Logging documentation reorganised.
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+=============
+Logging HOWTO
+=============
+
+:Author: Vinay Sajip <vinay_sajip at red-dove dot com>
+
+.. Contents::
+
+.. _logging-basic-tutorial:
+
+.. currentmodule:: logging
+
+Basic Logging Tutorial
+----------------------
+
+Logging is a means of tracking events that happen when some software runs. The
+software's developer adds logging calls to their code to indicate that certain
+events have occurred. An event is described by a descriptive message which can
+optionally contain variable data (i.e. data that is potentially different for
+each occurrence of the event). Events also have an importance which the
+developer ascribes to the event; the importance can also be called the *level*
+or *severity*.
+
+When to use logging
+^^^^^^^^^^^^^^^^^^^
+
+Logging provides a set of convenience functions for simple logging usage. These
+are :func:`debug`, :func:`info`, :func:`warning`, :func:`error` and
+:func:`critical`. To determine when to use logging, see the table below, which
+states, for each of a set of common tasks, the best tool to use for it.
+
++-------------------------------------+--------------------------------------+
+| Task you want to perform | The best tool for the task |
++=====================================+======================================+
+| Display console output for ordinary | :func:`print` |
+| usage of a command line script or | |
+| program | |
++-------------------------------------+--------------------------------------+
+| Report events that occur during | :func:`logging.info` (or |
+| normal operation of a program (e.g. | :func:`logging.debug` for very |
+| for status monitoring or fault | detailed output for diagnostic |
+| investigation) | purposes) |
++-------------------------------------+--------------------------------------+
+| Issue a warning regarding a | :func:`warnings.warn` in library |
+| particular runtime event | code if the issue is avoidable and |
+| | the client application should be |
+| | modified to eliminate the warning |
+| | |
+| | :func:`logging.warning` if there is |
+| | nothing the client application can do|
+| | about the situation, but the event |
+| | should still be noted |
++-------------------------------------+--------------------------------------+
+| Report an error regarding a | Raise an exception |
+| particular runtime event | |
++-------------------------------------+--------------------------------------+
+| Report suppression of an error | :func:`logging.error`, |
+| without raising an exception (e.g. | :func:`logging.exception` or |
+| error handler in a long-running | :func:`logging.critical` as |
+| server process) | appropriate for the specific error |
+| | and application domain |
++-------------------------------------+--------------------------------------+
+
+The logging functions are named after the level or severity of the events
+they are used to track. The standard levels and their applicability are
+described below (in increasing order of severity):
+
++--------------+---------------------------------------------+
+| Level | When it's used |
++==============+=============================================+
+| ``DEBUG`` | Detailed information, typically of interest |
+| | only when diagnosing problems. |
++--------------+---------------------------------------------+
+| ``INFO`` | Confirmation that things are working as |
+| | expected. |
++--------------+---------------------------------------------+
+| ``WARNING`` | An indication that something unexpected |
+| | happened, or indicative of some problem in |
+| | the near future (e.g. 'disk space low'). |
+| | The software is still working as expected. |
++--------------+---------------------------------------------+
+| ``ERROR`` | Due to a more serious problem, the software |
+| | has not been able to perform some function. |
++--------------+---------------------------------------------+
+| ``CRITICAL`` | A serious error, indicating that the program|
+| | itself may be unable to continue running. |
++--------------+---------------------------------------------+
+
+The default level is ``WARNING``, which means that only events of this level
+and above will be tracked, unless the logging package is configured to do
+otherwise.
+
+Events that are tracked can be handled in different ways. The simplest way of
+handling tracked events is to print them to the console. Another common way
+is to write them to a disk file.
+
+
+.. _howto-minimal-example:
+
+A simple example
+^^^^^^^^^^^^^^^^
+
+A very simple example is::
+
+ import logging
+ logging.warning('Watch out!') # will print a message to the console
+ logging.info('I told you so') # will not print anything
+
+If you type these lines into a script and run it, you'll see::
+
+ WARNING:root:Watch out!
+
+printed out on the console. The ``INFO`` message doesn't appear because the
+default level is ``WARNING``. The printed message includes the indication of
+the level and the description of the event provided in the logging call, i.e.
+'Watch out!'. Don't worry about the 'root' part for now: it will be explained
+later. The actual output can be formatted quite flexibly if you need that;
+formatting options will also be explained later.
+
+
+Logging to a file
+^^^^^^^^^^^^^^^^^
+
+A very common situation is that of recording logging events in a file, so let's
+look at that next::
+
+ import logging
+ logging.basicConfig(filename='example.log',level=logging.DEBUG)
+ logging.debug('This message should go to the log file')
+ logging.info('So should this')
+ logging.warning('And this, too')
+
+And now if we open the file and look at what we have, we should find the log
+messages::
+
+ DEBUG:root:This message should go to the log file
+ INFO:root:So should this
+ WARNING:root:And this, too
+
+This example also shows how you can set the logging level which acts as the
+threshold for tracking. In this case, because we set the threshold to
+``DEBUG``, all of the messages were printed.
