logging.rst

:mod:`logging` --- Logging facility for Python
==============================================

.. module:: logging
   :synopsis: Flexible event logging system for applications.

.. moduleauthor:: Vinay Sajip <vinay_sajip@red-dove.com>
.. sectionauthor:: Vinay Sajip <vinay_sajip@red-dove.com>

**Source code:** :source:`Lib/logging/__init__.py`

.. index:: pair: Errors; logging

.. sidebar:: Important

   This page contains the API reference information. For tutorial
   information and discussion of more advanced topics, see

   * :ref:`Basic Tutorial <logging-basic-tutorial>`
   * :ref:`Advanced Tutorial <logging-advanced-tutorial>`
   * :ref:`Logging Cookbook <logging-cookbook>`

--------------

This module defines functions and classes which implement a flexible event
logging system for applications and libraries.

The key benefit of having the logging API provided by a standard library module
is that all Python modules can participate in logging, so your application log
can include your own messages integrated with messages from third-party
modules.

The module provides a lot of functionality and flexibility.  If you are
unfamiliar with logging, the best way to get to grips with it is to see the
tutorials (see the links on the right).

The basic classes defined by the module, together with their functions, are
listed below.

* 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.


.. _logger:

Logger Objects
--------------

Loggers have the following attributes and methods.  Note that Loggers should
*NEVER* be instantiated directly, but always through the module-level function
``logging.getLogger(name)``.  Multiple calls to :func:`getLogger` with the same
name will always return a reference to the same Logger object.

The ``name`` is potentially a period-separated hierarchical value, like
``foo.bar.baz`` (though it could also be just plain ``foo``, for example).
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``.  The logger name hierarchy is analogous to the Python
package hierarchy, and identical to it if you organise your loggers on a
per-module basis using the recommended construction
``logging.getLogger(__name__)``.  That's because in a module, ``__name__``
is the module's name in the Python package namespace.


.. class:: Logger

   .. attribute:: Logger.propagate

      If this attribute evaluates to true, events logged to this logger will be
      passed to the handlers of higher level (ancestor) loggers, in addition to
      any handlers attached to this logger. Messages are passed directly to the
      ancestor loggers' handlers - neither the level nor filters of the ancestor
      loggers in question are considered.

      If this evaluates to false, logging messages are not passed to the handlers
      of ancestor loggers.

      Spelling it out with an example: If the propagate attribute of the logger named
      ``A.B.C`` evaluates to true, any event logged to ``A.B.C`` via a method call such as
      ``logging.getLogger('A.B.C').error(...)`` will [subject to passing that logger's
      level and filter settings] be passed in turn to any handlers attached to loggers
      named ``A.B``, ``A`` and the root logger, after first being passed to any handlers
      attached to ``A.B.C``. If any logger in the chain ``A.B.C``, ``A.B``, ``A`` has its
      ``propagate`` attribute set to false, then that is the last logger whose handlers
      are offered the event to handle, and propagation stops at that point.

      The constructor sets this attribute to ``True``.

      .. note:: If you attach a handler to a logger *and* one or more of its
         ancestors, it may emit the same record multiple times. In general, you
         should not need to attach a handler to more than one logger - if you just
         attach it to the appropriate logger which is highest in the logger
         hierarchy, then it will see all events logged by all descendant loggers,
         provided that their propagate setting is left set to ``True``. A common
         scenario is to attach handlers only to the root logger, and to let
         propagation take care of the rest.

   .. method:: Logger.setLevel(level)

      Sets the threshold for this logger to *level*. Logging messages which are less
      severe than *level* will be ignored; logging messages which have severity *level*
      or higher will be emitted by whichever handler or handlers service this logger,
      unless a handler's level has been set to a higher severity level than *level*.

      When a logger is created, the level is set to :const:`NOTSET` (which causes
      all messages to be processed when the logger is the root logger, or delegation
      to the parent when the logger is a non-root logger). Note that the root logger
      is created with level :const:`WARNING`.

      The term 'delegation to the parent' means that if a logger has a level of
      NOTSET, its chain of ancestor loggers is traversed until either an ancestor with
      a level other than NOTSET is found, or the root is reached.

      If an ancestor is found with a level other than NOTSET, then that ancestor's
      level is treated as the effective level of the logger where the ancestor search
      began, and is used to determine how a logging event is handled.

      If the root is reached, and it has a level of NOTSET, then all messages will be
      processed. Otherwise, the root's level will be used as the effective level.

      See :ref:`levels` for a list of levels.

      .. versionchanged:: 3.2
         The *level* parameter now accepts a string representation of the
         level such as 'INFO' as an alternative to the integer constants
         such as :const:`INFO`. Note, however, that levels are internally stored
         as integers, and methods such as e.g. :meth:`getEffectiveLevel` and
         :meth:`isEnabledFor` will return/expect to be passed integers.


