'logging' module - multiple messages

[Robin Munn]

David N. Welton <davidw at dedasys.com> wrote:
> 
> [ CC'ing replies to me appreciated - thanks. ]
> 
> When I do something like this:
> 
> 
> class WFLog:
>     def __init__(self, name="WF"):
>         self.log = logging.getLogger(name)
>         f = logging.Formatter("%(name)s|%(levelname)s|%(message)s" )
>         h = logging.StreamHandler()
>         h.setFormatter(f)
>         self.log.addHandler(h)
>         self.log.setLevel(logging.DEBUG)
>         self.log.debug("Logging enabled")
> 
> ....
> 
>     wl = WFLog("WF")
>     wl2 = WFLog("WF.SC")
>     wl.debug("testing, testing, 1 2 3")
>     wl2.debug("prova, prova, 1 2 3")
> 
> It appears that it prints out a copy for each logger that exists:
> 
> WF|DEBUG|Logging enabled
> WF.SC|DEBUG|Logging enabled
> WF.SC|DEBUG|Logging enabled
> WF|DEBUG|?|testing, testing, 1 2 3
> WF.SC|DEBUG|?|prova, prova, 1 2 3
> WF.SC|DEBUG|?|prova, prova, 1 2 3
> 
> Which is annoying.  Any way to get it to just do one copy per log
> instance?
> 
> Thanks for your time,

If you read the documentation for the logging module at

    http://www.python.org/dev/doc/devel/lib/module-logging.html

you will find this in italics at the end of the sixth paragraph: "In
addition to any handlers directly associated with a logger, all handlers
associated with all ancestors of the logger are called to dispatch the
message." The second paragraph of the documentation explains that
"[e]ach instance has a name, and they are conceptually arranged in a
name space hierarchy using dots (periods) as separators. For example, a
logger named 'scan' is the parent of loggers 'scan.text', 'scan.html'
and 'scan.pdf'."

So what's going on is that your "WF" logger is being considered the
parent of "WF.SC" -- so when WF.SC logs a message, WF receives it too.
This will usually be what you want - you could then attach different
handlers to the different elements in the tree (say WF.SC goes to the
screen via a StreamHandler with level CRITICAL, and WF goes to a log
file with level DEBUG, for example). If you really don't want that, you
could create a Filter that checks the name of the logger that produced
the message, like so:

    # "Selfish" filter that doesn't listen to its children
    class SelfishFilter(logging.Filter):
        """Filter that doesn't allow messages from children"""
        def filter(self, record):
            return self.name == record.name

Then you would attach this filter to your code like so:

    class WFLog:
        def __init__(self, name="WF"):
            self.log = logging.getLogger(name)
            f = logging.Formatter("%(name)s|%(levelname)s|%(message)s" )
            h = logging.StreamHandler()
            filter = SelfishFilter(name)
            h.setFormatter(f)
            self.log.addHandler(h)
            self.log.addFilter(filter)
            self.log.setLevel(logging.DEBUG)
            self.log.debug("Logging enabled")

That should give you the behavior you're looking for.

-- 
Robin Munn <rmunn at pobox.com>
http://www.rmunn.com/
PGP key ID: 0x6AFB6838    50FF 2478 CFFB 081A 8338  54F7 845D ACFD 6AFB 6838