+
+If you want to set the logging level from a command-line option such as::
+
+ --log=INFO
+
+and you have the value of the parameter passed for ``--log`` in some variable
+*loglevel*, you can use::
+
+ getattr(logging, loglevel.upper())
+
+to get the value which you'll pass to :func:`basicConfig` via the *level*
+argument. You may want to error check any user input value, perhaps as in the
+following example::
+
+ # assuming loglevel is bound to the string value obtained from the
+ # command line argument. Convert to upper case to allow the user to
+ # specify --log=DEBUG or --log=debug
+ numeric_level = getattr(logging, loglevel.upper(), None)
+ if not isinstance(numeric_level, int):
+ raise ValueError('Invalid log level: %s' % loglevel)
+ logging.basicConfig(level=numeric_level, ...)
+
+The call to :func:`basicConfig` should come *before* any calls to :func:`debug`,
+:func:`info` etc. As it's intended as a one-off simple configuration facility,
+only the first call will actually do anything: subsequent calls are effectively
+no-ops.
+
+If you run the above script several times, the messages from successive runs
+are appended to the file *example.log*. If you want each run to start afresh,
+not remembering the messages from earlier runs, you can specify the *filemode*
+argument, by changing the call in the above example to::
+
+ logging.basicConfig(filename='example.log', filemode='w', level=logging.DEBUG)
+
+The output will be the same as before, but the log file is no longer appended
+to, so the messages from earlier runs are lost.
+
+
+Logging from multiple modules
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+If your program consists of multiple modules, here's an example of how you
+could organize logging in it::
+
+ # myapp.py
+ import logging
+ import mylib
+
+ def main():
+ logging.basicConfig(filename='myapp.log', level=logging.INFO)
+ logging.info('Started')
+ mylib.do_something()
+ logging.info('Finished')
+
+ if __name__ == '__main__':
+ main()
+
+::
+
+ # mylib.py
+ import logging
+
+ def do_something():
+ logging.info('Doing something')
+
+If you run *myapp.py*, you should see this in *myapp.log*::
+
+ INFO:root:Started
+ INFO:root:Doing something
+ INFO:root:Finished
+
+which is hopefully what you were expecting to see. You can generalize this to
+multiple modules, using the pattern in *mylib.py*. Note that for this simple
+usage pattern, you won't know, by looking in the log file, *where* in your
+application your messages came from, apart from looking at the event
+description. If you want to track the location of your messages, you'll need
+to refer to the documentation beyond the tutorial level - see
+:ref:`advanced-logging-tutorial`.
+
+
+Logging variable data
+^^^^^^^^^^^^^^^^^^^^^
+
+To log variable data, use a format string for the event description message and
+append the variable data as arguments. For example::
+
+ import logging
+ logging.warning('%s before you %s', 'Look', 'leap!')
+
+will display::
+
+ WARNING:root:Look before you leap!
+
+As you can see, merging of variable data into the event description message
+uses the old, %-style of string formatting. This is for backwards
+compatibility: the logging package pre-dates newer formatting options such as
+:meth:`str.format` and :class:`string.Template`. These newer formatting
+options *are* supported, but exploring them is outside the scope of this
+tutorial.
+
+
+Changing the format of displayed messages
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+To change the format which is used to display messages, you need to
+specify the format you want to use::
+
+ import logging
+ logging.basicConfig(format='%(levelname)s:%(message)s', level=logging.DEBUG)
+ logging.debug('This message should appear on the console')
+ logging.info('So should this')
+ logging.warning('And this, too')
+
+which would print::
+
+ DEBUG:This message should appear on the console
+ INFO:So should this
+ WARNING:And this, too
+
+Notice that the 'root' which appeared in earlier examples has disappeared. For
+a full set of things that can appear in format strings, you can refer to the
+documentation for :ref:`logrecord-attributes`, but for simple usage, you just
+need the *levelname* (severity), *message* (event description, including
+variable data) and perhaps to display when the event occurred. This is
+described in the next section.
+
+
+Displaying the date/time in messages
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+To display the date and time of an event, you would place '%(asctime)s' in
+your format string::
+
+ import logging
+ logging.basicConfig(format='%(asctime)s %(message)s')
+ logging.warning('is when this event was logged.')
+
+which should print something like this::
+
+ 2010-12-12 11:41:42,612 is when this event was logged.
+
+The default format for date/time display (shown above) is ISO8601. If you need
+more control over the formatting of the date/time, provide a *datefmt*
+argument to ``basicConfig``, as in this example::
+
+ import logging
+ logging.basicConfig(format='%(asctime)s %(message)s', datefmt='%m/%d/%Y %I:%M:%S %p')
+ logging.warning('is when this event was logged.')
