16.7. "logging.config" — Logging configuration
**********************************************

**Source code:** Lib/logging/config.py


Important
^^^^^^^^^

This page contains only reference information. For tutorials, please
see

* Basic Tutorial

* Advanced Tutorial

* Logging Cookbook

======================================================================

This section describes the API for configuring the logging module.


16.7.1. Configuration functions
===============================

The following functions configure the logging module. They are located
in the "logging.config" module.  Their use is optional — you can
configure the logging module using these functions or by making calls
to the main API (defined in "logging" itself) and defining handlers
which are declared either in "logging" or "logging.handlers".

logging.config.dictConfig(config)

      Takes the logging configuration from a dictionary.  The contents
      of this dictionary are described in Configuration dictionary
      schema below.

      If an error is encountered during configuration, this function
      will raise a "ValueError", "TypeError", "AttributeError" or
      "ImportError" with a suitably descriptive message.  The
      following is a (possibly incomplete) list of conditions which
      will raise an error:

      * A "level" which is not a string or which is a string not
        corresponding to an actual logging level.

      * A "propagate" value which is not a boolean.

      * An id which does not have a corresponding destination.

      * A non-existent handler id found during an incremental call.

      * An invalid logger name.

      * Inability to resolve to an internal or external object.

      Parsing is performed by the "DictConfigurator" class, whose
      constructor is passed the dictionary used for configuration, and
      has a "configure()" method.  The "logging.config" module has a
      callable attribute "dictConfigClass" which is initially set to
      "DictConfigurator". You can replace the value of
      "dictConfigClass" with a suitable implementation of your own.

      "dictConfig()" calls "dictConfigClass" passing the specified
      dictionary, and then calls the "configure()" method on the
      returned object to put the configuration into effect:

         def dictConfig(config):
             dictConfigClass(config).configure()

      For example, a subclass of "DictConfigurator" could call
      "DictConfigurator.__init__()" in its own "__init__()", then set
      up custom prefixes which would be usable in the subsequent
      "configure()" call. "dictConfigClass" would be bound to this new
      subclass, and then "dictConfig()" could be called exactly as in
      the default, uncustomized state.

   New in version 3.2.

logging.config.fileConfig(fname, defaults=None, disable_existing_loggers=True)

   Reads the logging configuration from a "configparser"-format file.
   The format of the file should be as described in Configuration file
   format. This function can be called several times from an
   application, allowing an end user to select from various pre-canned
   configurations (if the developer provides a mechanism to present
   the choices and load the chosen configuration).

   Parameters:
      * **fname** – A filename, or a file-like object, or an
        instance derived from "RawConfigParser". If a
        "RawConfigParser"-derived instance is passed, it is used as
        is. Otherwise, a "Configparser" is instantiated, and the
        configuration read by it from the object passed in "fname". If
        that has a "readline()" method, it is assumed to be a file-
        like object and read using "read_file()"; otherwise, it is
        assumed to be a filename and passed to "read()".

      * **defaults** – Defaults to be passed to the ConfigParser can
        be specified in this argument.

      * **disable_existing_loggers** – If specified as "False",
        loggers which exist when this call is made are left enabled.
        The default is "True" because this enables old behaviour in a
        backward-compatible way. This behaviour is to disable any
        existing loggers unless they or their ancestors are explicitly
        named in the logging configuration.

   Changed in version 3.4: An instance of a subclass of
   "RawConfigParser" is now accepted as a value for "fname". This
   facilitates:

   * Use of a configuration file where logging configuration is just
     part of the overall application configuration.

   * Use of a configuration read from a file, and then modified by
     the using application (e.g. based on command-line parameters or
     other aspects of the runtime environment) before being passed to
     "fileConfig".

logging.config.listen(port=DEFAULT_LOGGING_CONFIG_PORT, verify=None)

   Starts up a socket server on the specified port, and listens for
   new configurations. If no port is specified, the module’s default
   "DEFAULT_LOGGING_CONFIG_PORT" is used. Logging configurations will
   be sent as a file suitable for processing by "dictConfig()" or
   "fileConfig()". Returns a "Thread" instance on which you can call
   "start()" to start the server, and which you can "join()" when
   appropriate. To stop the server, call "stopListening()".

