How does python variable inheritance work?

Question

I'm trying something very basic with Python inheritance:

class Parent:
    def __init__(self):
        self.text = 'parent'

    def getText(self):
        print self.text

class Child1(Parent):
    def __init__(self):
        self.x = 'x'

class Child2(Parent):
    def __init__(self):
        self.x = 'x'

if __name__ == "__main__": 
    parent = Parent()
    child1 = Child1()
    child2 = Child2()

    parent.getText()
    child1.getText()
    child2.getText()

but I keep getting

Child1 instance has no attribute 'text'

how are variables passed to children?

You need to call the constructor of the parent classes manually - Here, self.text is initialize in Parent constructor which is never called. — Holt
– Holt, Commented Mar 3, 2016 at 15:50
@Abdul I misread. For that purpose you should just use pass, that's what it's for (a dummy body). — deceze
– deceze ♦, Commented Mar 3, 2016 at 15:53
If you don't want python complaining about empty block, use pass, e.g. if a: pass (only for demonstration purpose). — Holt
– Holt, Commented Mar 3, 2016 at 15:54
In addition to pass, if the block is also a function, then adding a docstring also works to prevent python from complaining about the indentation being wrong. — mgilson
– mgilson, Commented Mar 3, 2016 at 16:03

Holt · Accepted Answer · 2016-03-03 15:53:03Z

11

You need to call the constructor of the parent classes manually - Here, self.text is initialize in Parent constructor which is never called:

class Child1(Parent):
    def __init__ (self):
        super(Child1, self).__init__ ()
        # or Parent.__init__ (self)
        self.x = 'x'

answered Mar 3, 2016 at 15:53

Holt

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4 Comments

Abdul Ahmad Over a year ago

any reason why I would use super vs Parent init?

Holt Over a year ago

@AbdulAhmad There are multiple posts on SO about this: stackoverflow.com/questions/222877/how-to-use-super-in-python, stackoverflow.com/questions/576169/…. I personally use super most of the time.

Abdul Ahmad Over a year ago

one more question, what's the correct way of altering the self.text from parent in a child? just self.text = 'new text'? or is there another way more suited for inheritance? (I will accept the answer once the timer is down, have 2 minutes left)

deceze Over a year ago

@AbdulAhmad You're just setting an attribute on the self object, it's no different than altering it in another method of the object. Just assign a new value to it.

Ragnar Kruse · Accepted Answer · 2020-07-09 18:24:39Z

Since python 3.6, we can now use the __init_subclass__ function, which is called automatically before __init__ of the Child.

class Parent:
    def __init__(self):
        self.text = 'parent'
    
    def __init_subclass__(self):
        Parent.__init__(self)

    def getText(self):
        print(self.text)

class Child1(Parent): pass
class Child2(Parent): pass

classes = [Parent(), Child1(), Child2()]

for item in classes:
    item.getText()

output

parent
parent
parent

If you use your Parent class more as a "interface", here is another example.

class Animal():
    def __init_subclass__(self, sound):
        self.sound = sound

    def make_sound(self):
        print(self.sound)

class Cat(Animal, sound='meow'): pass
class Dog(Animal, sound='woof'): pass

animals = [Cat(), Dog()]

for animal in animals:
    animal.make_sound()

output

meow
woof

lafferc · Accepted Answer · 2016-03-03 15:52:43Z

1

your init function needs to call the parent init

class Child1(Parent):
def __init__(self):
    self.x = 'x'
    Parent.__init__(self)

answered Mar 3, 2016 at 15:52

lafferc

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Comments

GLaDOS · Accepted Answer · 2016-03-03 15:55:32Z

1

In python when you override a function which was supposed to be inherited you override all of it, __init__ is no exception. You should call the functions super method to use the base initializer, or implement the attribute in the constructor you have rewrote.

class Parent:
    def __init__(self):
        self.text = 'parent'

    def getText(self):
        print self.text

class Child1(Parent):
    def __init__(self):
        super(Child1, self).__init__()
        self.x = 'x'

child1.getText()

Should work now.

answered Mar 3, 2016 at 15:55

GLaDOS

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