+
+which would display something like this::
+
+ 12/12/2010 11:46:36 AM is when this event was logged.
+
+The format of the *datefmt* argument is the same as supported by
+:func:`time.strftime`.
+
+
+Next Steps
+^^^^^^^^^^
+
+That concludes the basic tutorial. It should be enough to get you up and
+running with logging. There's a lot more that the logging package offers, but
+to get the best out of it, you'll need to invest a little more of your time in
+reading the following sections. If you're ready for that, grab some of your
+favourite beverage and carry on.
+
+If your logging needs are simple, then use the above examples to incorporate
+logging into your own scripts, and if you run into problems or don't
+understand something, please post a question on the comp.lang.python Usenet
+group (available at http://groups.google.com/group/comp.lang.python) and you
+should receive help before too long.
+
+Still here? There's no need to read the whole of the logging documentation in
+linear fashion, top to bottom (there's quite a lot of it still to come). You
+can carry on reading the next few sections, which provide a slightly more
+advanced/in-depth tutorial than the basic one above. After that, you can
+take a look at the :ref:`logging-cookbook`.
+
+.. _logging-advanced-tutorial:
+
+
+Advanced Logging Tutorial
+-------------------------
+
+The logging library takes a modular approach and offers several categories
+of components: loggers, handlers, filters, and formatters.
+
+* Loggers expose the interface that application code directly uses.
+* Handlers send the log records (created by loggers) to the appropriate
+ destination.
+* Filters provide a finer grained facility for determining which log records
+ to output.
+* Formatters specify the layout of log records in the final output.
+
+Logging is performed by calling methods on instances of the :class:`Logger`
+class (hereafter called :dfn:`loggers`). Each instance has a name, and they are
+conceptually arranged in a namespace hierarchy using dots (periods) as
+separators. For example, a logger named 'scan' is the parent of loggers
+'scan.text', 'scan.html' and 'scan.pdf'. Logger names can be anything you want,
+and indicate the area of an application in which a logged message originates.
+
+A good convention to use when naming loggers is to use a module-level logger,
+in each module which uses logging, named as follows::
+
+ logger = logging.getLogger(__name__)
+
+This means that logger names track the package/module hierarchy, and it's
+intuitively obvious where events are logged just from the logger name.
+
+The root of the hierarchy of loggers is called the root logger. That's the
+logger used by the functions :func:`debug`, :func:`info`, :func:`warning`,
+:func:`error` and :func:`critical`, which just call the same-named method of
+the root logger. The functions and the methods have the same signatures. The
+root logger's name is printed as 'root' in the logged output.
+
+It is, of course, possible to log messages to different destinations. Support
+is included in the package for writing log messages to files, HTTP GET/POST
+locations, email via SMTP, generic sockets, queues, or OS-specific logging
+mechanisms such as syslog or the Windows NT event log. Destinations are served
+by :dfn:`handler` classes. You can create your own log destination class if
+you have special requirements not met by any of the built-in handler classes.
+
+By default, no destination is set for any logging messages. You can specify
+a destination (such as console or file) by using :func:`basicConfig` as in the
+tutorial examples. If you call the functions :func:`debug`, :func:`info`,
+:func:`warning`, :func:`error` and :func:`critical`, they will check to see
+if no destination is set; and if one is not set, they will set a destination
+of the console (``sys.stderr``) and a default format for the displayed
+message before delegating to the root logger to do the actual message output.
+
+The default format set by :func:`basicConfig` for messages is::
+
+ severity:logger name:message
+
+You can change this by passing a format string to :func:`basicConfig` with the
+*format* keyword argument. For all options regarding how a format string is
+constructed, see :ref:`formatter-objects`.
+
+
+Loggers
+^^^^^^^
+
+:class:`Logger` objects have a threefold job. First, they expose several
+methods to application code so that applications can log messages at runtime.
+Second, logger objects determine which log messages to act upon based upon
+severity (the default filtering facility) or filter objects. Third, logger
+objects pass along relevant log messages to all interested log handlers.
+
+The most widely used methods on logger objects fall into two categories:
+configuration and message sending.
+
+These are the most common configuration methods:
+
+* :meth:`Logger.setLevel` specifies the lowest-severity log message a logger
+ will handle, where debug is the lowest built-in severity level and critical
+ is the highest built-in severity. For example, if the severity level is
+ INFO, the logger will handle only INFO, WARNING, ERROR, and CRITICAL messages
+ and will ignore DEBUG messages.
+
+* :meth:`Logger.addHandler` and :meth:`Logger.removeHandler` add and remove
+ handler objects from the logger object. Handlers are covered in more detail
+ in :ref:`handler-basic`.
