Doc/library/logging.handlers.rst
!logging.handlers --- Logging handlers.. module:: logging.handlers :synopsis: Handlers for the logging module.
Source code: :source:Lib/logging/handlers.py
.. sidebar:: Important
This page contains only reference information. For tutorials, please see
Basic Tutorial <logging-basic-tutorial>Advanced Tutorial <logging-advanced-tutorial>Logging Cookbook <logging-cookbook>.. currentmodule:: logging
The following useful handlers are provided in the package. Note that three of
the handlers (:class:StreamHandler, :class:FileHandler and
:class:NullHandler) are actually defined in the :mod:logging module itself,
but have been documented here along with the other handlers.
.. _stream-handler:
StreamHandler ^^^^^^^^^^^^^
The :class:StreamHandler class, located in the core :mod:logging package,
sends logging output to streams such as sys.stdout, sys.stderr or any
file-like object (or, more precisely, any object which supports :meth:write
and :meth:flush methods).
.. class:: StreamHandler(stream=None)
Returns a new instance of the :class:StreamHandler class. If stream is
specified, the instance will use it for logging output; otherwise, sys.stderr
will be used.
.. method:: emit(record)
If a formatter is specified, it is used to format the record. The record
is then written to the stream followed by :attr:`terminator`. If exception information
is present, it is formatted using :func:`traceback.print_exception` and
appended to the stream.
.. method:: flush()
Flushes the stream by calling its :meth:`flush` method. Note that the
:meth:`close` method is inherited from :class:`~logging.Handler` and so
does no output, so an explicit :meth:`flush` call may be needed at times.
.. method:: setStream(stream)
Sets the instance's stream to the specified value, if it is different.
The old stream is flushed before the new stream is set.
:param stream: The stream that the handler should use.
:return: the old stream, if the stream was changed, or ``None`` if it wasn't.
.. versionadded:: 3.7
.. attribute:: terminator
String used as the terminator when writing a formatted record to a stream.
Default value is ``'\n'``.
If you don't want a newline termination, you can set the handler instance's
``terminator`` attribute to the empty string.
In earlier versions, the terminator was hardcoded as ``'\n'``.
.. versionadded:: 3.2
.. _file-handler:
FileHandler ^^^^^^^^^^^
The :class:FileHandler class, located in the core :mod:logging package,
sends logging output to a disk file. It inherits the output functionality from
:class:StreamHandler.
.. class:: FileHandler(filename, mode='a', encoding=None, delay=False, errors=None)
Returns a new instance of the :class:FileHandler class. The specified file is
opened and used as the stream for logging. If mode is not specified,
'a' is used. If encoding is not None, it is used to open the file
with that encoding. If delay is true, then file opening is deferred until the
first call to :meth:emit. By default, the file grows indefinitely. If
errors is specified, it's used to determine how encoding errors are handled.
.. versionchanged:: 3.6
As well as string values, :class:~pathlib.Path objects are also accepted
for the filename argument.
.. versionchanged:: 3.9 The errors parameter was added.
.. method:: close()
Closes the file.
.. method:: emit(record)
Outputs the record to the file.
Note that if the file was closed due to logging shutdown at exit and the file
mode is 'w', the record will not be emitted (see :issue:`42378`).
.. _null-handler:
NullHandler ^^^^^^^^^^^
.. versionadded:: 3.1
The :class:NullHandler class, located in the core :mod:logging package,
does not do any formatting or output. It is essentially a 'no-op' handler
for use by library developers.
.. class:: NullHandler()
Returns a new instance of the :class:NullHandler class.
.. method:: emit(record)
This method does nothing.
.. method:: handle(record)
This method does nothing.
.. method:: createLock()
This method returns ``None`` for the lock, since there is no
underlying I/O to which access needs to be serialized.
See :ref:library-config for more information on how to use
:class:NullHandler.
.. _watched-file-handler:
WatchedFileHandler ^^^^^^^^^^^^^^^^^^
.. currentmodule:: logging.handlers
The :class:WatchedFileHandler class, located in the :mod:!logging.handlers
module, is a :class:FileHandler which watches the file it is logging to. If
the file changes, it is closed and reopened using the file name.
A file change can happen because of usage of programs such as newsyslog and logrotate which perform log file rotation. This handler, intended for use under Unix/Linux, watches the file to see if it has changed since the last emit. (A file is deemed to have changed if its device or inode have changed.) If the file has changed, the old file stream is closed, and the file opened to get a new stream.
This handler is not appropriate for use under Windows, because under Windows
open log files cannot be moved or renamed - logging opens the files with
exclusive locks - and so there is no need for such a handler. Furthermore,
ST_INO is not supported under Windows; :func:~os.stat always returns zero
for this value.
