On inner classes

Gonçalo Rodrigues op73418 at mail.telepac.pt
Mon Feb 25 18:43:03 EST 2002

On Mon, 25 Feb 2002 17:36:02 -0500, mcherm <mcherm at destiny.com> wrote:

>bjorn writes:
>> You can't. Think about it. Your inner_class is at class scope in
>> outer_class. What you're after is a value of an *instance* of
>> outer_class, however you have no handle to such an instance from within
>> inner_class, ergo you can't get to it.
>Maybe I can make this a little clearer. Your use of the term "inner 
>class" suggests that you might be coming from a Java background. In 
>java, there are things called inner classes, and the way that they work 
>is that each instance of the inner class is "magically" connected to an
>instance of the outer class.
>That's not what happens when you declare one class inside of another in 
>Python -- in Python the "inner" class is just another class, and its 
>instances are not "magically" bound to instances of the outer class. 
>It's as if (in Python language) they were declared to be STATIC inner 
>Most of the time, this is the behavior that you want -- inner classes in 
>Java were invented to solve problems that are really easy to handle in 
>Python because of the existance of bound methods. (I can explain what 
>that meant if you like.) But as you might expect, if you actually DO 
>want the java-like behavior, Python allows it, you just have to be 
>explicit and explain what you mean.
>Here's some code illustrating what I mean:
>Python 2.2c1 (#27, Dec 14 2001, 13:15:16) [MSC 32 bit (Intel)] on win32
>Type "copyright", "credits" or "license" for more information.
>IDLE 0.8 -- press F1 for help
> >>> class Outer:
>	def makeAnInner(self):
>		class Inner:
>			def __init__(self, myOuter):
>				self.myOuter = myOuter
>			def innerMethod(self):
>				print "I'm an inner, my outer is %s" % self.myOuter
>				self.myOuter.outerMethod()
>		anInner = Inner(self)
>		anInner.innerMethod()
>	def outerMethod(self):
>		print "This is an outer method."
> >>> anOuter = Outer()
> >>> anOuter.makeAnInner()
>I'm an inner, my outer is <__main__.Outer instance at 0x00954E90>
>This is an outer method.
>There are only two things you have to do differently here than in 
>Java... you have to declare the __init__() method of Inner to take a 
>myOuter parameter and store it, and you have to explicitly pass the 
>outer when you create it in the line "anInner = Inner(self)". By 
>requiring you to explicitly create this outer-inner relationship when 
>you want it, Python adds the ability to NOT have the relationship when 
>you DON'T need it.
>-- Michael Chermside

Thanx, Both Bjorn and Michael, great explanation!

Gonçalo Rodrigues

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