[Python-Dev] PEP: Post import hooks (original) (raw)

Phillip J. Eby pje at telecommunity.com
Fri Jan 11 00:10:08 CET 2008

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.

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