.. class:: WatchedFileHandler(filename, mode='a', encoding=None, delay=False, errors=None)
Returns a new instance of the :class:WatchedFileHandler class. The specified
file is opened and used as the stream for logging. If mode is not specified,
'a' is used. If encoding is not None, it is used to open the file
with that encoding. If delay is true, then file opening is deferred until the
first call to :meth:emit. By default, the file grows indefinitely. If
errors is provided, it determines how encoding errors are handled.
.. versionchanged:: 3.6
As well as string values, :class:~pathlib.Path objects are also accepted
for the filename argument.
.. versionchanged:: 3.9 The errors parameter was added.
.. method:: reopenIfNeeded()
Checks to see if the file has changed. If it has, the existing stream is
flushed and closed and the file opened again, typically as a precursor to
outputting the record to the file.
.. versionadded:: 3.6
.. method:: emit(record)
Outputs the record to the file, but first calls :meth:`reopenIfNeeded` to
reopen the file if it has changed.
.. _base-rotating-handler:
BaseRotatingHandler ^^^^^^^^^^^^^^^^^^^
The :class:BaseRotatingHandler class, located in the :mod:!logging.handlers
module, is the base class for the rotating file handlers,
:class:RotatingFileHandler and :class:TimedRotatingFileHandler. You should
not need to instantiate this class, but it has attributes and methods you may
need to override.
.. class:: BaseRotatingHandler(filename, mode, encoding=None, delay=False, errors=None)
The parameters are as for :class:FileHandler. The attributes are:
.. attribute:: namer
If this attribute is set to a callable, the :meth:`rotation_filename`
method delegates to this callable. The parameters passed to the callable
are those passed to :meth:`rotation_filename`.
.. note:: The namer function is called quite a few times during rollover,
so it should be as simple and as fast as possible. It should also
return the same output every time for a given input, otherwise the
rollover behaviour may not work as expected.
It's also worth noting that care should be taken when using a namer to
preserve certain attributes in the filename which are used during rotation.
For example, :class:`RotatingFileHandler` expects to have a set of log files
whose names contain successive integers, so that rotation works as expected,
and :class:`TimedRotatingFileHandler` deletes old log files (based on the
``backupCount`` parameter passed to the handler's initializer) by determining
the oldest files to delete. For this to happen, the filenames should be
sortable using the date/time portion of the filename, and a namer needs to
respect this. (If a namer is wanted that doesn't respect this scheme, it will
need to be used in a subclass of :class:`TimedRotatingFileHandler` which
overrides the :meth:`~TimedRotatingFileHandler.getFilesToDelete` method to
fit in with the custom naming scheme.)
.. versionadded:: 3.3
.. attribute:: BaseRotatingHandler.rotator
If this attribute is set to a callable, the :meth:`rotate` method
delegates to this callable. The parameters passed to the callable are
those passed to :meth:`rotate`.
.. versionadded:: 3.3
.. method:: BaseRotatingHandler.rotation_filename(default_name)
Modify the filename of a log file when rotating.
This is provided so that a custom filename can be provided.
The default implementation calls the 'namer' attribute of the handler,
if it's callable, passing the default name to it. If the attribute isn't
callable (the default is ``None``), the name is returned unchanged.
:param default_name: The default name for the log file.
.. versionadded:: 3.3
.. method:: BaseRotatingHandler.rotate(source, dest)
When rotating, rotate the current log.
The default implementation calls the 'rotator' attribute of the handler,
if it's callable, passing the source and dest arguments to it. If the
attribute isn't callable (the default is ``None``), the source is simply
renamed to the destination.
:param source: The source filename. This is normally the base
filename, e.g. 'test.log'.
:param dest: The destination filename. This is normally
what the source is rotated to, e.g. 'test.log.1'.
.. versionadded:: 3.3
The reason the attributes exist is to save you having to subclass - you can use
the same callables for instances of :class:RotatingFileHandler and
:class:TimedRotatingFileHandler. If either the namer or rotator callable
raises an exception, this will be handled in the same way as any other
exception during an :meth:emit call, i.e. via the :meth:handleError method
of the handler.
If you need to make more significant changes to rotation processing, you can override the methods.
For an example, see :ref:cookbook-rotator-namer.
.. _rotating-file-handler:
RotatingFileHandler ^^^^^^^^^^^^^^^^^^^
The :class:RotatingFileHandler class, located in the :mod:!logging.handlers
module, supports rotation of disk log files.
