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Subject: Re: Design thought for callbacks
From: Cem Karan <cfkaran2@gmail.com>
In-Reply-To: <CAPTjJmpEHQV2PpaZvszuTGn0_GNg0sWV6aLenbQsZrSXCy2YKQ@mail.gmail.com>
Date: Sat, 21 Feb 2015 08:13:08 -0500
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To: Chris Angelico <rosuav@gmail.com>
Cc: "comp.lang.python" <python-list@python.org>
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On Feb 21, 2015, at 12:42 AM, Chris Angelico <rosuav@gmail.com> wrote:

> On Sat, Feb 21, 2015 at 1:44 PM, Cem Karan <cfkaran2@gmail.com> wrote:
>> In order to inform users that certain bits of state have changed, I =
require them to register a callback with my code.  The problem is that =
when I store these callbacks, it naturally creates a strong reference to =
the objects, which means that if they are deleted without unregistering =
themselves first, my code will keep the callbacks alive.  Since this =
could lead to really weird and nasty situations, I would like to store =
all the callbacks in a WeakSet =
(https://docs.python.org/3/library/weakref.html#weakref.WeakSet).  That =
way, my code isn't the reason why the objects are kept alive, and if =
they are no longer alive, they are automatically removed from the =
WeakSet, preventing me from accidentally calling them when they are =
dead.  My question is simple; is this a good design?  If not, why not?  =
Are there any potential 'gotchas' I should be worried about?
>>=20
>=20
> No, it's not. I would advise using strong references - if the callback
> is a closure, for instance, you need to hang onto it, because there
> are unlikely to be any other references to it. If I register a
> callback with you, I expect it to be called; I expect, in fact, that
> that *will* keep my object alive.

OK, so it would violate the principle of least surprise for you.  =
Interesting.  Is this a general pattern in python?  That is, callbacks =
are owned by what they are registered with?

In the end, I want to make a library that offers as few surprises to the =
user as possible, and no matter how I think about callbacks, they are =
surprising to me.  If callbacks are strongly-held, then calling 'del =
foo' on a callable object may not make it go away, which can lead to =
weird and nasty situations.  Weakly-held callbacks mean that I (as the =
programmer), know that objects will go away after the next garbage =
collection (see Frank's earlier message), so I don't get 'dead' =
callbacks coming back from the grave to haunt me.

So, what's the consensus on the list, strongly-held callbacks, or =
weakly-held ones?

Thanks,
Cem Karan=