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Subject: Re: Design thought for callbacks
From: Cem Karan <cfkaran2@gmail.com>
In-Reply-To: <mc95pr$gau$1@ger.gmane.org>
Date: Sat, 21 Feb 2015 08:14:03 -0500
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References: <33677AE8-B2FA-49F9-9304-C8D93784255D@gmail.com> <mc95pr$gau$1@ger.gmane.org>
To: Frank Millman <frank@chagford.com>
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On Feb 21, 2015, at 12:41 AM, Frank Millman <frank@chagford.com> wrote:

>=20
> "Cem Karan" <cfkaran2@gmail.com> wrote in message=20
> news:33677AE8-B2FA-49F9-9304-C8D93784255D@gmail.com...
>> Hi all, I'm working on a project that will involve the use of =
callbacks,=20
>> and I want to bounce an idea I had off of everyone to make sure I'm =
not=20
>> developing a bad idea.  Note that this is for python 3.4 code; I =
don't=20
>> need to worry about any version of python earlier than that.
>>=20
>> In order to inform users that certain bits of state have changed, I=20=

>> require them to register a callback with my code.  The problem is =
that=20
>> when I store these callbacks, it naturally creates a strong reference =
to=20
>> the objects, which means that if they are deleted without =
unregistering=20
>> themselves first, my code will keep the callbacks alive.  Since this =
could=20
>> lead to really weird and nasty situations, I would like to store all =
the=20
>> callbacks in a WeakSet=20
>> (https://docs.python.org/3/library/weakref.html#weakref.WeakSet).  =
That=20
>> way, my code isn't the reason why the objects are kept alive, and if =
they=20
>> are no longer alive, they are automatically removed from the WeakSet,=20=

>> preventing me from accidentally calling them when they are dead.  My=20=

>> question is simple; is this a good design?  If not, why not?
>>  Are there any potential 'gotchas' I should be worried about?
>>=20
>=20
> I tried something similar a while ago, and I did find a gotcha.
>=20
> The problem lies in this phrase - "if they are no longer alive, they =
are=20
> automatically removed from the WeakSet, preventing me from =
accidentally=20
> calling them when they are dead."
>=20
> I found that the reference was not removed immediately, but was =
waiting to=20
> be garbage collected. During that window, I could call the callback, =
which=20
> resulted in an error.
>=20
> There may have been a simple workaround. Perhaps someone else can =
comment.

THAT would be one heck of a gotcha!  Must have been fun debugging that =
one!

Cem Karan=