.. class:: RotatingFileHandler(filename, mode='a', maxBytes=0, backupCount=0, encoding=None, delay=False, errors=None)
Returns a new instance of the :class:RotatingFileHandler class. The specified
file is opened and used as the stream for logging. If mode is not specified,
'a' is used. If encoding is not None, it is used to open the file
with that encoding. If delay is true, then file opening is deferred until the
first call to :meth:emit. By default, the file grows indefinitely. If
errors is provided, it determines how encoding errors are handled.
You can use the maxBytes and backupCount values to allow the file to
:dfn:rollover at a predetermined size. When the size is about to be exceeded,
the file is closed and a new file is silently opened for output. Rollover occurs
whenever the current log file is nearly maxBytes in length; but if either of
maxBytes or backupCount is zero, rollover never occurs, so you generally want
to set backupCount to at least 1, and have a non-zero maxBytes.
When backupCount is non-zero, the system will save old log files by appending
the extensions '.1', '.2' etc., to the filename. For example, with a backupCount
of 5 and a base file name of :file:app.log, you would get :file:app.log,
:file:app.log.1, :file:app.log.2, up to :file:app.log.5. The file being
written to is always :file:app.log. When this file is filled, it is closed
and renamed to :file:app.log.1, and if files :file:app.log.1,
:file:app.log.2, etc. exist, then they are renamed to :file:app.log.2,
:file:app.log.3 etc. respectively.
.. versionchanged:: 3.6
As well as string values, :class:~pathlib.Path objects are also accepted
for the filename argument.
.. versionchanged:: 3.9 The errors parameter was added.
.. method:: doRollover()
Does a rollover, as described above.
.. method:: emit(record)
Outputs the record to the file, catering for rollover as described
previously.
.. method:: shouldRollover(record)
See if the supplied record would cause the file to exceed the configured size limit.
.. _timed-rotating-file-handler:
TimedRotatingFileHandler ^^^^^^^^^^^^^^^^^^^^^^^^
The :class:TimedRotatingFileHandler class, located in the
:mod:!logging.handlers module, supports rotation of disk log files at certain
timed intervals.
.. class:: TimedRotatingFileHandler(filename, when='h', interval=1, backupCount=0, encoding=None, delay=False, utc=False, atTime=None, errors=None)
Returns a new instance of the :class:TimedRotatingFileHandler class. The
specified file is opened and used as the stream for logging. On rotating it also
sets the filename suffix. Rotating happens based on the product of when and
interval.
You can use the when to specify the type of interval. The list of possible values is below. Note that they are not case sensitive.
+----------------+----------------------------+-------------------------+
| Value | Type of interval | If/how atTime is used |
+================+============================+=========================+
| 'S' | Seconds | Ignored |
+----------------+----------------------------+-------------------------+
| 'M' | Minutes | Ignored |
+----------------+----------------------------+-------------------------+
| 'H' | Hours | Ignored |
+----------------+----------------------------+-------------------------+
| 'D' | Days | Ignored |
+----------------+----------------------------+-------------------------+
| 'W0'-'W6' | Weekday (0=Monday) | Used to compute initial |
| | | rollover time |
+----------------+----------------------------+-------------------------+
| 'midnight' | Roll over at midnight, if | Used to compute initial |
| | atTime not specified, | rollover time |
| | else at time atTime | |
+----------------+----------------------------+-------------------------+
When using weekday-based rotation, specify 'W0' for Monday, 'W1' for Tuesday, and so on up to 'W6' for Sunday. In this case, the value passed for interval isn't used.
The system will save old log files by appending extensions to the filename.
The extensions are date-and-time based, using the strftime format
%Y-%m-%d_%H-%M-%S or a leading portion thereof, depending on the
rollover interval.
When computing the next rollover time for the first time (when the handler is created), the last modification time of an existing log file, or else the current time, is used to compute when the next rotation will occur.
If the utc argument is true, times in UTC will be used; otherwise local time is used.
If backupCount is nonzero, at most backupCount files will be kept, and if more would be created when rollover occurs, the oldest one is deleted. The deletion logic uses the interval to determine which files to delete, so changing the interval may leave old files lying around.
If delay is true, then file opening is deferred until the first call to
:meth:emit.
If atTime is not None, it must be a datetime.time instance which
specifies the time of day when rollover occurs, for the cases where rollover
is set to happen "at midnight" or "on a particular weekday". Note that in
these cases, the atTime value is effectively used to compute the initial
rollover, and subsequent rollovers would be calculated via the normal
interval calculation.
If errors is specified, it's used to determine how encoding errors are handled.
