Passing contextual information when logging
vinay_sajip at yahoo.co.uk
Tue Jan 8 10:08:08 CET 2008
Some users of the logging package have raised an issue regarding the
difficulty of passing additional contextual information when logging.
For example, the developer of a networked application may want to log,
in addition to specifics related to to the network service being
provided, information about the IP address of the remote machine and
the username of the person logged into and using the service.
Python 2.4 introduced an 'extra' keyword argument which was intended
to hold a dict-like object containing additional information to be
added to a LogRecord. The additional information would then be printed
via placeholders in a format string.
While this works, it is a little unwieldy to use in practice, because
users need to provide the 'extra' parameter in every logging call.
This has led people in some instances to create context-specific
Logger instances (e.g. one logger for every connection). This has a
drawback in that a logger name can only provide limited contextual
information, and moreover, if the number of connections is effectively
unbounded over time, the number of Logger instances will also grow in
an unbounded way. (Logger instances are never garbage collected,
because references to them are always held in the logging package.
This alleviates a burden on users in that they never have to pass
loggers around, but means that creating a lot of Logger instances will
lead to a long-lived memory burden.)
One solution is to create a generic wrapper around loggers to which a
logger name and contextual information can be passed. The wrapper
would delegate logging calls to the logger with the specified name,
but would manipulate the arguments passed to the logging call to
insert the contextual information. I have created such a wrapper
class, called LoggerAdapter, which is in the example script located at
I would welcome your views on whether the LoggerAdapter class is
suitable for adding to the logging package in Python 2.6/3.0. Does it
do what might reasonably be expected out of the box? LoggerAdapters
are, of course, garbage collected in the normal way and so impose no
particular memory burden.
More information about the Python-list