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Subject: Re: Design thought for callbacks
From: Cem Karan <cfkaran2@gmail.com>
In-Reply-To: <mcargl$h3k$1@reader1.panix.com>
Date: Sun, 22 Feb 2015 08:21:35 -0500
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To: Grant Edwards <invalid@invalid.invalid>
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On Feb 21, 2015, at 3:57 PM, Grant Edwards <invalid@invalid.invalid> =
wrote:

> On 2015-02-21, Cem Karan <cfkaran2@gmail.com> wrote:
>>=20
>> On Feb 21, 2015, at 12:42 AM, Chris Angelico <rosuav@gmail.com> =
wrote:
>>=20
>>> 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.
>>=20
>> OK, so it would violate the principle of least surprise for you.
>=20
> And me as well.  I would expect to be able to pass a closure as a
> callback and not have to keep a reference to it.  Perhaps that just a
> leftover from working with other languages (javascript, scheme, etc.).
> It doesn't matter if it's a string, a float, a callback, a graphic or
> whatever: if I pass your function/library an object, I expect _you_ to
> keep track of it until you're done with it.
>=20
>> Interesting.  Is this a general pattern in python?  That is,
>> callbacks are owned by what they are registered with?
>=20
> I'm not sure what you mean by "owned" or why it matters that it's a
> callback: it's an object that was passed to you: you need to hold onto
> a reference to it until you're done with it, and the polite thing to
> do is to delete references to it when you're done with it.

I tend to structure my code as a tree or DAG of objects.  The owner =
refers to the owned object, but the owned object has no reference to its =
owner.  With callbacks, you get cycles, where the owned owns the owner.  =
As a result, if you forget where your object has been registered, it may =
be kept alive when you aren't expecting it.  My hope was that with =
WeakSets I could continue to preserve the DAG or tree while still having =
the benefits of callbacks.  However, it looks like that is too =
surprising to most people.

Thanks,
Cem Karan=