.. note:: Calculation of the initial rollover time is done when the handler is initialised. Calculation of subsequent rollover times is done only when rollover occurs, and rollover occurs only when emitting output. If this is not kept in mind, it might lead to some confusion. For example, if an interval of "every minute" is set, that does not mean you will always see log files with times (in the filename) separated by a minute; if, during application execution, logging output is generated more frequently than once a minute, then you can expect to see log files with times separated by a minute. If, on the other hand, logging messages are only output once every five minutes (say), then there will be gaps in the file times corresponding to the minutes where no output (and hence no rollover) occurred.
.. versionchanged:: 3.4 atTime parameter was added.
.. versionchanged:: 3.6
As well as string values, :class:~pathlib.Path objects are also accepted
for the filename argument.
.. versionchanged:: 3.9 The errors parameter was added.
.. method:: doRollover()
Does a rollover, as described above.
.. method:: emit(record)
Outputs the record to the file, catering for rollover as described above.
.. method:: getFilesToDelete()
Returns a list of filenames which should be deleted as part of rollover. These
are the absolute paths of the oldest backup log files written by the handler.
.. method:: shouldRollover(record)
See if enough time has passed for a rollover to occur and if it has, compute
the next rollover time.
.. _socket-handler:
SocketHandler ^^^^^^^^^^^^^
The :class:SocketHandler class, located in the :mod:!logging.handlers module,
sends logging output to a network socket. The base class uses a TCP socket.
.. class:: SocketHandler(host, port)
Returns a new instance of the :class:SocketHandler class intended to
communicate with a remote machine whose address is given by host and port.
.. versionchanged:: 3.4
If port is specified as None, a Unix domain socket is created
using the value in host - otherwise, a TCP socket is created.
.. method:: close()
Closes the socket.
.. method:: emit()
Pickles the record's attribute dictionary and writes it to the socket in
binary format. If there is an error with the socket, silently drops the
packet. If the connection was previously lost, re-establishes the
connection. To unpickle the record at the receiving end into a
:class:`~logging.LogRecord`, use the :func:`~logging.makeLogRecord`
function.
.. method:: handleError()
Handles an error which has occurred during :meth:`emit`. The most likely
cause is a lost connection. Closes the socket so that we can retry on the
next event.
.. method:: makeSocket()
This is a factory method which allows subclasses to define the precise
type of socket they want. The default implementation creates a TCP socket
(:const:`socket.SOCK_STREAM`).
.. method:: makePickle(record)
Pickles the record's attribute dictionary in binary format with a length
prefix, and returns it ready for transmission across the socket. The
details of this operation are equivalent to::
data = pickle.dumps(record_attr_dict, 1)
datalen = struct.pack('>L', len(data))
return datalen + data
Note that pickles aren't completely secure. If you are concerned about
security, you may want to override this method to implement a more secure
mechanism. For example, you can sign pickles using HMAC and then verify
them on the receiving end, or alternatively you can disable unpickling of
global objects on the receiving end.
.. method:: send(packet)
Send a pickled byte-string *packet* to the socket. The format of the sent
byte-string is as described in the documentation for
:meth:`~SocketHandler.makePickle`.
This function allows for partial sends, which can happen when the network
is busy.
.. method:: createSocket()
Tries to create a socket; on failure, uses an exponential back-off
algorithm. On initial failure, the handler will drop the message it was
trying to send. When subsequent messages are handled by the same
instance, it will not try connecting until some time has passed. The
default parameters are such that the initial delay is one second, and if
after that delay the connection still can't be made, the handler will
double the delay each time up to a maximum of 30 seconds.
This behaviour is controlled by the following handler attributes:
* ``retryStart`` (initial delay, defaulting to 1.0 seconds).
* ``retryFactor`` (multiplier, defaulting to 2.0).
* ``retryMax`` (maximum delay, defaulting to 30.0 seconds).
This means that if the remote listener starts up *after* the handler has
been used, you could lose messages (since the handler won't even attempt
a connection until the delay has elapsed, but just silently drop messages
during the delay period).
.. _datagram-handler:
DatagramHandler ^^^^^^^^^^^^^^^
The :class:DatagramHandler class, located in the :mod:!logging.handlers
module, inherits from :class:SocketHandler to support sending logging messages
over UDP sockets.
.. class:: DatagramHandler(host, port)
Returns a new instance of the :class:DatagramHandler class intended to
communicate with a remote machine whose address is given by host and port.
.. note:: As UDP is not a streaming protocol, there is no persistent connection between an instance of this handler and host. For this reason, when using a network socket, a DNS lookup might have to be made each time an event is logged, which can introduce some latency into the system. If this affects you, you can do a lookup yourself and initialize this handler using the looked-up IP address rather than the hostname.