+
+* :meth:`Logger.addFilter` and :meth:`Logger.removeFilter` add and remove filter
+ objects from the logger object. Filters are covered in more detail in
+ :ref:`filter`.
+
+You don't need to always call these methods on every logger you create. See the
+last two paragraphs in this section.
+
+With the logger object configured, the following methods create log messages:
+
+* :meth:`Logger.debug`, :meth:`Logger.info`, :meth:`Logger.warning`,
+ :meth:`Logger.error`, and :meth:`Logger.critical` all create log records with
+ a message and a level that corresponds to their respective method names. The
+ message is actually a format string, which may contain the standard string
+ substitution syntax of :const:`%s`, :const:`%d`, :const:`%f`, and so on. The
+ rest of their arguments is a list of objects that correspond with the
+ substitution fields in the message. With regard to :const:`**kwargs`, the
+ logging methods care only about a keyword of :const:`exc_info` and use it to
+ determine whether to log exception information.
+
+* :meth:`Logger.exception` creates a log message similar to
+ :meth:`Logger.error`. The difference is that :meth:`Logger.exception` dumps a
+ stack trace along with it. Call this method only from an exception handler.
+
+* :meth:`Logger.log` takes a log level as an explicit argument. This is a
+ little more verbose for logging messages than using the log level convenience
+ methods listed above, but this is how to log at custom log levels.
+
+:func:`getLogger` returns a reference to a logger instance with the specified
+name if it is provided, or ``root`` if not. The names are period-separated
+hierarchical structures. Multiple calls to :func:`getLogger` with the same name
+will return a reference to the same logger object. Loggers that are further
+down in the hierarchical list are children of loggers higher up in the list.
+For example, given a logger with a name of ``foo``, loggers with names of
+``foo.bar``, ``foo.bar.baz``, and ``foo.bam`` are all descendants of ``foo``.
+
+Loggers have a concept of *effective level*. If a level is not explicitly set
+on a logger, the level of its parent is used instead as its effective level.
+If the parent has no explicit level set, *its* parent is examined, and so on -
+all ancestors are searched until an explicitly set level is found. The root
+logger always has an explicit level set (``WARNING`` by default). When deciding
+whether to process an event, the effective level of the logger is used to
+determine whether the event is passed to the logger's handlers.
+
+Child loggers propagate messages up to the handlers associated with their
+ancestor loggers. Because of this, it is unnecessary to define and configure
+handlers for all the loggers an application uses. It is sufficient to
+configure handlers for a top-level logger and create child loggers as needed.
+(You can, however, turn off propagation by setting the *propagate*
+attribute of a logger to *False*.)
+
+
+.. _handler-basic:
+
+Handlers
+^^^^^^^^
+
+:class:`Handler` objects are responsible for dispatching the appropriate log
+messages (based on the log messages' severity) to the handler's specified
+destination. Logger objects can add zero or more handler objects to themselves
+with an :func:`addHandler` method. As an example scenario, an application may
+want to send all log messages to a log file, all log messages of error or higher
+to stdout, and all messages of critical to an email address. This scenario
+requires three individual handlers where each handler is responsible for sending
+messages of a specific severity to a specific location.
+
+The standard library includes quite a few handler types (see
+:ref:`useful-handlers`); the tutorials use mainly :class:`StreamHandler` and
+:class:`FileHandler` in its examples.
+
+There are very few methods in a handler for application developers to concern
+themselves with. The only handler methods that seem relevant for application
+developers who are using the built-in handler objects (that is, not creating
+custom handlers) are the following configuration methods:
+
+* The :meth:`Handler.setLevel` method, just as in logger objects, specifies the
+ lowest severity that will be dispatched to the appropriate destination. Why
+ are there two :func:`setLevel` methods? The level set in the logger
+ determines which severity of messages it will pass to its handlers. The level
+ set in each handler determines which messages that handler will send on.
+
+* :func:`setFormatter` selects a Formatter object for this handler to use.
+
+* :func:`addFilter` and :func:`removeFilter` respectively configure and
+ deconfigure filter objects on handlers.
+
+Application code should not directly instantiate and use instances of
+:class:`Handler`. Instead, the :class:`Handler` class is a base class that
+defines the interface that all handlers should have and establishes some
+default behavior that child classes can use (or override).
+
+
+Formatters
+^^^^^^^^^^
+
+Formatter objects configure the final order, structure, and contents of the log
+message. Unlike the base :class:`logging.Handler` class, application code may
+instantiate formatter classes, although you could likely subclass the formatter
+if your application needs special behavior. The constructor takes three
+optional arguments -- a message format string, a date format string and a style
+indicator.
+
+.. method:: logging.Formatter.__init__(fmt=None, datefmt=None, style='%')
+
+If there is no message format string, the default is to use the
+raw message. If there is no date format string, the default date format is::
+
+ %Y-%m-%d %H:%M:%S
+
+with the milliseconds tacked on at the end. The ``style`` is one of `%`, '{'
+or '$'. If one of these is not specified, then '%' will be used.