   The "verify" argument, if specified, should be a callable which
   should verify whether bytes received across the socket are valid
   and should be processed. This could be done by encrypting and/or
   signing what is sent across the socket, such that the "verify"
   callable can perform signature verification and/or decryption. The
   "verify" callable is called with a single argument - the bytes
   received across the socket - and should return the bytes to be
   processed, or "None" to indicate that the bytes should be
   discarded. The returned bytes could be the same as the passed in
   bytes (e.g. when only verification is done), or they could be
   completely different (perhaps if decryption were performed).

   To send a configuration to the socket, read in the configuration
   file and send it to the socket as a sequence of bytes preceded by a
   four-byte length string packed in binary using "struct.pack('>L',
   n)".

   Note: Because portions of the configuration are passed through
     "eval()", use of this function may open its users to a security
     risk. While the function only binds to a socket on "localhost",
     and so does not accept connections from remote machines, there
     are scenarios where untrusted code could be run under the account
     of the process which calls "listen()". Specifically, if the
     process calling "listen()" runs on a multi-user machine where
     users cannot trust each other, then a malicious user could
     arrange to run essentially arbitrary code in a victim user’s
     process, simply by connecting to the victim’s "listen()" socket
     and sending a configuration which runs whatever code the attacker
     wants to have executed in the victim’s process. This is
     especially easy to do if the default port is used, but not hard
     even if a different port is used). To avoid the risk of this
     happening, use the "verify" argument to "listen()" to prevent
     unrecognised configurations from being applied.

   Changed in version 3.4: The "verify" argument was added.

   Note: If you want to send configurations to the listener which
     don’t disable existing loggers, you will need to use a JSON
     format for the configuration, which will use "dictConfig()" for
     configuration. This method allows you to specify
     "disable_existing_loggers" as "False" in the configuration you
     send.

logging.config.stopListening()

   Stops the listening server which was created with a call to
   "listen()". This is typically called before calling "join()" on the
   return value from "listen()".


16.7.2. Configuration dictionary schema
=======================================

Describing a logging configuration requires listing the various
objects to create and the connections between them; for example, you
may create a handler named ‘console’ and then say that the logger
named ‘startup’ will send its messages to the ‘console’ handler. These
objects aren’t limited to those provided by the "logging" module
because you might write your own formatter or handler class. The
parameters to these classes may also need to include external objects
such as "sys.stderr".  The syntax for describing these objects and
connections is defined in Object connections below.


16.7.2.1. Dictionary Schema Details
-----------------------------------

The dictionary passed to "dictConfig()" must contain the following
keys:

* *version* - to be set to an integer value representing the schema
  version.  The only valid value at present is 1, but having this key
  allows the schema to evolve while still preserving backwards
  compatibility.

All other keys are optional, but if present they will be interpreted
as described below.  In all cases below where a ‘configuring dict’ is
mentioned, it will be checked for the special "'()'" key to see if a
custom instantiation is required.  If so, the mechanism described in
User-defined objects below is used to create an instance; otherwise,
the context is used to determine what to instantiate.

* *formatters* - the corresponding value will be a dict in which
  each key is a formatter id and each value is a dict describing how
  to configure the corresponding "Formatter" instance.

  The configuring dict is searched for keys "format" and "datefmt"
  (with defaults of "None") and these are used to construct a
  "Formatter" instance.

* *filters* - the corresponding value will be a dict in which each
  key is a filter id and each value is a dict describing how to
  configure the corresponding Filter instance.

  The configuring dict is searched for the key "name" (defaulting to
  the empty string) and this is used to construct a "logging.Filter"
  instance.

* *handlers* - the corresponding value will be a dict in which each
  key is a handler id and each value is a dict describing how to
  configure the corresponding Handler instance.

  The configuring dict is searched for the following keys:

  * "class" (mandatory).  This is the fully qualified name of the
    handler class.

  * "level" (optional).  The level of the handler.

  * "formatter" (optional).  The id of the formatter for this
    handler.

  * "filters" (optional).  A list of ids of the filters for this
    handler.

  All *other* keys are passed through as keyword arguments to the
  handler’s constructor.  For example, given the snippet:

     handlers:
       console:
         class : logging.StreamHandler
         formatter: brief
         level   : INFO
         filters: [allow_foo]
         stream  : ext://sys.stdout
       file:
         class : logging.handlers.RotatingFileHandler
         formatter: precise
         filename: logconfig.log
         maxBytes: 1024
         backupCount: 3

  the handler with id "console" is instantiated as a
  "logging.StreamHandler", using "sys.stdout" as the underlying
  stream.  The handler with id "file" is instantiated as a
  "logging.handlers.RotatingFileHandler" with the keyword arguments
  "filename='logconfig.log', maxBytes=1024, backupCount=3".

