Simple questions on use of objects (probably faq)

[Steven D'Aprano]

On Thu, 09 Mar 2006 11:54:11 +0100, Brian Elmegaard wrote:

>> What is "going wrong" exactly ?
> 
>   def _add_instance(cls, instance):
>       _add_instance=classmethod(_add_instance)
>       cls._instances.append(instance)
> 
> gives me:
> d:/DTU/80494 $ python.exe ooo.py 
> Traceback (most recent call last):
>   File "ooo.py", line 36, in ?
>     Foo(value)
>   File "ooo.py", line 6, in __init__
>     self._add_instance(self)
>   File "ooo.py", line 9, in _add_instance
>     _add_instance=classmethod(_add_instance)
> UnboundLocalError: local variable '_add_instance' referenced before assignment

This isn't a bug in version 2.3. It is a bug in your code.  Follow the
code at runtime when you call the method:

Calling x._add_instance(foo) calls the _add_instance method with arguments
x.__class__ and foo. The first instruction that gets executed is:

_add_instance=classmethod(_add_instance)

but _add_instance doesn't exist inside the method's local scope, so you
get an UnboundLocalError exception.

What you probably think you want is something like this:

class Foo:
    _instances = []

    def _add_instance(cls, instance):
        cls._instances.append(instance)

    _add_instances = classmethod(_add_instances)


I say "think you want" because I don't know what problem you are trying to
solve with this messy, self-referential, piece of code. If you could
explain what your goal is, there is probably a better way of reaching it.


-- 
Steven.