+
+If the ``style`` is '%', the message format string uses
+``%(<dictionary key>)s`` styled string substitution; the possible keys are
+documented in :ref:`logrecord-attributes`. If the style is '{', the message
+format string is assumed to be compatible with :meth:`str.format` (using
+keyword arguments), while if the style is '$' then the message format string
+should conform to what is expected by :meth:`string.Template.substitute`.
+
+.. versionchanged:: 3.2
+ Added the ``style`` parameter.
+
+The following message format string will log the time in a human-readable
+format, the severity of the message, and the contents of the message, in that
+order::
+
+ '%(asctime)s - %(levelname)s - %(message)s'
+
+Formatters use a user-configurable function to convert the creation time of a
+record to a tuple. By default, :func:`time.localtime` is used; to change this
+for a particular formatter instance, set the ``converter`` attribute of the
+instance to a function with the same signature as :func:`time.localtime` or
+:func:`time.gmtime`. To change it for all formatters, for example if you want
+all logging times to be shown in GMT, set the ``converter`` attribute in the
+Formatter class (to ``time.gmtime`` for GMT display).
+
+
+Configuring Logging
+^^^^^^^^^^^^^^^^^^^
+
+Programmers can configure logging in three ways:
+
+1. Creating loggers, handlers, and formatters explicitly using Python
+ code that calls the configuration methods listed above.
+2. Creating a logging config file and reading it using the :func:`fileConfig`
+ function.
+3. Creating a dictionary of configuration information and passing it
+ to the :func:`dictConfig` function.
+
+For the reference documentation on the last two options, see :ref:`config-ref`.
+The following example configures a very simple logger, a console handler, and
+a simple formatter using Python code::
+
+ import logging
+
+ # create logger
+ logger = logging.getLogger('simple_example')
+ logger.setLevel(logging.DEBUG)
+
+ # create console handler and set level to debug
+ ch = logging.StreamHandler()
+ ch.setLevel(logging.DEBUG)
+
+ # create formatter
+ formatter = logging.Formatter('%(asctime)s - %(name)s - %(levelname)s - %(message)s')
+
+ # add formatter to ch
+ ch.setFormatter(formatter)
+
+ # add ch to logger
+ logger.addHandler(ch)
+
+ # 'application' code
+ logger.debug('debug message')
+ logger.info('info message')
+ logger.warn('warn message')
+ logger.error('error message')
+ logger.critical('critical message')
+
+Running this module from the command line produces the following output::
+
+ $ python simple_logging_module.py
+ 2005-03-19 15:10:26,618 - simple_example - DEBUG - debug message
+ 2005-03-19 15:10:26,620 - simple_example - INFO - info message
+ 2005-03-19 15:10:26,695 - simple_example - WARNING - warn message
+ 2005-03-19 15:10:26,697 - simple_example - ERROR - error message
+ 2005-03-19 15:10:26,773 - simple_example - CRITICAL - critical message
+
+The following Python module creates a logger, handler, and formatter nearly
+identical to those in the example listed above, with the only difference being
+the names of the objects::
+
+ import logging
+ import logging.config
+
+ logging.config.fileConfig('logging.conf')
+
+ # create logger
+ logger = logging.getLogger('simpleExample')
+
+ # 'application' code
+ logger.debug('debug message')
+ logger.info('info message')
+ logger.warn('warn message')
+ logger.error('error message')
+ logger.critical('critical message')
+
+Here is the logging.conf file::
+
+ [loggers]
+ keys=root,simpleExample
+
+ [handlers]
+ keys=consoleHandler
+
+ [formatters]
+ keys=simpleFormatter
+
+ [logger_root]
+ level=DEBUG
+ handlers=consoleHandler
+
+ [logger_simpleExample]
+ level=DEBUG
+ handlers=consoleHandler
+ qualname=simpleExample
+ propagate=0
+
+ [handler_consoleHandler]
+ class=StreamHandler
+ level=DEBUG
+ formatter=simpleFormatter
+ args=(sys.stdout,)
+
+ [formatter_simpleFormatter]
+ format=%(asctime)s - %(name)s - %(levelname)s - %(message)s
+ datefmt=
+
+The output is nearly identical to that of the non-config-file-based example::
+
+ $ python simple_logging_config.py
+ 2005-03-19 15:38:55,977 - simpleExample - DEBUG - debug message
+ 2005-03-19 15:38:55,979 - simpleExample - INFO - info message
+ 2005-03-19 15:38:56,054 - simpleExample - WARNING - warn message
+ 2005-03-19 15:38:56,055 - simpleExample - ERROR - error message
+ 2005-03-19 15:38:56,130 - simpleExample - CRITICAL - critical message
+
+You can see that the config file approach has a few advantages over the Python
+code approach, mainly separation of configuration and code and the ability of
+noncoders to easily modify the logging properties.
