Overriding a method at the instance level on a subclass of a builtin type

[Aaron Brady]

On Dec 2, 6:58 pm, "Zac Burns" <zac... at gmail.com> wrote:
> Sorry for the long subject.
>
> I'm trying to create a subclass dictionary that runs extra init code
> on the first __getitem__ call. However, the performance of __getitem__
> is quite important - so I'm trying in the subclassed __getitem__
> method to first run some code and then patch in the original dict
> method for the instance to avoid even the check to see if the init
> code has been run. Various recipes using instancemethod and the like
> have failed me.
>
> Curiously if __slots__ is not specified no error occurs when setting
> self.__getitem__ but the function is not overriden. If __slots__ is
> ['__getitem__'] however it complains that __getitem__ is read only. I
> do not understand that behavior.
>
> --
> Zachary Burns
> (407)590-4814
> Aim - Zac256FL
> Production Engineer (Digital Overlord)
> Zindagi Games

That sounds like the State Pattern, from GoF.  http://en.wikipedia.org/wiki/State_pattern

I like the idea of 'renaming', not redefining, but reassigning methods
at different points during an object's lifetime.  I often wish I had
more experience with it, and more docs talked about it.

It's hard on memory usage, since each instance has its own function
attribute, even if there's still only one instance of the function.
Without it, the function attribute is just looked up on the class.

Not thoroughly tested:

>>> class A:
...     def methA( self ):
...             print 'methA'
...             self.meth= self.methB
...     meth= methA
...     def methB( self ):
...             print 'methB'
...
>>> a= A()
>>> a.meth()
methA
>>> a.meth()
methB