   .. method:: Logger.isEnabledFor(level)

      Indicates if a message of severity *level* would be processed by this logger.
      This method checks first the module-level level set by
      ``logging.disable(level)`` and then the logger's effective level as determined
      by :meth:`getEffectiveLevel`.


   .. method:: Logger.getEffectiveLevel()

      Indicates the effective level for this logger. If a value other than
      :const:`NOTSET` has been set using :meth:`setLevel`, it is returned. Otherwise,
      the hierarchy is traversed towards the root until a value other than
      :const:`NOTSET` is found, and that value is returned. The value returned is
      an integer, typically one of :const:`logging.DEBUG`, :const:`logging.INFO`
      etc.


   .. method:: Logger.getChild(suffix)

      Returns a logger which is a descendant to this logger, as determined by the suffix.
      Thus, ``logging.getLogger('abc').getChild('def.ghi')`` would return the same
      logger as would be returned by ``logging.getLogger('abc.def.ghi')``. This is a
      convenience method, useful when the parent logger is named using e.g. ``__name__``
      rather than a literal string.

      .. versionadded:: 3.2


   .. method:: Logger.debug(msg, *args, **kwargs)

      Logs a message with level :const:`DEBUG` on this logger. The *msg* is the
      message format string, and the *args* are the arguments which are merged into
      *msg* using the string formatting operator. (Note that this means that you can
      use keywords in the format string, together with a single dictionary argument.)
      No % formatting operation is performed on *msg* when no *args* are supplied.

      There are four keyword arguments in *kwargs* which are inspected:
      *exc_info*, *stack_info*, *stacklevel* and *extra*.

      If *exc_info* does not evaluate as false, it causes exception information to be
      added to the logging message. If an exception tuple (in the format returned by
      :func:`sys.exc_info`) or an exception instance is provided, it is used;
      otherwise, :func:`sys.exc_info` is called to get the exception information.

      The second optional keyword argument is *stack_info*, which defaults to
      ``False``. If true, stack information is added to the logging
      message, including the actual logging call. Note that this is not the same
      stack information as that displayed through specifying *exc_info*: The
      former is stack frames from the bottom of the stack up to the logging call
      in the current thread, whereas the latter is information about stack frames
      which have been unwound, following an exception, while searching for
      exception handlers.

      You can specify *stack_info* independently of *exc_info*, e.g. to just show
      how you got to a certain point in your code, even when no exceptions were
      raised. The stack frames are printed following a header line which says:

      .. code-block:: none

          Stack (most recent call last):

      This mimics the ``Traceback (most recent call last):`` which is used when
      displaying exception frames.

      The third optional keyword argument is *stacklevel*, which defaults to ``1``.
      If greater than 1, the corresponding number of stack frames are skipped
      when computing the line number and function name set in the :class:`LogRecord`
      created for the logging event. This can be used in logging helpers so that
      the function name, filename and line number recorded are not the information
      for the helper function/method, but rather its caller. The name of this
      parameter mirrors the equivalent one in the :mod:`warnings` module.

      The fourth keyword argument is *extra* which can be used to pass a
      dictionary which is used to populate the __dict__ of the :class:`LogRecord`
      created for the logging event with user-defined attributes. These custom
      attributes can then be used as you like. For example, they could be
      incorporated into logged messages. For example::

         FORMAT = '%(asctime)s %(clientip)-15s %(user)-8s %(message)s'
         logging.basicConfig(format=FORMAT)
         d = {'clientip': '192.168.0.1', 'user': 'fbloggs'}
         logger = logging.getLogger('tcpserver')
         logger.warning('Protocol problem: %s', 'connection reset', extra=d)

      would print something like

      .. code-block:: none

         2006-02-08 22:20:02,165 192.168.0.1 fbloggs  Protocol problem: connection reset

      The keys in the dictionary passed in *extra* should not clash with the keys used
      by the logging system. (See the :class:`Formatter` documentation for more
      information on which keys are used by the logging system.)

      If you choose to use these attributes in logged messages, you need to exercise
      some care. In the above example, for instance, the :class:`Formatter` has been
      set up with a format string which expects 'clientip' and 'user' in the attribute
      dictionary of the :class:`LogRecord`. If these are missing, the message will
      not be logged because a string formatting exception will occur. So in this case,
      you always need to pass the *extra* dictionary with these keys.

      While this might be annoying, this feature is intended for use in specialized
      circumstances, such as multi-threaded servers where the same code executes in
      many contexts, and interesting conditions which arise are dependent on this
      context (such as remote client IP address and authenticated user name, in the
      above example). In such circumstances, it is likely that specialized
      :class:`Formatter`\ s would be used with particular :class:`Handler`\ s.