* *loggers* - the corresponding value will be a dict in which each
  key is a logger name and each value is a dict describing how to
  configure the corresponding Logger instance.

  The configuring dict is searched for the following keys:

  * "level" (optional).  The level of the logger.

  * "propagate" (optional).  The propagation setting of the logger.

  * "filters" (optional).  A list of ids of the filters for this
    logger.

  * "handlers" (optional).  A list of ids of the handlers for this
    logger.

  The specified loggers will be configured according to the level,
  propagation, filters and handlers specified.

* *root* - this will be the configuration for the root logger.
  Processing of the configuration will be as for any logger, except
  that the "propagate" setting will not be applicable.

* *incremental* - whether the configuration is to be interpreted as
  incremental to the existing configuration.  This value defaults to
  "False", which means that the specified configuration replaces the
  existing configuration with the same semantics as used by the
  existing "fileConfig()" API.

  If the specified value is "True", the configuration is processed as
  described in the section on Incremental Configuration.

* *disable_existing_loggers* - whether any existing loggers are to
  be disabled. This setting mirrors the parameter of the same name in
  "fileConfig()". If absent, this parameter defaults to "True". This
  value is ignored if *incremental* is "True".


16.7.2.2. Incremental Configuration
-----------------------------------

It is difficult to provide complete flexibility for incremental
configuration.  For example, because objects such as filters and
formatters are anonymous, once a configuration is set up, it is not
possible to refer to such anonymous objects when augmenting a
configuration.

Furthermore, there is not a compelling case for arbitrarily altering
the object graph of loggers, handlers, filters, formatters at run-
time, once a configuration is set up; the verbosity of loggers and
handlers can be controlled just by setting levels (and, in the case of
loggers, propagation flags).  Changing the object graph arbitrarily in
a safe way is problematic in a multi-threaded environment; while not
impossible, the benefits are not worth the complexity it adds to the
implementation.

Thus, when the "incremental" key of a configuration dict is present
and is "True", the system will completely ignore any "formatters" and
"filters" entries, and process only the "level" settings in the
"handlers" entries, and the "level" and "propagate" settings in the
"loggers" and "root" entries.

Using a value in the configuration dict lets configurations to be sent
over the wire as pickled dicts to a socket listener. Thus, the logging
verbosity of a long-running application can be altered over time with
no need to stop and restart the application.


16.7.2.3. Object connections
----------------------------

The schema describes a set of logging objects - loggers, handlers,
formatters, filters - which are connected to each other in an object
graph.  Thus, the schema needs to represent connections between the
objects.  For example, say that, once configured, a particular logger
has attached to it a particular handler.  For the purposes of this
discussion, we can say that the logger represents the source, and the
handler the destination, of a connection between the two.  Of course
in the configured objects this is represented by the logger holding a
reference to the handler.  In the configuration dict, this is done by
giving each destination object an id which identifies it
unambiguously, and then using the id in the source object’s
configuration to indicate that a connection exists between the source
and the destination object with that id.

So, for example, consider the following YAML snippet:

   formatters:
     brief:
       # configuration for formatter with id 'brief' goes here
     precise:
       # configuration for formatter with id 'precise' goes here
   handlers:
     h1: #This is an id
      # configuration of handler with id 'h1' goes here
      formatter: brief
     h2: #This is another id
      # configuration of handler with id 'h2' goes here
      formatter: precise
   loggers:
     foo.bar.baz:
       # other configuration for logger 'foo.bar.baz'
       handlers: [h1, h2]

(Note: YAML used here because it’s a little more readable than the
equivalent Python source form for the dictionary.)

The ids for loggers are the logger names which would be used
programmatically to obtain a reference to those loggers, e.g.
"foo.bar.baz".  The ids for Formatters and Filters can be any string
value (such as "brief", "precise" above) and they are transient, in
that they are only meaningful for processing the configuration
dictionary and used to determine connections between objects, and are
not persisted anywhere when the configuration call is complete.

The above snippet indicates that logger named "foo.bar.baz" should
have two handlers attached to it, which are described by the handler
ids "h1" and "h2". The formatter for "h1" is that described by id
"brief", and the formatter for "h2" is that described by id "precise".


16.7.2.4. User-defined objects
------------------------------

The schema supports user-defined objects for handlers, filters and
formatters.  (Loggers do not need to have different types for
different instances, so there is no support in this configuration
schema for user-defined logger classes.)

