How to inherit from 2 classes without having to change the original classes?

Question

Whenever I run this code I always get a TypeError that says __init__() missing 1 required positional argument: 'hours' , but I am not trying to change anything from the original class that the ScientificSwimmer class is inheriting from…if that makes sense.

Here is the code:

class Human:
    def __init__(self, name, age):
        self.name = name
        self.age = age

    def hobby(self):
        print("Likes watching Netflix")

    def info(self):
        print(self.name , "is ",self.age," years old")


class Scientist(Human):
    def __init__(self, name, age, lab):
        super().__init__(name, age)
        self.lab = lab

    def hobby(self):
        print("Likes doing scientific experiments")

    def labName(self,lab):
        print("works at the " ,lab, "laboratory")


class Swimmer(Human):
    def __init__(self, name, age, hours):
        super().__init__(name,age)
        self.hours = hours

    def hobby(self):
        print("likes swimmming in the lake" )

    def hoursSwimming(self, hours):
        print("swims ", hours , "hours per week")


class ScientificSwimmer(Scientist, Swimmer):

    def __init__ (self, name, age, lab, hours):
       Scientist.__init__(self,name, age, lab)
       Swimmer.__init__(self, name, age, hours)


scienswim = ScientificSwimmer("\nJohn Smith", 30, "nuclear", 100)
scienswim.info()
scienswim.hobby()
scienswim.labName("nuclear")
scienswim.hoursSwimming(100)

My desired result is for it to print:

John smith is 30 years old.
likes doing scientific experiments.
works at the nuclear laboratory.
swims 100 hours per week.

Answer 1

Your original classes simply aren't designed correctly to support cooperative inheritance, which is what super is designed for. The number one rule for using super is this: you can't assume you know which class super() will refer to. That's determined by the runtime type of self , not the classes you statically inherit from.

Use keyword arguments exclusively to ensure that the method signatures remain compatible, and always call super().__init__ with any unknown keyword arguments (namely, those not explicitly mentioned in the signature).

class Human: 
    def __init__(self, *, name, age, **kwargs):
        super().__init__(**kwargs)
        self.name = name
        self.age = age
        
    def hobby(self):
        print("Likes watching Netflix")
        
    def info(self):
        print(self.name , "is ",self.age," years old")
        

class Scientist(Human): 
    def __init__(self, *, lab, **kwargs):
        super().__init__(**kwargs)
        self.lab = lab
        
    def hobby(self):
        print("Likes doing scientific experiments")
    
    def labName(self):
        print("works at the " , self.lab, "laboratory")
  

class Swimmer(Human):
    def __init__(self, *, hours, **kwargs):
        super().__init__(**kwargs)
        self.hours = hours
    
    def hobby(self):
        print("likes swimmming in the lake" )
    
    def hoursSwimming(self):
        print("swims ", self.hours , "hours per week")
     

class ScientificSwimmer(Scientist, Swimmer):
    pass


scienswim = ScientificSwimmer(name="John Smith", age=30, lab="nuclear", hours=100)
scienswim.info()
scienswim.hobby()
scienswim.labName()
scienswim.hoursSwimming()

The method resolution order for ScientificSwimmer is [ScientificSwimmer, Scientist, Swimmer, Human, object] . This results in the following:

1. ScientificSwimmer need not be defined, as the other __init__ methods will take care of everything.
2. Scientist.__init__ extracts the lab keyword argument and passes the rest on to the next class.
3. Swimmer.__init__ extracts the hours keyword argument and passes the rest on to the next class.
4. Human.__init__ extracts the name and age keyword arguments and passes the rest on to the next class.
5. object.__init__ receives no keyword arguments, all defined arguments having been correctly extracted by downstream classes.