      .. versionchanged:: 3.2
         The *stack_info* parameter was added.

      .. versionchanged:: 3.5
         The *exc_info* parameter can now accept exception instances.

      .. versionchanged:: 3.8
         The *stacklevel* parameter was added.


   .. method:: Logger.info(msg, *args, **kwargs)

      Logs a message with level :const:`INFO` on this logger. The arguments are
      interpreted as for :meth:`debug`.


   .. method:: Logger.warning(msg, *args, **kwargs)

      Logs a message with level :const:`WARNING` on this logger. The arguments are
      interpreted as for :meth:`debug`.

      .. note:: There is an obsolete method ``warn`` which is functionally
         identical to ``warning``. As ``warn`` is deprecated, please do not use
         it - use ``warning`` instead.

   .. method:: Logger.error(msg, *args, **kwargs)

      Logs a message with level :const:`ERROR` on this logger. The arguments are
      interpreted as for :meth:`debug`.


   .. method:: Logger.critical(msg, *args, **kwargs)

      Logs a message with level :const:`CRITICAL` on this logger. The arguments are
      interpreted as for :meth:`debug`.


   .. method:: Logger.log(level, msg, *args, **kwargs)

      Logs a message with integer level *level* on this logger. The other arguments are
      interpreted as for :meth:`debug`.


   .. method:: Logger.exception(msg, *args, **kwargs)

      Logs a message with level :const:`ERROR` on this logger. The arguments are
      interpreted as for :meth:`debug`. Exception info is added to the logging
      message. This method should only be called from an exception handler.


   .. method:: Logger.addFilter(filter)

      Adds the specified filter *filter* to this logger.


   .. method:: Logger.removeFilter(filter)

      Removes the specified filter *filter* from this logger.


   .. method:: Logger.filter(record)

      Apply this logger's filters to the record and return ``True`` if the
      record is to be processed. The filters are consulted in turn, until one of
      them returns a false value. If none of them return a false value, the record
      will be processed (passed to handlers). If one returns a false value, no
      further processing of the record occurs.


   .. method:: Logger.addHandler(hdlr)

      Adds the specified handler *hdlr* to this logger.


   .. method:: Logger.removeHandler(hdlr)

      Removes the specified handler *hdlr* from this logger.


   .. method:: Logger.findCaller(stack_info=False, stacklevel=1)

      Finds the caller's source filename and line number. Returns the filename, line
      number, function name and stack information as a 4-element tuple. The stack
      information is returned as ``None`` unless *stack_info* is ``True``.

      The *stacklevel* parameter is passed from code calling the :meth:`debug`
      and other APIs. If greater than 1, the excess is used to skip stack frames
      before determining the values to be returned. This will generally be useful
      when calling logging APIs from helper/wrapper code, so that the information
      in the event log refers not to the helper/wrapper code, but to the code that
      calls it.


   .. method:: Logger.handle(record)

      Handles a record by passing it to all handlers associated with this logger and
      its ancestors (until a false value of *propagate* is found). This method is used
      for unpickled records received from a socket, as well as those created locally.
      Logger-level filtering is applied using :meth:`~Logger.filter`.


   .. method:: Logger.makeRecord(name, level, fn, lno, msg, args, exc_info, func=None, extra=None, sinfo=None)

      This is a factory method which can be overridden in subclasses to create
      specialized :class:`LogRecord` instances.

   .. method:: Logger.hasHandlers()

      Checks to see if this logger has any handlers configured. This is done by
      looking for handlers in this logger and its parents in the logger hierarchy.
      Returns ``True`` if a handler was found, else ``False``. The method stops searching
      up the hierarchy whenever a logger with the 'propagate' attribute set to
      false is found - that will be the last logger which is checked for the
      existence of handlers.

      .. versionadded:: 3.2

   .. versionchanged:: 3.7
      Loggers can now be pickled and unpickled.

.. _levels:

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             |
+--------------+---------------+


.. _handler:

Handler Objects
---------------

Handlers have the following attributes and methods. Note that :class:`Handler`
is never instantiated directly; this class acts as a base for more useful
subclasses. However, the :meth:`__init__` method in subclasses needs to call
:meth:`Handler.__init__`.

.. class:: Handler

   .. method:: Handler.__init__(level=NOTSET)

      Initializes the :class:`Handler` instance by setting its level, setting the list
      of filters to the empty list and creating a lock (using :meth:`createLock`) for
      serializing access to an I/O mechanism.


   .. method:: Handler.createLock()

      Initializes a thread lock which can be used to serialize access to underlying
      I/O functionality which may not be threadsafe.