.. versionchanged:: 3.4
If port is specified as None, a Unix domain socket is created
using the value in host - otherwise, a UDP socket is created.
.. method:: emit()
Pickles the record's attribute dictionary and writes it to the socket in
binary format. If there is an error with the socket, silently drops the
packet. To unpickle the record at the receiving end into a
:class:`~logging.LogRecord`, use the :func:`~logging.makeLogRecord`
function.
.. method:: makeSocket()
The factory method of :class:`SocketHandler` is here overridden to create
a UDP socket (:const:`socket.SOCK_DGRAM`).
.. method:: send(s)
Send a pickled byte-string to a socket. The format of the sent byte-string
is as described in the documentation for :meth:`SocketHandler.makePickle`.
.. _syslog-handler:
SysLogHandler ^^^^^^^^^^^^^
The :class:SysLogHandler class, located in the :mod:!logging.handlers module,
supports sending logging messages to a remote or local Unix syslog.
.. class:: SysLogHandler(address=('localhost', SYSLOG_UDP_PORT), facility=LOG_USER, socktype=socket.SOCK_DGRAM, timeout=None)
Returns a new instance of the :class:SysLogHandler class intended to
communicate with a remote Unix machine whose address is given by address in
the form of a (host, port) tuple. If address is not specified,
('localhost', 514) is used. The address is used to open a socket. An
alternative to providing a (host, port) tuple is providing an address as a
string, for example '/dev/log'. In this case, a Unix domain socket is used to
send the message to the syslog. If facility is not specified,
:const:LOG_USER is used. The type of socket opened depends on the
socktype argument, which defaults to :const:socket.SOCK_DGRAM and thus
opens a UDP socket. To open a TCP socket (for use with the newer syslog
daemons such as rsyslog), specify a value of :const:socket.SOCK_STREAM.
If timeout is specified, it sets a timeout (in seconds) for the socket operations.
This can help prevent the program from hanging indefinitely if the syslog server is
unreachable. By default, timeout is None, meaning no timeout is applied.
Note that if your server is not listening on UDP port 514,
:class:SysLogHandler may appear not to work. In that case, check what
address you should be using for a domain socket - it's system dependent.
For example, on Linux it's usually '/dev/log' but on OS/X it's
'/var/run/syslog'. You'll need to check your platform and use the
appropriate address (you may need to do this check at runtime if your
application needs to run on several platforms). On Windows, you pretty
much have to use the UDP option.
.. note:: On macOS 12.x (Monterey), Apple has changed the behaviour of their
syslog daemon - it no longer listens on a domain socket. Therefore, you cannot
expect :class:SysLogHandler to work on this system.
See :gh:`91070` for more information.
.. versionchanged:: 3.2 socktype was added.
.. versionchanged:: 3.14 timeout was added.
.. method:: close()
Closes the socket to the remote host.
.. method:: createSocket()
Tries to create a socket and, if it's not a datagram socket, connect it
to the other end. This method is called during handler initialization,
but it's not regarded as an error if the other end isn't listening at
this point - the method will be called again when emitting an event, if
there is no socket at that point.
.. versionadded:: 3.11
.. method:: emit(record)
The record is formatted, and then sent to the syslog server. If exception
information is present, it is *not* sent to the server.
.. versionchanged:: 3.2.1
(See: :issue:`12168`.) In earlier versions, the message sent to the
syslog daemons was always terminated with a NUL byte, because early
versions of these daemons expected a NUL terminated message - even
though it's not in the relevant specification (:rfc:`5424`). More recent
versions of these daemons don't expect the NUL byte but strip it off
if it's there, and even more recent daemons (which adhere more closely
to RFC 5424) pass the NUL byte on as part of the message.
To enable easier handling of syslog messages in the face of all these
differing daemon behaviours, the appending of the NUL byte has been
made configurable, through the use of a class-level attribute,
``append_nul``. This defaults to ``True`` (preserving the existing
behaviour) but can be set to ``False`` on a ``SysLogHandler`` instance
in order for that instance to *not* append the NUL terminator.
.. versionchanged:: 3.3
(See: :issue:`12419`.) In earlier versions, there was no facility for
an "ident" or "tag" prefix to identify the source of the message. This
can now be specified using a class-level attribute, defaulting to
``""`` to preserve existing behaviour, but which can be overridden on
a ``SysLogHandler`` instance in order for that instance to prepend
the ident to every message handled. Note that the provided ident must
be text, not bytes, and is prepended to the message exactly as is.
.. method:: encodePriority(facility, priority)
Encodes the facility and priority into an integer. You can pass in strings
or integers - if strings are passed, internal mapping dictionaries are
used to convert them to integers.