Objects to be configured are described by dictionaries which detail
their configuration.  In some places, the logging system will be able
to infer from the context how an object is to be instantiated, but
when a user-defined object is to be instantiated, the system will not
know how to do this.  In order to provide complete flexibility for
user-defined object instantiation, the user needs to provide a
‘factory’ - a callable which is called with a configuration dictionary
and which returns the instantiated object. This is signalled by an
absolute import path to the factory being made available under the
special key "'()'".  Here’s a concrete example:

   formatters:
     brief:
       format: '%(message)s'
     default:
       format: '%(asctime)s %(levelname)-8s %(name)-15s %(message)s'
       datefmt: '%Y-%m-%d %H:%M:%S'
     custom:
         (): my.package.customFormatterFactory
         bar: baz
         spam: 99.9
         answer: 42

The above YAML snippet defines three formatters.  The first, with id
"brief", is a standard "logging.Formatter" instance with the specified
format string.  The second, with id "default", has a longer format and
also defines the time format explicitly, and will result in a
"logging.Formatter" initialized with those two format strings.  Shown
in Python source form, the "brief" and "default" formatters have
configuration sub-dictionaries:

   {
     'format' : '%(message)s'
   }

and:

   {
     'format' : '%(asctime)s %(levelname)-8s %(name)-15s %(message)s',
     'datefmt' : '%Y-%m-%d %H:%M:%S'
   }

respectively, and as these dictionaries do not contain the special key
"'()'", the instantiation is inferred from the context: as a result,
standard "logging.Formatter" instances are created.  The configuration
sub-dictionary for the third formatter, with id "custom", is:

   {
     '()' : 'my.package.customFormatterFactory',
     'bar' : 'baz',
     'spam' : 99.9,
     'answer' : 42
   }

and this contains the special key "'()'", which means that user-
defined instantiation is wanted.  In this case, the specified factory
callable will be used. If it is an actual callable it will be used
directly - otherwise, if you specify a string (as in the example) the
actual callable will be located using normal import mechanisms. The
callable will be called with the **remaining** items in the
configuration sub-dictionary as keyword arguments.  In the above
example, the formatter with id "custom" will be assumed to be returned
by the call:

   my.package.customFormatterFactory(bar='baz', spam=99.9, answer=42)

The key "'()'" has been used as the special key because it is not a
valid keyword parameter name, and so will not clash with the names of
the keyword arguments used in the call.  The "'()'" also serves as a
mnemonic that the corresponding value is a callable.


16.7.2.5. Access to external objects
------------------------------------

There are times where a configuration needs to refer to objects
external to the configuration, for example "sys.stderr".  If the
configuration dict is constructed using Python code, this is
straightforward, but a problem arises when the configuration is
provided via a text file (e.g. JSON, YAML).  In a text file, there is
no standard way to distinguish "sys.stderr" from the literal string
"'sys.stderr'".  To facilitate this distinction, the configuration
system looks for certain special prefixes in string values and treat
them specially.  For example, if the literal string
"'ext://sys.stderr'" is provided as a value in the configuration, then
the "ext://" will be stripped off and the remainder of the value
processed using normal import mechanisms.

The handling of such prefixes is done in a way analogous to protocol
handling: there is a generic mechanism to look for prefixes which
match the regular expression "^(?P<prefix>[a-z]+)://(?P<suffix>.*)$"
whereby, if the "prefix" is recognised, the "suffix" is processed in a
prefix-dependent manner and the result of the processing replaces the
string value.  If the prefix is not recognised, then the string value
will be left as-is.


16.7.2.6. Access to internal objects
------------------------------------

As well as external objects, there is sometimes also a need to refer
to objects in the configuration.  This will be done implicitly by the
configuration system for things that it knows about.  For example, the
string value "'DEBUG'" for a "level" in a logger or handler will
automatically be converted to the value "logging.DEBUG", and the
"handlers", "filters" and "formatter" entries will take an object id
and resolve to the appropriate destination object.