   .. method:: Handler.acquire()

      Acquires the thread lock created with :meth:`createLock`.


   .. method:: Handler.release()

      Releases the thread lock acquired with :meth:`acquire`.


   .. method:: Handler.setLevel(level)

      Sets the threshold for this handler to *level*. Logging messages which are
      less severe than *level* will be ignored. When a handler is created, the
      level is set to :const:`NOTSET` (which causes all messages to be
      processed).

      See :ref:`levels` for a list of levels.

      .. versionchanged:: 3.2
         The *level* parameter now accepts a string representation of the
         level such as 'INFO' as an alternative to the integer constants
         such as :const:`INFO`.


   .. method:: Handler.setFormatter(fmt)

      Sets the :class:`Formatter` for this handler to *fmt*.


   .. method:: Handler.addFilter(filter)

      Adds the specified filter *filter* to this handler.


   .. method:: Handler.removeFilter(filter)

      Removes the specified filter *filter* from this handler.


   .. method:: Handler.filter(record)

      Apply this handler's filters to the record and return ``True`` if the
      record is to be processed. The filters are consulted in turn, until one of
      them returns a false value. If none of them return a false value, the record
      will be emitted. If one returns a false value, the handler will not emit the
      record.


   .. method:: Handler.flush()

      Ensure all logging output has been flushed. This version does nothing and is
      intended to be implemented by subclasses.


   .. method:: Handler.close()

      Tidy up any resources used by the handler. This version does no output but
      removes the handler from an internal list of handlers which is closed when
      :func:`shutdown` is called. Subclasses should ensure that this gets called
      from overridden :meth:`close` methods.


   .. method:: Handler.handle(record)

      Conditionally emits the specified logging record, depending on filters which may
      have been added to the handler. Wraps the actual emission of the record with
      acquisition/release of the I/O thread lock.


   .. method:: Handler.handleError(record)

      This method should be called from handlers when an exception is encountered
      during an :meth:`emit` call. If the module-level attribute
      ``raiseExceptions`` is ``False``, exceptions get silently ignored. This is
      what is mostly wanted for a logging system - most users will not care about
      errors in the logging system, they are more interested in application
      errors. You could, however, replace this with a custom handler if you wish.
      The specified record is the one which was being processed when the exception
      occurred. (The default value of ``raiseExceptions`` is ``True``, as that is
      more useful during development).


   .. method:: Handler.format(record)

      Do formatting for a record - if a formatter is set, use it. Otherwise, use the
      default formatter for the module.


   .. method:: Handler.emit(record)

      Do whatever it takes to actually log the specified logging record. This version
      is intended to be implemented by subclasses and so raises a
      :exc:`NotImplementedError`.

For a list of handlers included as standard, see :mod:`logging.handlers`.

.. _formatter-objects:

Formatter Objects
-----------------

.. currentmodule:: logging

:class:`Formatter` objects have the following attributes and methods. They are
responsible for converting a :class:`LogRecord` to (usually) a string which can
be interpreted by either a human or an external system. The base
:class:`Formatter` allows a formatting string to be specified. If none is
supplied, the default value of ``'%(message)s'`` is used, which just includes
the message in the logging call. To have additional items of information in the
formatted output (such as a timestamp), keep reading.

A Formatter can be initialized with a format string which makes use of knowledge
of the :class:`LogRecord` attributes - such as the default value mentioned above
making use of the fact that the user's message and arguments are pre-formatted
into a :class:`LogRecord`'s *message* attribute.  This format string contains
standard Python %-style mapping keys. See section :ref:`old-string-formatting`
for more information on string formatting.

The useful mapping keys in a :class:`LogRecord` are given in the section on
:ref:`logrecord-attributes`.


.. class:: Formatter(fmt=None, datefmt=None, style='%', validate=True, *, defaults=None)

   Returns a new instance of the :class:`Formatter` class.  The instance is
   initialized with a format string for the message as a whole, as well as a
   format string for the date/time portion of a message.  If no *fmt* is
   specified, ``'%(message)s'`` is used.  If no *datefmt* is specified, a format
   is used which is described in the :meth:`formatTime` documentation.

   The *style* parameter can be one of '%', '{' or '$' and determines how
   the format string will be merged with its data: using one of %-formatting,
   :meth:`str.format` or :class:`string.Template`. This only applies to the
   format string *fmt* (e.g. ``'%(message)s'`` or ``{message}``), not to the
   actual log messages passed to ``Logger.debug`` etc; see
   :ref:`formatting-styles` for more information on using {- and $-formatting
   for log messages.

   The *defaults* parameter can be a dictionary with default values to use in
   custom fields. For example:
   ``logging.Formatter('%(ip)s %(message)s', defaults={"ip": None})``

   .. versionchanged:: 3.2
      The *style* parameter was added.