The symbolic ``LOG_`` values are defined in :class:`SysLogHandler` and
mirror the values defined in the ``sys/syslog.h`` header file.
**Priorities**
+--------------------------+---------------+
| Name (string) | Symbolic value|
+==========================+===============+
| ``alert`` | LOG_ALERT |
+--------------------------+---------------+
| ``crit`` or ``critical`` | LOG_CRIT |
+--------------------------+---------------+
| ``debug`` | LOG_DEBUG |
+--------------------------+---------------+
| ``emerg`` or ``panic`` | LOG_EMERG |
+--------------------------+---------------+
| ``err`` or ``error`` | LOG_ERR |
+--------------------------+---------------+
| ``info`` | LOG_INFO |
+--------------------------+---------------+
| ``notice`` | LOG_NOTICE |
+--------------------------+---------------+
| ``warn`` or ``warning`` | LOG_WARNING |
+--------------------------+---------------+
**Facilities**
+---------------+---------------+
| Name (string) | Symbolic value|
+===============+===============+
| ``auth`` | LOG_AUTH |
+---------------+---------------+
| ``authpriv`` | LOG_AUTHPRIV |
+---------------+---------------+
| ``cron`` | LOG_CRON |
+---------------+---------------+
| ``daemon`` | LOG_DAEMON |
+---------------+---------------+
| ``ftp`` | LOG_FTP |
+---------------+---------------+
| ``kern`` | LOG_KERN |
+---------------+---------------+
| ``lpr`` | LOG_LPR |
+---------------+---------------+
| ``mail`` | LOG_MAIL |
+---------------+---------------+
| ``news`` | LOG_NEWS |
+---------------+---------------+
| ``syslog`` | LOG_SYSLOG |
+---------------+---------------+
| ``user`` | LOG_USER |
+---------------+---------------+
| ``uucp`` | LOG_UUCP |
+---------------+---------------+
| ``local0`` | LOG_LOCAL0 |
+---------------+---------------+
| ``local1`` | LOG_LOCAL1 |
+---------------+---------------+
| ``local2`` | LOG_LOCAL2 |
+---------------+---------------+
| ``local3`` | LOG_LOCAL3 |
+---------------+---------------+
| ``local4`` | LOG_LOCAL4 |
+---------------+---------------+
| ``local5`` | LOG_LOCAL5 |
+---------------+---------------+
| ``local6`` | LOG_LOCAL6 |
+---------------+---------------+
| ``local7`` | LOG_LOCAL7 |
+---------------+---------------+
.. method:: mapPriority(levelname)
Maps a logging level name to a syslog priority name.
You may need to override this if you are using custom levels, or
if the default algorithm is not suitable for your needs. The
default algorithm maps ``DEBUG``, ``INFO``, ``WARNING``, ``ERROR`` and
``CRITICAL`` to the equivalent syslog names, and all other level
names to 'warning'.
.. _nt-eventlog-handler:
NTEventLogHandler ^^^^^^^^^^^^^^^^^
The :class:NTEventLogHandler class, located in the :mod:!logging.handlers
module, supports sending logging messages to a local Windows NT, Windows 2000 or
Windows XP event log. Before you can use it, you need Mark Hammond's Win32
extensions for Python installed.
.. class:: NTEventLogHandler(appname, dllname=None, logtype='Application')
Returns a new instance of the :class:NTEventLogHandler class. The appname is
used to define the application name as it appears in the event log. An
appropriate registry entry is created using this name. The dllname should give
the fully qualified pathname of a .dll or .exe which contains message
definitions to hold in the log (if not specified, 'win32service.pyd' is used
'Application', 'System' or 'Security', and
defaults to 'Application'... method:: close()
At this point, you can remove the application name from the registry as a
source of event log entries. However, if you do this, you will not be able
to see the events as you intended in the Event Log Viewer - it needs to be
able to access the registry to get the .dll name. The current version does
not do this.
.. method:: emit(record)
Determines the message ID, event category and event type, and then logs
the message in the NT event log.
.. method:: getEventCategory(record)
Returns the event category for the record. Override this if you want to
specify your own categories. This version returns 0.
.. method:: getEventType(record)
Returns the event type for the record. Override this if you want to
specify your own types. This version does a mapping using the handler's
typemap attribute, which is set up in :meth:`__init__` to a dictionary
which contains mappings for :const:`DEBUG`, :const:`INFO`,
:const:`WARNING`, :const:`ERROR` and :const:`CRITICAL`. If you are using
your own levels, you will either need to override this method or place a
suitable dictionary in the handler's *typemap* attribute.
