__.User, object], so super knows it has a hybrid object, in the Archer class, so it proxies to the next class in the Hybrid MRO, i.e. Wizard.__init__, but you only passed a single argument, and the second positional argument is missing — juanpa.arrivillaga
– juanpa.arrivillaga, Commented Apr 27, 2022 at 20:38
Also, check out the great PyCon talk: youtube.com/watch?v=EiOglTERPEo which goes into a lot of details and builds up examples of how to use cooperative inheritance. — juanpa.arrivillaga
– juanpa.arrivillaga, Commented Apr 27, 2022 at 20:40
In the Archer and Wizard classes if you replace super().__init__(name) with User.__init__(self, name) it will fix your problem. I don't think this is the proper way of fixing it though; someone else will probably come along with a better answer. — hostingutilities.com
– hostingutilities.com, Commented Apr 27, 2022 at 20:54

flakes · Accepted Answer · 2022-04-27 20:55:10Z

1

The classes as you have them now aren't great for use in multiple inheritance. When using multiple inheritance I prefer having constructors that agree on the same contract (use the same arguments) or no arguments at all. Here powers and arrows differ which makes calling each constructor awkward.

IMO A better way to design this class would be mixins. The mixins would have no constructors and depend on particular values being present in the classes which extend them.

Example Mixins:

class UserMixin:
    name: str

    def greet(self):
        return f"Welcome, {self.name.title()}."


class ArcherMixin(UserMixin):
    arrows: int

    def shoot_arrow(self):
        self.arrows -= 1
        return "You shot an arrow!"

    def arrows_left(self):
        return f"You have {self.arrows} arrows remaining."


class WizardMixin(UserMixin):
    power: int

    def cast_spell(self):
        return f"You cast a spell with a power of {self.power}"

Example Implementations:

class User(UserMixin):
    def __init__(self, name):
        self.name = name


class Archer(ArcherMixin):
    def __init__(self, name, arrows):
        self.name = name
        self.arrows = arrows


class Wizard(WizardMixin):
    def __init__(self, name, power):
        self.name = name
        self.power = power


class Hybrid(ArcherMixin, WizardMixin):
    def __init__(self, name, arrows, power):
        self.name = name
        self.arrows = arrows
        self.power = power

    def powerful(self):
        return "Hybrids are super powerful!"

Example usage:

merlin = Wizard('merlin', 1000)
robin = Archer('robin', 150)
hawk = Hybrid('hawk', 200, 650)

print(merlin.greet())
print(merlin.cast_spell())

print(robin.greet())
print(robin.arrows_left())
print(robin.shoot_arrow())
print(robin.arrows_left())

print(hawk.greet())
print(hawk.cast_spell())
print(hawk.arrows_left())
print(hawk.shoot_arrow())
print(hawk.arrows_left())

Welcome, Merlin.
You cast a spell with a power of 1000
Welcome, Robin.
You have 150 arrows remaining.
You shot an arrow!
You have 149 arrows remaining.
Welcome, Hawk.
You cast a spell with a power of 650
You have 200 arrows remaining.
You shot an arrow!
You have 199 arrows remaining.

answered Apr 27, 2022 at 20:55

flakes

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5 Comments

juanpa.arrivillaga Over a year ago

Yep, the mixin pattern is usually much, much clearner.

ScottMButler Over a year ago

I like this approach - Thank you! Now I've just got to get to grips with it so I can use this comfortably at will.

ScottMButler Over a year ago

Been practising this and it's really sinking in - makes a lot of sense. One question I have though is when stating the expected data type within the mixin classes, is there a reason why you use ':' as apposed to '=' For example when writing the UserMixin class you put 'name: str'

flakes Over a year ago

@ScottMButler : is used to apply a typehint annotation to the attribute name. It doesn't assign it to anything, which is what = is for. All it does is declare that there will be a attribute with that name/type available on concrete instances of the class.

ScottMButler Over a year ago

@flakes Thanks so much for your help - greatly appreciated!

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