+
+Note that the class names referenced in config files need to be either relative
+to the logging module, or absolute values which can be resolved using normal
+import mechanisms. Thus, you could use either
+:class:`handlers.WatchedFileHandler` (relative to the logging module) or
+``mypackage.mymodule.MyHandler`` (for a class defined in package ``mypackage``
+and module ``mymodule``, where ``mypackage`` is available on the Python import
+path).
+
+In Python 3.2, a new means of configuring logging has been introduced, using
+dictionaries to hold configuration information. This provides a superset of the
+functionality of the config-file-based approach outlined above, and is the
+recommended configuration method for new applications and deployments. Because
+a Python dictionary is used to hold configuration information, and since you
+can populate that dictionary using different means, you have more options for
+configuration. For example, you can use a configuration file in JSON format,
+or, if you have access to YAML processing functionality, a file in YAML
+format, to populate the configuration dictionary. Or, of course, you can
+construct the dictionary in Python code, receive it in pickled form over a
+socket, or use whatever approach makes sense for your application.
+
+Here's an example of the same configuration as above, in YAML format for
+the new dictionary-based approach::
+
+ version: 1
+ formatters:
+ simple:
+ format: format=%(asctime)s - %(name)s - %(levelname)s - %(message)s
+ handlers:
+ console:
+ class: logging.StreamHandler
+ level: DEBUG
+ formatter: simple
+ stream: ext://sys.stdout
+ loggers:
+ simpleExample:
+ level: DEBUG
+ handlers: [console]
+ propagate: no
+ root:
+ level: DEBUG
+ handlers: [console]
+
+For more information about logging using a dictionary, see
+:ref:`logging-config-api`.
+
+What happens if no configuration is provided
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+If no logging configuration is provided, it is possible to have a situation
+where a logging event needs to be output, but no handlers can be found to
+output the event. The behaviour of the logging package in these
+circumstances is dependent on the Python version.
+
+For versions of Python prior to 3.2, the behaviour is as follows:
+
+* If *logging.raiseExceptions* is *False* (production mode), the event is
+ silently dropped.
+
+* If *logging.raiseExceptions* is *True* (development mode), a message
+ 'No handlers could be found for logger X.Y.Z' is printed once.
+
+In Python 3.2 and later, the behaviour is as follows:
+
+* The event is output using a 'handler of last resort', stored in
+ ``logging.lastResort``. This internal handler is not associated with any
+ logger, and acts like a :class:`StreamHandler` which writes the event
+ description message to the current value of ``sys.stderr`` (therefore
+ respecting any redirections which may be in effect). No formatting is
+ done on the message - just the bare event description message is printed.
+ The handler's level is set to ``WARNING``, so all events at this and
+ greater severities will be output.
+
+To obtain the pre-3.2 behaviour, ``logging.lastResort`` can be set to *None*.
+
+.. _library-config:
+
+Configuring Logging for a Library
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+When developing a library which uses logging, you should take care to
+document how the library uses logging - for example, the names of loggers
+used. Some consideration also needs to be given to its logging configuration.
+If the using application does not use logging, and library code makes logging
+calls, then (as described in the previous section) events of severity
+``WARNING`` and greater will be printed to ``sys.stderr``. This is regarded as
+the best default behaviour.
+
+If for some reason you *don't* want these messages printed in the absence of
+any logging configuration, you can attach a do-nothing handler to the top-level
+logger for your library. This avoids the message being printed, since a handler
+will be always be found for the library's events: it just doesn't produce any
+output. If the library user configures logging for application use, presumably
+that configuration will add some handlers, and if levels are suitably
+configured then logging calls made in library code will send output to those
+handlers, as normal.
+
+A do-nothing handler is included in the logging package: :class:`NullHandler`
+(since Python 3.1). An instance of this handler could be added to the top-level
+logger of the logging namespace used by the library (*if* you want to prevent
+your library's logged events being output to ``sys.stderr`` in the absence of
+logging configuration). If all logging by a library *foo* is done using loggers
+with names matching 'foo.x', 'foo.x.y', etc. then the code::
+
+ import logging
+ logging.getLogger('foo').addHandler(logging.NullHandler())
+
+should have the desired effect. If an organisation produces a number of
+libraries, then the logger name specified can be 'orgname.foo' rather than
+just 'foo'.
+
+**PLEASE NOTE:** It is strongly advised that you *do not add any handlers other
+than* :class:`NullHandler` *to your library's loggers*. This is because the
+configuration of handlers is the prerogative of the application developer who
+uses your library. The application developer knows their target audience and
+what handlers are most appropriate for their application: if you add handlers
+'under the hood', you might well interfere with their ability to carry out
+unit tests and deliver logs which suit their requirements.