   .. versionchanged:: 3.8
      The *validate* parameter was added. Incorrect or mismatched style and fmt
      will raise a ``ValueError``.
      For example: ``logging.Formatter('%(asctime)s - %(message)s', style='{')``.

   .. versionchanged:: 3.10
      The *defaults* parameter was added.

   .. method:: format(record)

      The record's attribute dictionary is used as the operand to a string
      formatting operation. Returns the resulting string. Before formatting the
      dictionary, a couple of preparatory steps are carried out. The *message*
      attribute of the record is computed using *msg* % *args*. If the
      formatting string contains ``'(asctime)'``, :meth:`formatTime` is called
      to format the event time. If there is exception information, it is
      formatted using :meth:`formatException` and appended to the message. Note
      that the formatted exception information is cached in attribute
      *exc_text*. This is useful because the exception information can be
      pickled and sent across the wire, but you should be careful if you have
      more than one :class:`Formatter` subclass which customizes the formatting
      of exception information. In this case, you will have to clear the cached
      value (by setting the *exc_text* attribute to ``None``) after a formatter
      has done its formatting, so that the next formatter to handle the event
      doesn't use the cached value, but recalculates it afresh.

      If stack information is available, it's appended after the exception
      information, using :meth:`formatStack` to transform it if necessary.


   .. method:: formatTime(record, datefmt=None)

      This method should be called from :meth:`format` by a formatter which
      wants to make use of a formatted time. This method can be overridden in
      formatters to provide for any specific requirement, but the basic behavior
      is as follows: if *datefmt* (a string) is specified, it is used with
      :func:`time.strftime` to format the creation time of the
      record. Otherwise, the format '%Y-%m-%d %H:%M:%S,uuu' is used, where the
      uuu part is a millisecond value and the other letters are as per the
      :func:`time.strftime` documentation.  An example time in this format is
      ``2003-01-23 00:29:50,411``.  The resulting string is returned.

      This function uses a user-configurable function to convert the creation
      time to a tuple. By default, :func:`time.localtime` is used; to change
      this for a particular formatter instance, set the ``converter`` attribute
      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.

      .. versionchanged:: 3.3
         Previously, the default format was hard-coded as in this example:
         ``2010-09-06 22:38:15,292`` where the part before the comma is
         handled by a strptime format string (``'%Y-%m-%d %H:%M:%S'``), and the
         part after the comma is a millisecond value. Because strptime does not
         have a format placeholder for milliseconds, the millisecond value is
         appended using another format string, ``'%s,%03d'`` --- and both of these
         format strings have been hardcoded into this method. With the change,
         these strings are defined as class-level attributes which can be
         overridden at the instance level when desired. The names of the
         attributes are ``default_time_format`` (for the strptime format string)
         and ``default_msec_format`` (for appending the millisecond value).

      .. versionchanged:: 3.9
         The ``default_msec_format`` can be ``None``.

   .. method:: formatException(exc_info)

      Formats the specified exception information (a standard exception tuple as
      returned by :func:`sys.exc_info`) as a string. This default implementation
      just uses :func:`traceback.print_exception`. The resulting string is
      returned.

   .. method:: formatStack(stack_info)

      Formats the specified stack information (a string as returned by
      :func:`traceback.print_stack`, but with the last newline removed) as a
      string. This default implementation just returns the input value.

.. _filter:

Filter Objects
--------------

``Filters`` can be used by ``Handlers`` and ``Loggers`` for more sophisticated
filtering than is provided by levels. The base filter class only allows events
which are below a certain point in the logger hierarchy. For example, a filter
initialized with 'A.B' will allow events logged by loggers 'A.B', 'A.B.C',
'A.B.C.D', 'A.B.D' etc. but not 'A.BB', 'B.A.B' etc. If initialized with the
empty string, all events are passed.


.. class:: Filter(name='')

   Returns an instance of the :class:`Filter` class. If *name* is specified, it
   names a logger which, together with its children, will have its events allowed
   through the filter. If *name* is the empty string, allows every event.


   .. method:: filter(record)

      Is the specified record to be logged? Returns zero for no, nonzero for
      yes. If deemed appropriate, the record may be modified in-place by this
      method.

Note that filters attached to handlers are consulted before an event is
emitted by the handler, whereas filters attached to loggers are consulted
whenever an event is logged (using :meth:`debug`, :meth:`info`,
etc.), before sending an event to handlers. This means that events which have
been generated by descendant loggers will not be filtered by a logger's filter
setting, unless the filter has also been applied to those descendant loggers.

You don't actually need to subclass ``Filter``: you can pass any instance
which has a ``filter`` method with the same semantics.

