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Python 3.6 : Either I miss something either generic typing breaks super chaining for inheritance

First I ran the following code, which went real fine :

class Monster:
def __init__(self):
self._can_do =
print("created a monster")
super().__init__()

class Race(Monster):
""" all races must derive from this """
def __init__(self):
super().__init__()
print("created a race x")

class Human(Race):
def __init__(self):
super().__init__()
self._can_do.append("Do nothing special !")
print("created a human")

class Elf(Race):
def __init__(self):
super().__init__()
self._can_do.append("Avoid sleep")
print("created an elf")

class Class:
""" all classes must derive from this """
def __init__(self):
super().__init__()
print("created a class x")

class Fighter(Class):
def __init__(self):
super().__init__()
self._can_do.append("Hit hard")
print("created a fighter")

class Wizard(Class):
def __init__(self):
super().__init__()
self._can_do.append("Cast spells")
print("created a wizard")


class Hero(Human, Fighter):
def __init__(self):
x = super()
print(f"super = x")
super().__init__()
def speak(self):
for action in self._can_do:
print(f"I can action !")

print("creating hero 1 :")
hero1 = Hero()

print("hero 1 human fighter says :")
hero1.speak()


Result was :

creating hero 1 :
created a monster
created a class x
created a fighter
created a race x
created a human
hero 1 human fighter says :
I can Hit hard !
I can Do nothing special ! !


Then I had another go, changing very slightly the code, as below, because that is were I want to go :

(made the Hero class heriting dynamically instead of statically)

import typing

class Monster:
def __init__(self):
self._can_do =
print("created a monster")
super().__init__()

class Race(Monster):
""" all races must derive from this """
def __init__(self):
super().__init__()
print("created a race x")

class Human(Race):
def __init__(self):
super().__init__()
self._can_do.append("Do nothing special !")
print("created a human")

class Elf(Race):
def __init__(self):
super().__init__()
self._can_do.append("Avoid sleep")
print("created an elf")

class Class:
""" all classes must derive from this """
def __init__(self):
super().__init__()
print("created a class x")

class Fighter(Class):
def __init__(self):
super().__init__()
self._can_do.append("Hit hard")
print("created a fighter")

class Wizard(Class):
def __init__(self):
super().__init__()
self._can_do.append("Cast spells")
print("created a wizard")

RaceT = typing.TypeVar('RaceT', bound=Race)
ClassT = typing.TypeVar('ClassT', bound=Class)

class Hero(typing.Generic[RaceT, ClassT]):
def __init__(self):
super().__init__()
def speak(self):
for action in self._can_do:
print(f"I can action !")

print("creating hero 1 :")
hero1 = Hero[Human,Fighter]()

print("hero 1 human fighter says :")
hero1.speak()


This time, all went wrong :

creating hero 1 :
hero 1 human fighter says :
Traceback (most recent call last):
File "./test2.py", line 61, in <module>
hero1.speak()
File "./test2.py", line 54, in speak
for action in self._can_do:
AttributeError: 'Hero' object has no attribute '_can_do'


Seems using generic class creation makes super unable to find parent class initializer, non ?

Did I miss something ?

typing.Generic

List[int]

Tuple[str, float]

2 Answers
2

That's not what generic typing means. When you declare

class Hero(typing.Generic[RaceT, ClassT]):
...


then that means that Hero takes two type parameters. It doesn't mean that Hero[Human, Fighter] is a subclass of Human or Fighter, any more than List[int] is a subclass of int. Generic typing is not a way to dynamically adjust a class's superclasses.

Hero

Hero[Human, Fighter]

Human

Fighter

List[int]

int

Here's a metaclass solution that allows you to dynamically generate classes from a Race, Class pair

Race, Class

class Hero(type):
instances = # All Elf Wizards should have the same class, for example
@classmethod
def class_factory(metacls, race, class_):
if not (issubclass(race, Race) and issubclass(class_, Class)):
raise ValueError("Needs race and class, got and ".format(race, class_))
name = "0.__name__1.__name__".format(race, class_)
if name in metacls.instances:
return metacls.instances[name]
cls = metacls(name, (race, class_), )
metacls.instances[name] = cls
return cls

ElfWizard = Hero.class_factory(Elf, Wizard)
assert ElfWizard is Hero.class_factory(Elf, Wizard)
tim = ElfWizard()


prints

created a monster
created a class x
created a wizard
created a race x
created an elf


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