Call to `init` of super class is missed

Origin

If a parent class declare __init__ method explicitly, even if that method is empty

then the sub class’s __init__ method need to invoke parent’s __init__

if not, a warning Call to __init__ of super class is missed will be thrown by PyCharm.

Tracing

Why? Please follow me, look at the first demo

Error Demo

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
class Animal:
    def __init__(self):
        self.color = 'black'


class Cat(Animal):
    def __init__(self):
        self.age = 1


if __name__ == '__main__':
    ci = Cat()
    print(ci.color, ci.age)

# it will throw error: AttributeError: 'Cat' object has no attribute 'color'

Fine 1

If a sub class do not override __init__ method, parent’s attributes will be inherited.

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
class Animal:
    def __init__(self):
        self.color = 'black'


class Cat(Animal):
    pass


if __name__ == '__main__':
    ci = Cat()
    print(ci.color) # black

# this works fine, Animal's attr is be inherited.

Fine 2

If a sub class want to declare its own attributes and inherit its parent’s attributes, do as follows:

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
class Animal:
    def __init__(self):
        self.color = 'black'


class Cat(Animal):
    def __init__(self):
        super().__init__()
        self.age = 1


if __name__ == '__main__':
    ci = Cat()
    print(ci.color, ci.age) # black 1

Fine 3: Multiple Inheritance - solution 1

If a sub class inherits from multiple parent class, you should do like as follows.

If you do not explicitly invoke parent’s __init__ for each parent class in Subclass’s __init__, it will only inherit the first parent class’s attributes.(The first parent class in the following code is Engine class.)

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
class Engine:
    def __init__(self):
        self.performance = 80


class Skeleton:
    def __init__(self):
        self.shape = 'Rectangle'


class Car(Engine, Skeleton):
    def __init__(self):
        Engine.__init__(self)
        Skeleton.__init__(self)

    def deliver(self):
        print(self.performance)
        print(self.shape)


if __name__ == '__main__':
    BMW = Car()
    BMW.deliver()

Fine 3: Multiple Inheritance - solution 2

Or like this:

(If you are not sure the parent class’s behavior, Solution 1 is very good.)

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
class Engine:
    def __init__(self):
        super().__init__()
        self.performance = 80


class Skeleton:
    def __init__(self):
        super().__init__()
        self.shape = 'Rectangle'


class Car(Engine, Skeleton):
    def __init__(self):
        super().__init__()

    def deliver(self):
        print(self.performance)
        print(self.shape)

Conclusion

At the most time(Or always. Unless you know why you need not invoke init), you should invoke parent’s init in every subclass init method.