[Python-Dev] Changing existing class instances

[Jim Fulton]

"A.M. Kuchling" wrote:
> 
> Currently, when you replace a class definition with an updated
> version, it's really difficult to change existing class instances;
> you'd have to essentially sweep every Python object and check if it's
> an instance, starting at roots such as __main__ and sys.modules.  This
> makes developing code in a long-running process difficult, Zope being
> the best example of this.  When you modify a class definition used by
> Zope code, you can't update existing instances floating around in
> memory.

In the case of Zope, if the objects that you care about happen to be 
persistent objects, then it's relatively easy to arrange to get the
objects flushed from memory and reloaded with the new classes.
(There are some subtle issues to deal with, like worrying about 
multiple threads, but in a development environment, you can deal with
these, for example, by limiting the server to one thread.)

Note that this is really only a special case of a much larger problem.

Reloading a module redefines the global variables in a module.
It doesn't update any references to those global references
from other places, such as instances or *other* modules.

For example, imports like:

  from foo import spam

are not updated when foo is reloaded.

Maybe you are expecting too much from reload.

Jim      

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