.. versionchanged:: 3.2
   You don't need to create specialized ``Filter`` classes, or use other
   classes with a ``filter`` method: you can use a function (or other
   callable) as a filter. The filtering logic will check to see if the filter
   object has a ``filter`` attribute: if it does, it's assumed to be a
   ``Filter`` and its :meth:`~Filter.filter` method is called. Otherwise, it's
   assumed to be a callable and called with the record as the single
   parameter. The returned value should conform to that returned by
   :meth:`~Filter.filter`.

Although filters are used primarily to filter records based on more
sophisticated criteria than levels, they get to see every record which is
processed by the handler or logger they're attached to: this can be useful if
you want to do things like counting how many records were processed by a
particular logger or handler, or adding, changing or removing attributes in
the :class:`LogRecord` being processed. Obviously changing the LogRecord needs
to be done with some care, but it does allow the injection of contextual
information into logs (see :ref:`filters-contextual`).

.. _log-record:

LogRecord Objects
-----------------

:class:`LogRecord` instances are created automatically by the :class:`Logger`
every time something is logged, and can be created manually via
:func:`makeLogRecord` (for example, from a pickled event received over the
wire).


.. class:: LogRecord(name, level, pathname, lineno, msg, args, exc_info, func=None, sinfo=None)

   Contains all the information pertinent to the event being logged.

   The primary information is passed in :attr:`msg` and :attr:`args`, which
   are combined using ``msg % args`` to create the :attr:`message` field of the
   record.

   :param name:  The name of the logger used to log the event represented by
                 this LogRecord. Note that this name will always have this
                 value, even though it may be emitted by a handler attached to
                 a different (ancestor) logger.
   :param level: The numeric level of the logging event (one of DEBUG, INFO etc.)
                 Note that this is converted to *two* attributes of the LogRecord:
                 ``levelno`` for the numeric value and ``levelname`` for the
                 corresponding level name.
   :param pathname: The full pathname of the source file where the logging call
                    was made.
   :param lineno: The line number in the source file where the logging call was
                  made.
   :param msg: The event description message, possibly a format string with
               placeholders for variable data.
   :param args: Variable data to merge into the *msg* argument to obtain the
                event description.
   :param exc_info: An exception tuple with the current exception information,
                    or ``None`` if no exception information is available.
   :param func: The name of the function or method from which the logging call
                was invoked.
   :param sinfo: A text string representing stack information from the base of
                 the stack in the current thread, up to the logging call.

   .. method:: getMessage()

      Returns the message for this :class:`LogRecord` instance after merging any
      user-supplied arguments with the message. If the user-supplied message
      argument to the logging call is not a string, :func:`str` is called on it to
      convert it to a string. This allows use of user-defined classes as
      messages, whose ``__str__`` method can return the actual format string to
      be used.

   .. versionchanged:: 3.2
      The creation of a :class:`LogRecord` has been made more configurable by
      providing a factory which is used to create the record. The factory can be
      set using :func:`getLogRecordFactory` and :func:`setLogRecordFactory`
      (see this for the factory's signature).

   This functionality can be used to inject your own values into a
   :class:`LogRecord` at creation time. You can use the following pattern::

      old_factory = logging.getLogRecordFactory()

      def record_factory(*args, **kwargs):
          record = old_factory(*args, **kwargs)
          record.custom_attribute = 0xdecafbad
          return record

      logging.setLogRecordFactory(record_factory)

   With this pattern, multiple factories could be chained, and as long
   as they don't overwrite each other's attributes or unintentionally
   overwrite the standard attributes listed above, there should be no
   surprises.


.. _logrecord-attributes:

LogRecord attributes
--------------------

The LogRecord has a number of attributes, most of which are derived from the
parameters to the constructor. (Note that the names do not always correspond
exactly between the LogRecord constructor parameters and the LogRecord
attributes.) These attributes can be used to merge data from the record into
the format string. The following table lists (in alphabetical order) the
attribute names, their meanings and the corresponding placeholder in a %-style
format string.

If you are using {}-formatting (:func:`str.format`), you can use
``{attrname}`` as the placeholder in the format string. If you are using
$-formatting (:class:`string.Template`), use the form ``${attrname}``. In
both cases, of course, replace ``attrname`` with the actual attribute name
you want to use.

In the case of {}-formatting, you can specify formatting flags by placing them
after the attribute name, separated from it with a colon. For example: a
placeholder of ``{msecs:03d}`` would format a millisecond value of ``4`` as
``004``. Refer to the :meth:`str.format` documentation for full details on
the options available to you.