.. method:: getMessageID(record)
Returns the message ID for the record. If you are using your own messages,
you could do this by having the *msg* passed to the logger being an ID
rather than a format string. Then, in here, you could use a dictionary
lookup to get the message ID. This version returns 1, which is the base
message ID in :file:`win32service.pyd`.
.. _smtp-handler:
SMTPHandler ^^^^^^^^^^^
The :class:SMTPHandler class, located in the :mod:!logging.handlers module,
supports sending logging messages to an email address via SMTP.
.. class:: SMTPHandler(mailhost, fromaddr, toaddrs, subject, credentials=None, secure=None, timeout=1.0)
Returns a new instance of the :class:SMTPHandler class. The instance is
initialized with the from and to addresses and subject line of the email. The
toaddrs should be a list of strings. To specify a non-standard SMTP port, use
the (host, port) tuple format for the mailhost argument. If you use a string,
the standard SMTP port is used. If your SMTP server requires authentication, you
can specify a (username, password) tuple for the credentials argument.
To specify the use of a secure protocol (TLS), pass in a tuple to the
secure argument. This will only be used when authentication credentials are
supplied. The tuple should be either an empty tuple, or a single-value tuple
with the name of a keyfile, or a 2-value tuple with the names of the keyfile
and certificate file. (This tuple is passed to the
:meth:smtplib.SMTP.starttls method.)
A timeout can be specified for communication with the SMTP server using the timeout argument.
.. versionchanged:: 3.3 Added the timeout parameter.
.. method:: emit(record)
Formats the record and sends it to the specified addressees.
.. method:: getSubject(record)
If you want to specify a subject line which is record-dependent, override
this method.
.. _memory-handler:
MemoryHandler ^^^^^^^^^^^^^
The :class:MemoryHandler class, located in the :mod:!logging.handlers module,
supports buffering of logging records in memory, periodically flushing them to a
:dfn:target handler. Flushing occurs whenever the buffer is full, or when an
event of a certain severity or greater is seen.
:class:MemoryHandler is a subclass of the more general
:class:BufferingHandler, which is an abstract class. This buffers logging
records in memory. Whenever each record is added to the buffer, a check is made
by calling :meth:shouldFlush to see if the buffer should be flushed. If it
should, then :meth:flush is expected to do the flushing.
.. class:: BufferingHandler(capacity)
Initializes the handler with a buffer of the specified capacity. Here, capacity means the number of logging records buffered.
.. method:: emit(record)
Append the record to the buffer. If :meth:`shouldFlush` returns true,
call :meth:`flush` to process the buffer.
.. method:: flush()
For a :class:`BufferingHandler` instance, flushing means that it sets the
buffer to an empty list. This method can be overwritten to implement more useful
flushing behavior.
.. method:: shouldFlush(record)
Return ``True`` if the buffer is up to capacity. This method can be
overridden to implement custom flushing strategies.
.. class:: MemoryHandler(capacity, flushLevel=ERROR, target=None, flushOnClose=True)
Returns a new instance of the :class:MemoryHandler class. The instance is
initialized with a buffer size of capacity (number of records buffered).
If flushLevel is not specified, :const:ERROR is used. If no target is
specified, the target will need to be set using :meth:setTarget before this
handler does anything useful. If flushOnClose is specified as False,
then the buffer is not flushed when the handler is closed. If not specified
or specified as True, the previous behaviour of flushing the buffer will
occur when the handler is closed.
.. versionchanged:: 3.6 The flushOnClose parameter was added.
.. method:: close()
Calls :meth:`flush`, sets the target to ``None`` and clears the
buffer.
.. method:: flush()
For a :class:`MemoryHandler` instance, flushing means just sending the buffered
records to the target, if there is one. The buffer is also cleared when
buffered records are sent to the target. Override if you want different behavior.
.. method:: setTarget(target)
Sets the target handler for this handler.
.. method:: shouldFlush(record)
Checks for buffer full or a record at the *flushLevel* or higher.
.. _http-handler:
HTTPHandler ^^^^^^^^^^^
The :class:HTTPHandler class, located in the :mod:!logging.handlers module,
supports sending logging messages to a web server, using either GET or
POST semantics.
.. class:: HTTPHandler(host, url, method='GET', secure=False, credentials=None, context=None)
Returns a new instance of the :class:HTTPHandler class. The host can be
of the form host:port, should you need to use a specific port number. If
no method is specified, GET is used. If secure is true, a HTTPS
connection will be used. The context parameter may be set to a
:class:ssl.SSLContext instance to configure the SSL settings used for the
HTTPS connection. If credentials is specified, it should be a 2-tuple
consisting of userid and password, which will be placed in a HTTP
'Authorization' header using Basic authentication. If you specify
credentials, you should also specify secure=True so that your userid and
password are not passed in cleartext across the wire.
