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Call to __init__ of super class is missed

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

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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.

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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:

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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.)

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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.)

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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.