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Subject: Re: Design thought for callbacks
From: Cem Karan <cfkaran2@gmail.com>
In-Reply-To: <mcf6el$ogp$1@ger.gmane.org>
Date: Tue, 24 Feb 2015 06:00:55 -0500
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To: "Frank Millman" <frank@chagford.com>
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On Feb 23, 2015, at 7:29 AM, "Frank Millman" <frank@chagford.com> wrote:

>=20
> "Cem Karan" <cfkaran2@gmail.com> wrote in message=20
> news:A3C11A70-5846-4915-BB26-B23793B65670@gmail.com...
>>=20
>>=20
>> Good questions!  That was why I was asking about 'gotchas' with =
WeakSets=20
>> originally.  Honestly, the only way to know for sure would be to =
write two=20
>> APIs for doing similar things, and then see how people react to them. =
 The=20
>> problem is, how do you set up such a study so it is statistically =
valid?
>>=20
>=20
> Just in case you missed Steven's comment on my 'gotcha', and my reply, =
it is=20
> worth repeating that what I reported as a gotcha was not what it =
seemed.
>=20
> If you set up the callback as a weakref, and the listening object goes =
out=20
> of scope, it will wait to be garbage collected. However, as far as I =
can=20
> tell, the weakref is removed at the same time as the object is gc'd, =
so=20
> there is no 'window' where the weakref exists but the object it is=20
> referencing does not exist.
>=20
> My problem was that I had performed a cleanup operation on the =
listening=20
> object before letting it go out of scope, and it was no longer in a =
valid=20
> state to deal with the callback, resulting in an error. If you do not =
have=20
> that situation, your original idea may well work.

Thank you Frank, I did read Steve's comment to your reply earlier, but =
what you said in your original reply made sense to me.  I don't have =
control over user code.  That means that if someone wants to write code =
such that they perform some kind of cleanup and are no longer able to =
handle the callback, they are free to do so.  While I can't prevent this =
from happening, I can make it as obvious as possible in my code that =
before you perform any cleanup, you also need to unregister from the =
library.  That is my main goal in developing pythonic/obvious methods of =
registering callbacks.

Thanks,
Cem Karan=