+----------------+-------------------------+-----------------------------------------------+
| Attribute name | Format                  | Description                                   |
+================+=========================+===============================================+
| args           | You shouldn't need to   | The tuple of arguments merged into ``msg`` to |
|                | format this yourself.   | produce ``message``, or a dict whose values   |
|                |                         | are used for the merge (when there is only one|
|                |                         | argument, and it is a dictionary).            |
+----------------+-------------------------+-----------------------------------------------+
| asctime        | ``%(asctime)s``         | Human-readable time when the                  |
|                |                         | :class:`LogRecord` was created.  By default   |
|                |                         | this is of the form '2003-07-08 16:49:45,896' |
|                |                         | (the numbers after the comma are millisecond  |
|                |                         | portion of the time).                         |
+----------------+-------------------------+-----------------------------------------------+
| created        | ``%(created)f``         | Time when the :class:`LogRecord` was created  |
|                |                         | (as returned by :func:`time.time`).           |
+----------------+-------------------------+-----------------------------------------------+
| exc_info       | You shouldn't need to   | Exception tuple (à la ``sys.exc_info``) or,   |
|                | format this yourself.   | if no exception has occurred, ``None``.       |
+----------------+-------------------------+-----------------------------------------------+
| filename       | ``%(filename)s``        | Filename portion of ``pathname``.             |
+----------------+-------------------------+-----------------------------------------------+
| funcName       | ``%(funcName)s``        | Name of function containing the logging call. |
+----------------+-------------------------+-----------------------------------------------+
| levelname      | ``%(levelname)s``       | Text logging level for the message            |
|                |                         | (``'DEBUG'``, ``'INFO'``, ``'WARNING'``,      |
|                |                         | ``'ERROR'``, ``'CRITICAL'``).                 |
+----------------+-------------------------+-----------------------------------------------+
| levelno        | ``%(levelno)s``         | Numeric logging level for the message         |
|                |                         | (:const:`DEBUG`, :const:`INFO`,               |
|                |                         | :const:`WARNING`, :const:`ERROR`,             |
|                |                         | :const:`CRITICAL`).                           |
+----------------+-------------------------+-----------------------------------------------+
| lineno         | ``%(lineno)d``          | Source line number where the logging call was |
|                |                         | issued (if available).                        |
+----------------+-------------------------+-----------------------------------------------+
| message        | ``%(message)s``         | The logged message, computed as ``msg %       |
|                |                         | args``. This is set when                      |
|                |                         | :meth:`Formatter.format` is invoked.          |
+----------------+-------------------------+-----------------------------------------------+
| module         | ``%(module)s``          | Module (name portion of ``filename``).        |
+----------------+-------------------------+-----------------------------------------------+
| msecs          | ``%(msecs)d``           | Millisecond portion of the time when the      |
|                |                         | :class:`LogRecord` was created.               |
+----------------+-------------------------+-----------------------------------------------+
| msg            | You shouldn't need to   | The format string passed in the original      |
|                | format this yourself.   | logging call. Merged with ``args`` to         |
|                |                         | produce ``message``, or an arbitrary object   |
|                |                         | (see :ref:`arbitrary-object-messages`).       |
+----------------+-------------------------+-----------------------------------------------+
| name           | ``%(name)s``            | Name of the logger used to log the call.      |
+----------------+-------------------------+-----------------------------------------------+
| pathname       | ``%(pathname)s``        | Full pathname of the source file where the    |
|                |                         | logging call was issued (if available).       |
+----------------+-------------------------+-----------------------------------------------+
| process        | ``%(process)d``         | Process ID (if available).                    |
+----------------+-------------------------+-----------------------------------------------+
| processName    | ``%(processName)s``     | Process name (if available).                  |
+----------------+-------------------------+-----------------------------------------------+
| relativeCreated| ``%(relativeCreated)d`` | Time in milliseconds when the LogRecord was   |
|                |                         | created, relative to the time the logging     |
|                |                         | module was loaded.                            |
+----------------+-------------------------+-----------------------------------------------+
| stack_info     | You shouldn't need to   | Stack frame information (where available)     |
|                | format this yourself.   | from the bottom of the stack in the current   |
|                |                         | thread, up to and including the stack frame   |
|                |                         | of the logging call which resulted in the     |
|                |                         | creation of this record.                      |
+----------------+-------------------------+-----------------------------------------------+
| thread         | ``%(thread)d``          | Thread ID (if available).                     |
+----------------+-------------------------+-----------------------------------------------+
| threadName     | ``%(threadName)s``      | Thread name (if available).                   |
+----------------+-------------------------+-----------------------------------------------+

.. versionchanged:: 3.1
   *processName* was added.


.. _logger-adapter:

LoggerAdapter Objects
---------------------

:class:`LoggerAdapter` instances are used to conveniently pass contextual
information into logging calls. For a usage example, see the section on
:ref:`adding contextual information to your logging output <context-info>`.