+
+
+Logging Levels
+--------------
+
+The numeric values of logging levels are given in the following table. These are
+primarily of interest if you want to define your own levels, and need them to
+have specific values relative to the predefined levels. If you define a level
+with the same numeric value, it overwrites the predefined value; the predefined
+name is lost.
+
++--------------+---------------+
+| Level | Numeric value |
++==============+===============+
+| ``CRITICAL`` | 50 |
++--------------+---------------+
+| ``ERROR`` | 40 |
++--------------+---------------+
+| ``WARNING`` | 30 |
++--------------+---------------+
+| ``INFO`` | 20 |
++--------------+---------------+
+| ``DEBUG`` | 10 |
++--------------+---------------+
+| ``NOTSET`` | 0 |
++--------------+---------------+
+
+Levels can also be associated with loggers, being set either by the developer or
+through loading a saved logging configuration. When a logging method is called
+on a logger, the logger compares its own level with the level associated with
+the method call. If the logger's level is higher than the method call's, no
+logging message is actually generated. This is the basic mechanism controlling
+the verbosity of logging output.
+
+Logging messages are encoded as instances of the :class:`LogRecord` class. When
+a logger decides to actually log an event, a :class:`LogRecord` instance is
+created from the logging message.
+
+Logging messages are subjected to a dispatch mechanism through the use of
+:dfn:`handlers`, which are instances of subclasses of the :class:`Handler`
+class. Handlers are responsible for ensuring that a logged message (in the form
+of a :class:`LogRecord`) ends up in a particular location (or set of locations)
+which is useful for the target audience for that message (such as end users,
+support desk staff, system administrators, developers). Handlers are passed
+:class:`LogRecord` instances intended for particular destinations. Each logger
+can have zero, one or more handlers associated with it (via the
+:meth:`addHandler` method of :class:`Logger`). 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 (unless the *propagate* flag
+for a logger is set to a false value, at which point the passing to ancestor
+handlers stops).
+
+Just as for loggers, handlers can have levels associated with them. A handler's
+level acts as a filter in the same way as a logger's level does. If a handler
+decides to actually dispatch an event, the :meth:`emit` method is used to send
+the message to its destination. Most user-defined subclasses of :class:`Handler`
+will need to override this :meth:`emit`.
+
+.. _custom-levels:
+
+Custom Levels
+^^^^^^^^^^^^^
+
+Defining your own levels is possible, but should not be necessary, as the
+existing levels have been chosen on the basis of practical experience.
+However, if you are convinced that you need custom levels, great care should
+be exercised when doing this, and it is possibly *a very bad idea to define
+custom levels if you are developing a library*. That's because if multiple
+library authors all define their own custom levels, there is a chance that
+the logging output from such multiple libraries used together will be
+difficult for the using developer to control and/or interpret, because a
+given numeric value might mean different things for different libraries.
+
+.. _useful-handlers:
+
+Useful Handlers
+---------------
+
+In addition to the base :class:`Handler` class, many useful subclasses are
+provided:
+
+#. :class:`StreamHandler` instances send messages to streams (file-like
+ objects).
+
+#. :class:`FileHandler` instances send messages to disk files.
+
+.. currentmodule:: logging.handlers
+
+#. :class:`BaseRotatingHandler` is the base class for handlers that
+ rotate log files at a certain point. It is not meant to be instantiated
+ directly. Instead, use :class:`RotatingFileHandler` or
+ :class:`TimedRotatingFileHandler`.
+
+#. :class:`RotatingFileHandler` instances send messages to disk
+ files, with support for maximum log file sizes and log file rotation.
+
+#. :class:`TimedRotatingFileHandler` instances send messages to
+ disk files, rotating the log file at certain timed intervals.
+
+#. :class:`SocketHandler` instances send messages to TCP/IP
+ sockets.
+
+#. :class:`DatagramHandler` instances send messages to UDP
+ sockets.
+
+#. :class:`SMTPHandler` instances send messages to a designated
+ email address.
+
+#. :class:`SysLogHandler` instances send messages to a Unix
+ syslog daemon, possibly on a remote machine.
+
+#. :class:`NTEventLogHandler` instances send messages to a
+ Windows NT/2000/XP event log.
+
+#. :class:`MemoryHandler` instances send messages to a buffer
+ in memory, which is flushed whenever specific criteria are met.
+
+#. :class:`HTTPHandler` instances send messages to an HTTP
+ server using either ``GET`` or ``POST`` semantics.
+
+#. :class:`WatchedFileHandler` instances watch the file they are
+ logging to. If the file changes, it is closed and reopened using the file
+ name. This handler is only useful on Unix-like systems; Windows does not
+ support the underlying mechanism used.
+
+#. :class:`QueueHandler` instances send messages to a queue, such as
+ those implemented in the :mod:`queue` or :mod:`multiprocessing` modules.
