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Python Enhancement Proposals

PEP 337 – Logging Usage in the Standard Library

Michael P. Dubner <dubnerm at>
Standards Track

Table of Contents


This PEP defines a standard for using the logging system (PEP 282) in the standard library.

Implementing this PEP will simplify development of daemon applications. As a downside this PEP requires slight modifications (however in a back-portable way) to a large number of standard modules.

After implementing this PEP one can use following filtering scheme:


PEP Deferral

Further exploration of the concepts covered in this PEP has been deferred for lack of a current champion interested in promoting the goals of the PEP and collecting and incorporating feedback, and with sufficient available time to do so effectively.


There are a couple of situations when output to stdout or stderr is impractical:

  • Daemon applications where the framework doesn’t allow the redirection of standard output to some file, but assumes use of some other form of logging. Examples are syslog under *nix’es and EventLog under WinNT+.
  • GUI applications which want to output every new log entry in separate pop-up window (i.e. fading OSD).

Also sometimes applications want to filter output entries based on their source or severity. This requirement can’t be implemented using simple redirection.

Finally sometimes output needs to be marked with event timestamps, which can be accomplished with ease using the logging system.


Every module usable for daemon and GUI applications should be rewritten to use the logging system instead of print or sys.stdout.write.

There should be code like this included in the beginning of every modified module:

import logging

_log = logging.getLogger('py.<module-name>')

A prefix of py. [2] must be used by all modules included in the standard library distributed along with Python, and only by such modules (unverifiable). The use of _log is intentional as we don’t want to auto-export it. For modules that use log only in one class a logger can be created inside the class definition as follows:

class XXX:

    __log = logging.getLogger('py.<module-name>')

Then this class can create access methods to log to this private logger.

So print and sys.std{out|err}.write statements should be replaced with _log.{debug|info}, and traceback.print_exception with _log.exception or sometimes _log.debug('...', exc_info=1).

Module List

Here is a (possibly incomplete) list of modules to be reworked:

  • asyncore (dispatcher.log, dispatcher.log_info)
  • BaseHTTPServer (BaseHTTPRequestHandler.log_request, BaseHTTPRequestHandler.log_error, BaseHTTPRequestHandler.log_message)
  • cgi (possibly - is cgi.log used by somebody?)
  • ftplib (if FTP.debugging)
  • gopherlib (get_directory)
  • httplib (HTTPResponse, HTTPConnection)
  • ihooks (_Verbose)
  • imaplib (IMAP4._mesg)
  • mhlib (MH.error)
  • nntplib (NNTP)
  • pipes (Template.makepipeline)
  • pkgutil (extend_path)
  • platform (_syscmd_ver)
  • poplib (if POP3._debugging)
  • profile (if Profile.verbose)
  • robotparser (_debug)
  • smtplib (if SGMLParser.verbose)
  • shlex (if shlex.debug)
  • smtpd (SMTPChannel/PureProxy where print >> DEBUGSTREAM)
  • smtplib (if SMTP.debuglevel)
  • SocketServer (BaseServer.handle_error)
  • telnetlib (if Telnet.debuglevel)
  • threading? (_Verbose._note, Thread.__bootstrap)
  • timeit (Timer.print_exc)
  • trace
  • uu (decode)

Additionally there are a couple of modules with commented debug output or modules where debug output should be added. For example:

  • urllib

Finally possibly some modules should be extended to provide more debug information.

Doubtful Modules

Listed here are modules that the community will propose for addition to the module list and modules that the community say should be removed from the module list.

  • tabnanny (check)

Guidelines for Logging Usage

Also we can provide some recommendation to authors of library modules so they all follow the same format of naming loggers. I propose that non-standard library modules should use loggers named after their full names, so a module “spam” in sub-package “junk” of package “dummy” will be named “dummy.junk.spam” and, of course, the __init__ module of the same sub-package will have the logger name “dummy.junk”.



Last modified: 2023-09-09 17:39:29 GMT