.. class:: LoggerAdapter(logger, extra)

   Returns an instance of :class:`LoggerAdapter` initialized with an
   underlying :class:`Logger` instance and a dict-like object.

   .. method:: process(msg, kwargs)

      Modifies the message and/or keyword arguments passed to a logging call in
      order to insert contextual information. This implementation takes the object
      passed as *extra* to the constructor and adds it to *kwargs* using key
      'extra'. The return value is a (*msg*, *kwargs*) tuple which has the
      (possibly modified) versions of the arguments passed in.

In addition to the above, :class:`LoggerAdapter` supports the following
methods of :class:`Logger`: :meth:`~Logger.debug`, :meth:`~Logger.info`,
:meth:`~Logger.warning`, :meth:`~Logger.error`, :meth:`~Logger.exception`,
:meth:`~Logger.critical`, :meth:`~Logger.log`, :meth:`~Logger.isEnabledFor`,
:meth:`~Logger.getEffectiveLevel`, :meth:`~Logger.setLevel` and
:meth:`~Logger.hasHandlers`. These methods have the same signatures as their
counterparts in :class:`Logger`, so you can use the two types of instances
interchangeably.

.. versionchanged:: 3.2
   The :meth:`~Logger.isEnabledFor`, :meth:`~Logger.getEffectiveLevel`,
   :meth:`~Logger.setLevel` and :meth:`~Logger.hasHandlers` methods were added
   to :class:`LoggerAdapter`.  These methods delegate to the underlying logger.

.. versionchanged:: 3.6
   Attribute :attr:`manager` and method :meth:`_log` were added, which
   delegate to the underlying logger and allow adapters to be nested.


Thread Safety
-------------

The logging module is intended to be thread-safe without any special work
needing to be done by its clients. It achieves this though using threading
locks; there is one lock to serialize access to the module's shared data, and
each handler also creates a lock to serialize access to its underlying I/O.

If you are implementing asynchronous signal handlers using the :mod:`signal`
module, you may not be able to use logging from within such handlers. This is
because lock implementations in the :mod:`threading` module are not always
re-entrant, and so cannot be invoked from such signal handlers.


Module-Level Functions
----------------------

In addition to the classes described above, there are a number of module-level
functions.


.. function:: getLogger(name=None)

   Return a logger with the specified name or, if name is ``None``, return a
   logger which is the root logger of the hierarchy. If specified, the name is
   typically a dot-separated hierarchical name like *'a'*, *'a.b'* or *'a.b.c.d'*.
   Choice of these names is entirely up to the developer who is using logging.

   All calls to this function with a given name return the same logger instance.
   This means that logger instances never need to be passed between different parts
   of an application.


.. function:: getLoggerClass()

   Return either the standard :class:`Logger` class, or the last class passed to
   :func:`setLoggerClass`. This function may be called from within a new class
   definition, to ensure that installing a customized :class:`Logger` class will
   not undo customizations already applied by other code. For example::

      class MyLogger(logging.getLoggerClass()):
          # ... override behaviour here


.. function:: getLogRecordFactory()

   Return a callable which is used to create a :class:`LogRecord`.

   .. versionadded:: 3.2
      This function has been provided, along with :func:`setLogRecordFactory`,
      to allow developers more control over how the :class:`LogRecord`
      representing a logging event is constructed.

   See :func:`setLogRecordFactory` for more information about the how the
   factory is called.

.. function:: debug(msg, *args, **kwargs)

   Logs a message with level :const:`DEBUG` on the root logger. The *msg* is the
   message format string, and the *args* are the arguments which are merged into
   *msg* using the string formatting operator. (Note that this means that you can
   use keywords in the format string, together with a single dictionary argument.)

   There are three keyword arguments in *kwargs* which are inspected: *exc_info*
   which, if it does not evaluate as false, causes exception information to be
   added to the logging message. If an exception tuple (in the format returned by
   :func:`sys.exc_info`) or an exception instance is provided, it is used;
   otherwise, :func:`sys.exc_info` is called to get the exception information.

   The second optional keyword argument is *stack_info*, which defaults to
   ``False``. If true, stack information is added to the logging
   message, including the actual logging call. Note that this is not the same
   stack information as that displayed through specifying *exc_info*: The
   former is stack frames from the bottom of the stack up to the logging call
   in the current thread, whereas the latter is information about stack frames
   which have been unwound, following an exception, while searching for
   exception handlers.

   You can specify *stack_info* independently of *exc_info*, e.g. to just show
   how you got to a certain point in your code, even when no exceptions were
   raised. The stack frames are printed following a header line which says:

   .. code-block:: none