[Tutor] inheritance, python and pygame

[Steven D'Aprano]

rail shafigulin wrote:
> i'm trying to learn OOP, python and pygame at the same time. so far it has
> been ok, but i stumbled onto a problem

Hugo has already pointed out that you mispelled __init__, and that in 
Python 3 you don't need to explicitly provide arguments to super().

> class BaseSprite(pygame.sprite.Sprite):
>   def __init(self, imagefile):
>     super(type(self), self).__init__()

Should be written like this in Python 3:

     def __init__(self, imagefile):
         super().__init__()


However, for those still using Python 2.x, you might be tempted to write 
this:

     def __init__(self, imagefile):
         super(type(self), self).__init__()  # NO! Don't do this!


This does not work! But you might be fooled into thinking it does work, 
because it *seems* to work if you only check a simple example:

class Base(object):
     def __init__(self):
         print "Base called"

class Subclass(Base):
     def __init__(self):
         print "Subclass called"
         super(type(self), self).__init__()  # Wrong!


It seems to work:

 >>> x = Subclass()
Subclass called
Base called


But watch what happens when we subclass again:

class AnotherSubclass(Subclass):
     def __init__(self):
         print "AnotherSubclass called"
         super(type(self), self).__init__()


 >>> y = AnotherSubclass()
AnotherSubclass called
Subclass called
Subclass called
Subclass called
...
Traceback (most recent call last):
   File "<stdin>", line 1, in <module>
   File "<stdin>", line 4, in __init__
   File "<stdin>", line 4, in __init__
   File "<stdin>", line 4, in __init__
   File "<stdin>", line 4, in __init__
   ...
RuntimeError: maximum recursion depth exceeded



The problem is that inside Subclass.__init__ the call to 
super(type(self), self) does not necessary give super(Subclass, self). 
If the instance being created is from AnotherSubclass, you get 
super(AnotherSubclass, self) inside the Subclass.__init__, which then 
loops forever.

So in Python 2.x, you must explicitly give the class. You can't use 
type(self).



-- 
Steven