However, a more generic mechanism is needed for user-defined objects
which are not known to the "logging" module.  For example, consider
"logging.handlers.MemoryHandler", which takes a "target" argument
which is another handler to delegate to. Since the system already
knows about this class, then in the configuration, the given "target"
just needs to be the object id of the relevant target handler, and the
system will resolve to the handler from the id.  If, however, a user
defines a "my.package.MyHandler" which has an "alternate" handler, the
configuration system would not know that the "alternate" referred to a
handler.  To cater for this, a generic resolution system allows the
user to specify:

   handlers:
     file:
       # configuration of file handler goes here

     custom:
       (): my.package.MyHandler
       alternate: cfg://handlers.file

The literal string "'cfg://handlers.file'" will be resolved in an
analogous way to strings with the "ext://" prefix, but looking in the
configuration itself rather than the import namespace.  The mechanism
allows access by dot or by index, in a similar way to that provided by
"str.format".  Thus, given the following snippet:

   handlers:
     email:
       class: logging.handlers.SMTPHandler
       mailhost: localhost
       fromaddr: my_app@domain.tld
       toaddrs:
         - support_team@domain.tld
         - dev_team@domain.tld
       subject: Houston, we have a problem.

in the configuration, the string "'cfg://handlers'" would resolve to
the dict with key "handlers", the string "'cfg://handlers.email" would
resolve to the dict with key "email" in the "handlers" dict, and so
on.  The string "'cfg://handlers.email.toaddrs[1]" would resolve to
"'dev_team.domain.tld'" and the string
"'cfg://handlers.email.toaddrs[0]'" would resolve to the value
"'support_team@domain.tld'". The "subject" value could be accessed
using either "'cfg://handlers.email.subject'" or, equivalently,
"'cfg://handlers.email[subject]'".  The latter form only needs to be
used if the key contains spaces or non-alphanumeric characters.  If an
index value consists only of decimal digits, access will be attempted
using the corresponding integer value, falling back to the string
value if needed.

Given a string "cfg://handlers.myhandler.mykey.123", this will resolve
to "config_dict['handlers']['myhandler']['mykey']['123']". If the
string is specified as "cfg://handlers.myhandler.mykey[123]", the
system will attempt to retrieve the value from
"config_dict['handlers']['myhandler']['mykey'][123]", and fall back to
"config_dict['handlers']['myhandler']['mykey']['123']" if that fails.


16.7.2.7. Import resolution and custom importers
------------------------------------------------

Import resolution, by default, uses the builtin "__import__()"
function to do its importing. You may want to replace this with your
own importing mechanism: if so, you can replace the "importer"
attribute of the "DictConfigurator" or its superclass, the
"BaseConfigurator" class. However, you need to be careful because of
the way functions are accessed from classes via descriptors. If you
are using a Python callable to do your imports, and you want to define
it at class level rather than instance level, you need to wrap it with
"staticmethod()". For example:

   from importlib import import_module
   from logging.config import BaseConfigurator

   BaseConfigurator.importer = staticmethod(import_module)

You don’t need to wrap with "staticmethod()" if you’re setting the
import callable on a configurator *instance*.


16.7.3. Configuration file format
=================================

The configuration file format understood by "fileConfig()" is based on
"configparser" functionality. The file must contain sections called
"[loggers]", "[handlers]" and "[formatters]" which identify by name
the entities of each type which are defined in the file. For each such
entity, there is a separate section which identifies how that entity
is configured.  Thus, for a logger named "log01" in the "[loggers]"
section, the relevant configuration details are held in a section
"[logger_log01]". Similarly, a handler called "hand01" in the
"[handlers]" section will have its configuration held in a section
called "[handler_hand01]", while a formatter called "form01" in the
"[formatters]" section will have its configuration specified in a
section called "[formatter_form01]". The root logger configuration
must be specified in a section called "[logger_root]".

Note: The "fileConfig()" API is older than the "dictConfig()" API
  and does not provide functionality to cover certain aspects of
  logging. For example, you cannot configure "Filter" objects, which
  provide for filtering of messages beyond simple integer levels,
  using "fileConfig()". If you need to have instances of "Filter" in
  your logging configuration, you will need to use "dictConfig()".
  Note that future enhancements to configuration functionality will be
  added to "dictConfig()", so it’s worth considering transitioning to
  this newer API when it’s convenient to do so.

Examples of these sections in the file are given below.

   [loggers]
   keys=root,log02,log03,log04,log05,log06,log07

   [handlers]
   keys=hand01,hand02,hand03,hand04,hand05,hand06,hand07,hand08,hand09

   [formatters]
   keys=form01,form02,form03,form04,form05,form06,form07,form08,form09

The root logger must specify a level and a list of handlers. An
example of a root logger section is given below.

