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?

• Holt: You need to call the constructor of the parent classes manually - Here, self.text is initialize in Parent constructor which is never called.
• Abdul Ahmad: @Holt what would that be? super().__init__() ?
• deceze: @Abdul I misread. For that purpose you should just use pass, that's what it's for (a dummy body).
• Holt: If you don't want python complaining about empty block, use pass, e.g. if a: pass (only for demonstration purpose).
• mgilson: 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.

Answer 1

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'

• Abdul Ahmad: any reason why I would use super vs Parent init?
• Holt: @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: 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: @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.

Answer 2

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

Answer 3

your init function needs to call the parent init

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

Answer 4

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.