+
+.. currentmodule:: logging
+
+#. :class:`NullHandler` instances do nothing with error messages. They are used
+ by library developers who want to use logging, but want to avoid the 'No
+ handlers could be found for logger XXX' message which can be displayed if
+ the library user has not configured logging. See :ref:`library-config` for
+ more information.
+
+.. versionadded:: 3.1
+ The :class:`NullHandler` class.
+
+.. versionadded:: 3.2
+ The :class:`~logging.handlers.QueueHandler` class.
+
+The :class:`NullHandler`, :class:`StreamHandler` and :class:`FileHandler`
+classes are defined in the core logging package. The other handlers are
+defined in a sub- module, :mod:`logging.handlers`. (There is also another
+sub-module, :mod:`logging.config`, for configuration functionality.)
+
+Logged messages are formatted for presentation through instances of the
+:class:`Formatter` class. They are initialized with a format string suitable for
+use with the % operator and a dictionary.
+
+For formatting multiple messages in a batch, instances of
+:class:`BufferingFormatter` can be used. In addition to the format string (which
+is applied to each message in the batch), there is provision for header and
+trailer format strings.
+
+When filtering based on logger level and/or handler level is not enough,
+instances of :class:`Filter` can be added to both :class:`Logger` and
+:class:`Handler` instances (through their :meth:`addFilter` method). Before
+deciding to process a message further, both loggers and handlers consult all
+their filters for permission. If any filter returns a false value, the message
+is not processed further.
+
+The basic :class:`Filter` functionality allows filtering by specific logger
+name. If this feature is used, messages sent to the named logger and its
+children are allowed through the filter, and all others dropped.
+
+
+.. _logging-exceptions:
+
+Exceptions raised during logging
+--------------------------------
+
+The logging package is designed to swallow exceptions which occur while logging
+in production. This is so that errors which occur while handling logging events
+- such as logging misconfiguration, network or other similar errors - do not
+cause the application using logging to terminate prematurely.
+
+:class:`SystemExit` and :class:`KeyboardInterrupt` exceptions are never
+swallowed. Other exceptions which occur during the :meth:`emit` method of a
+:class:`Handler` subclass are passed to its :meth:`handleError` method.
+
+The default implementation of :meth:`handleError` in :class:`Handler` checks
+to see if a module-level variable, :data:`raiseExceptions`, is set. If set, a
+traceback is printed to :data:`sys.stderr`. If not set, the exception is swallowed.
+
+**Note:** The default value of :data:`raiseExceptions` is ``True``. This is because
+during development, you typically want to be notified of any exceptions that
+occur. It's advised that you set :data:`raiseExceptions` to ``False`` for production
+usage.
+
+.. currentmodule:: logging
+
+.. _arbitrary-object-messages:
+
+Using arbitrary objects as messages
+-----------------------------------
+
+In the preceding sections and examples, it has been assumed that the message
+passed when logging the event is a string. However, this is not the only
+possibility. You can pass an arbitrary object as a message, and its
+:meth:`__str__` method will be called when the logging system needs to convert
+it to a string representation. In fact, if you want to, you can avoid
+computing a string representation altogether - for example, the
+:class:`SocketHandler` emits an event by pickling it and sending it over the
+wire.
+
+
+Optimization
+------------
+
+Formatting of message arguments is deferred until it cannot be avoided.
+However, computing the arguments passed to the logging method can also be
+expensive, and you may want to avoid doing it if the logger will just throw
+away your event. To decide what to do, you can call the :meth:`isEnabledFor`
+method which takes a level argument and returns true if the event would be
+created by the Logger for that level of call. You can write code like this::
+
+ if logger.isEnabledFor(logging.DEBUG):
+ logger.debug('Message with %s, %s', expensive_func1(),
+ expensive_func2())
+
+so that if the logger's threshold is set above ``DEBUG``, the calls to
+:func:`expensive_func1` and :func:`expensive_func2` are never made.
+
+There are other optimizations which can be made for specific applications which
+need more precise control over what logging information is collected. Here's a
+list of things you can do to avoid processing during logging which you don't
+need:
+
++-----------------------------------------------+----------------------------------------+
+| What you don't want to collect | How to avoid collecting it |
++===============================================+========================================+
+| Information about where calls were made from. | Set ``logging._srcfile`` to ``None``. |
++-----------------------------------------------+----------------------------------------+
+| Threading information. | Set ``logging.logThreads`` to ``0``. |
++-----------------------------------------------+----------------------------------------+
+| Process information. | Set ``logging.logProcesses`` to ``0``. |
++-----------------------------------------------+----------------------------------------+
+
+Also note that the core logging module only includes the basic handlers. If
+you don't import :mod:`logging.handlers` and :mod:`logging.config`, they won't
+take up any memory.
+