Aren't Django signals a little like COMEFROM? | Hacker News Reader

Aren't Django signals a little like COMEFROM?

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10 pointsteilo9 comments

I've always just looked at them the way I look at any other event-based hooks -- they're a way to say "when this thing happens, I want to know about it and run some code".

Some programming languages even have this built in :)

But doesn't it lead itself to "Action at distance" anti pattern?

Like everyone else, we use django-regsitration a lot. Here in your code, http://bitbucket.org/ubernostrum/django-registration/src/tip... you send a custom signal. Now any one can break the normal flow of control and call control _from any where in the project to anywhere in the project_.

Signals defines a fairly concise interface for interacting with events.

You're not "stealing" control from the loop. Rather, think of it more as listening and attaching something to an event.

For me, it helps to think of signals less as interrupts in a traditional program loop, but as something more akin to events in event-driven environments.

   break the normal flow of control

But after the signal is fired, and handlers do their handling, normal flow is restored and the code just after the signal resumes (assuming no exceptions were thrown). COMEFROM doesn't make that guarantee, making it much harder to follow.

It isn't that code can jump to an arbitrary point in the program (a function call can do that) but that the code after that point won't get called, barring a corresponding GOTO back to the label.

It's better because it doesn't require messing around with, or even being aware of the internals of the code it's being connected to. It's actually a lot like a Common Lisp :before method, which I've found really bloody useful on a few occasions.

CL's customizable generic dispatch is just more than :before :-) the whole thing is just tasty.

Indeed. If I ever design a general-purpose language, you can bet on it having something of that sort.

I think it's fine to say that you don't like signals. But I wouldn't do so without proposing a better alternative.

No they aren't. They are an implementation of the template method pattern, perhaps with some cross polination from an observer pattern.

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