[Python-Dev] PEP: Post import hooks

[Phillip J. Eby]

At 11:45 PM 1/10/2008 +0100, Christian Heimes wrote:
>In my version a hook is immediately called when the the registry value
>is set to None. When a hook is registered for a module during the
>execution of the callback then the hook is fired directly and not after
>the existing hooks are called. Is this a problem for you?

Yes, because it violates the invariant that hooks for a given module 
are called in the same order that they were registered in.

In case you're wondering why it's a problem, it's because if a hook 
imports something that registers a hook, when previously that thing 
wasn't imported until later, all of a sudden your callback order is 
very different than what it was before.

More succinctly: if making small changes to your program can cause 
large differences in the result, it's hard to debug.  (Especially if 
you have no idea what 3rd party module changed its import order and 
messed things up.)

Believe me, it's a lot easier to debug if there is a globally 
understandable hook order, even if it's still a partial ordering.