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Date: Tue, 17 Jan 2012 10:48:22 +0000 (GMT)
From: Vinay Sajip <vinay_sajip@yahoo.co.uk>
Subject: Re: python logging filter limitation, looks intentional?
To: Chris Withers <chris@simplistix.co.uk>
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> From: Chris Withers <chris@simplistix.co.uk>=0A=0A> How breaking code? Co=
nfiguration, maybe, but I can't see anyone being upset =0A> that filtering =
would begin working "the same as everything else".=0A> This just feels like=
 a bug...=0A=0AWell, it means that filters that don't get called now would =
get called - and that's a change in behaviour.=0A=0AIt's not a bug, because=
 it's like that by design. I understand that you don't agree with that part=
icular design decision, but it's out there now. What you're asking for isn'=
t unreasonable, but not achievable without a change in behaviour. I don't w=
ant people to have to code differently for Python 3.3 and for older version=
s.=0A=A0=0A> Actually, the filter I want to work on (log_nerf) is precisely=
 for 3rd party =0A> libraries; if a library author thinks a message should =
be error but you think it =0A> should be debug, and vice versa, having a fi=
lter that alters the level is the =0A> only way to sort things out without =
hacking the 3rd party library. I've also =0A> seen this technique used succ=
essfully where the filter looks to a database for =0A> config and nerfs mes=
sages during a certain time period.=0A=0AOkay,=A0 then applying a filter on=
 handlers is a way of achieving this.=0A=0A> Again, this doesn't help if th=
e __name__-ish loggers are in third party =0A> packages that you don't want=
 to have to fork ;-)=0A=0ATrue, but as I said earlier, you can attach a fil=
ter to your handlers. It's rather unlikely that you would have more than ha=
lf-a-dozen handlers (since handlers ~=3D potential audiences for log events=
), but even so, I can't believe it's especially onerous to set up filter co=
nfiguration for those handlers - and even easier if you add a DelegatingHan=
dler to the mix. In https://gist.github.com/1623702 I've shown a Delegating=
Handler; it's quite short.=0A=A0=0A> Both use cases are valid, and a Delega=
tingHandler just feels like a hack to work =0A> around a deficiency in the =
framework...=0A=0ARather, the approach I've taken is not to assume I'll mee=
t everyone's needs out of the box, but that the right pieces are in place f=
or people to use logging in useful ways in scenarios that I haven't thought=
 of - which shortcomings might well be termed "deficiencies" according to y=
our point of view - by deriving and overriding classes, or by composing usi=
ng filters, or setting module-level flags. There are numerous examples on e=
.g. Stack Overflow where a small amount of customisation (e.g. a custom han=
dler like DelegatingHandler) has been suggested to meet a specific user req=
uirement. I don't think of these as hacks - I occasionally have to subclass=
 stdlib classes in other areas to get the behaviour I want, and that's part=
 and parcel of having OO at your disposal.=0A=0ARegards,=0A=0AVinay Sajip=
=0A