   [logger_root]
   level=NOTSET
   handlers=hand01

The "level" entry can be one of "DEBUG, INFO, WARNING, ERROR,
CRITICAL" or "NOTSET". For the root logger only, "NOTSET" means that
all messages will be logged. Level values are "eval()"uated in the
context of the "logging" package’s namespace.

The "handlers" entry is a comma-separated list of handler names, which
must appear in the "[handlers]" section. These names must appear in
the "[handlers]" section and have corresponding sections in the
configuration file.

For loggers other than the root logger, some additional information is
required. This is illustrated by the following example.

   [logger_parser]
   level=DEBUG
   handlers=hand01
   propagate=1
   qualname=compiler.parser

The "level" and "handlers" entries are interpreted as for the root
logger, except that if a non-root logger’s level is specified as
"NOTSET", the system consults loggers higher up the hierarchy to
determine the effective level of the logger. The "propagate" entry is
set to 1 to indicate that messages must propagate to handlers higher
up the logger hierarchy from this logger, or 0 to indicate that
messages are **not** propagated to handlers up the hierarchy. The
"qualname" entry is the hierarchical channel name of the logger, that
is to say the name used by the application to get the logger.

Sections which specify handler configuration are exemplified by the
following.

   [handler_hand01]
   class=StreamHandler
   level=NOTSET
   formatter=form01
   args=(sys.stdout,)

The "class" entry indicates the handler’s class (as determined by
"eval()" in the "logging" package’s namespace). The "level" is
interpreted as for loggers, and "NOTSET" is taken to mean ‘log
everything’.

The "formatter" entry indicates the key name of the formatter for this
handler. If blank, a default formatter ("logging._defaultFormatter")
is used. If a name is specified, it must appear in the "[formatters]"
section and have a corresponding section in the configuration file.

The "args" entry, when "eval()"uated in the context of the "logging"
package’s namespace, is the list of arguments to the constructor for
the handler class. Refer to the constructors for the relevant
handlers, or to the examples below, to see how typical entries are
constructed.

   [handler_hand02]
   class=FileHandler
   level=DEBUG
   formatter=form02
   args=('python.log', 'w')

   [handler_hand03]
   class=handlers.SocketHandler
   level=INFO
   formatter=form03
   args=('localhost', handlers.DEFAULT_TCP_LOGGING_PORT)

   [handler_hand04]
   class=handlers.DatagramHandler
   level=WARN
   formatter=form04
   args=('localhost', handlers.DEFAULT_UDP_LOGGING_PORT)

   [handler_hand05]
   class=handlers.SysLogHandler
   level=ERROR
   formatter=form05
   args=(('localhost', handlers.SYSLOG_UDP_PORT), handlers.SysLogHandler.LOG_USER)

   [handler_hand06]
   class=handlers.NTEventLogHandler
   level=CRITICAL
   formatter=form06
   args=('Python Application', '', 'Application')

   [handler_hand07]
   class=handlers.SMTPHandler
   level=WARN
   formatter=form07
   args=('localhost', 'from@abc', ['user1@abc', 'user2@xyz'], 'Logger Subject')

   [handler_hand08]
   class=handlers.MemoryHandler
   level=NOTSET
   formatter=form08
   target=
   args=(10, ERROR)

   [handler_hand09]
   class=handlers.HTTPHandler
   level=NOTSET
   formatter=form09
   args=('localhost:9022', '/log', 'GET')

Sections which specify formatter configuration are typified by the
following.

   [formatter_form01]
   format=F1 %(asctime)s %(levelname)s %(message)s
   datefmt=
   class=logging.Formatter

The "format" entry is the overall format string, and the "datefmt"
entry is the "strftime()"-compatible date/time format string.  If
empty, the package substitutes something which is almost equivalent to
specifying the date format string "'%Y-%m-%d %H:%M:%S'".  This format
also specifies milliseconds, which are appended to the result of using
the above format string, with a comma separator.  An example time in
this format is "2003-01-23 00:29:50,411".

The "class" entry is optional.  It indicates the name of the
formatter’s class (as a dotted module and class name.)  This option is
useful for instantiating a "Formatter" subclass.  Subclasses of
"Formatter" can present exception tracebacks in an expanded or
condensed format.

Note: Due to the use of "eval()" as described above, there are
  potential security risks which result from using the "listen()" to
  send and receive configurations via sockets. The risks are limited
  to where multiple users with no mutual trust run code on the same
  machine; see the "listen()" documentation for more information.

See also:

  Module "logging"
     API reference for the logging module.

  Module "logging.handlers"
     Useful handlers included with the logging module.