.. versionchanged:: 3.5 The context parameter was added.
.. method:: mapLogRecord(record)
Provides a dictionary, based on ``record``, which is to be URL-encoded
and sent to the web server. The default implementation just returns
``record.__dict__``. This method can be overridden if e.g. only a
subset of :class:`~logging.LogRecord` is to be sent to the web server, or
if more specific customization of what's sent to the server is required.
.. method:: emit(record)
Sends the record to the web server as a URL-encoded dictionary. The
:meth:`mapLogRecord` method is used to convert the record to the
dictionary to be sent.
.. note:: Since preparing a record for sending it to a web server is not
the same as a generic formatting operation, using
:meth:~logging.Handler.setFormatter to specify a
:class:~logging.Formatter for a :class:HTTPHandler has no effect.
Instead of calling :meth:~logging.Handler.format, this handler calls
:meth:mapLogRecord and then :func:urllib.parse.urlencode to encode the
dictionary in a form suitable for sending to a web server.
.. _queue-handler:
QueueHandler ^^^^^^^^^^^^
.. versionadded:: 3.2
The :class:QueueHandler class, located in the :mod:!logging.handlers module,
supports sending logging messages to a queue, such as those implemented in the
:mod:queue or :mod:multiprocessing modules.
Along with the :class:QueueListener class, :class:QueueHandler can be used
to let handlers do their work on a separate thread from the one which does the
logging. This is important in web applications and also other service
applications where threads servicing clients need to respond as quickly as
possible, while any potentially slow operations (such as sending an email via
:class:SMTPHandler) are done on a separate thread.
.. class:: QueueHandler(queue)
Returns a new instance of the :class:QueueHandler class. The instance is
initialized with the queue to send messages to. The queue can be any
queue-like object; it's used as-is by the :meth:enqueue method, which
needs to know how to send messages to it. The queue is not required to
have the task tracking API, which means that you can use
:class:~queue.SimpleQueue instances for queue.
.. note:: If you are using :mod:multiprocessing, you should avoid using
:class:~queue.SimpleQueue and instead use :class:multiprocessing.Queue.
.. warning::
The :mod:`multiprocessing` module uses an internal logger created and
accessed via :meth:`~multiprocessing.get_logger`.
:class:`multiprocessing.Queue` will log ``DEBUG`` level messages upon
items being queued. If those log messages are processed by a
:class:`QueueHandler` using the same :class:`multiprocessing.Queue` instance,
it will cause a deadlock or infinite recursion.
.. method:: emit(record)
Enqueues the result of preparing the LogRecord. Should an exception
occur (e.g. because a bounded queue has filled up), the
:meth:`~logging.Handler.handleError` method is called to handle the
error. This can result in the record silently being dropped (if
:data:`logging.raiseExceptions` is ``False``) or a message printed to
``sys.stderr`` (if :data:`logging.raiseExceptions` is ``True``).
.. method:: prepare(record)
Prepares a record for queuing. The object returned by this
method is enqueued.
The base implementation formats the record to merge the message,
arguments, exception and stack information, if present. It also removes
unpickleable items from the record in-place. Specifically, it overwrites
the record's :attr:`msg` and :attr:`message` attributes with the merged
message (obtained by calling the handler's :meth:`format` method), and
sets the :attr:`args`, :attr:`exc_info` and :attr:`exc_text` attributes
to ``None``.
You might want to override this method if you want to convert
the record to a dict or JSON string, or send a modified copy
of the record while leaving the original intact.
.. note:: The base implementation formats the message with arguments, sets
the ``message`` and ``msg`` attributes to the formatted message and
sets the ``args`` and ``exc_text`` attributes to ``None`` to allow
pickling and to prevent further attempts at formatting. This means
that a handler on the :class:`QueueListener` side won't have the
information to do custom formatting, e.g. of exceptions. You may wish
to subclass ``QueueHandler`` and override this method to e.g. avoid
setting ``exc_text`` to ``None``. Note that the ``message`` / ``msg``
/ ``args`` changes are related to ensuring the record is pickleable,
and you might or might not be able to avoid doing that depending on
whether your ``args`` are pickleable. (Note that you may have to
consider not only your own code but also code in any libraries that
you use.)
.. method:: enqueue(record)
Enqueues the record on the queue using ``put_nowait()``; you may
want to override this if you want to use blocking behaviour, or a
timeout, or a customized queue implementation.
.. attribute:: listener
When created via configuration using :func:`~logging.config.dictConfig`, this
attribute will contain a :class:`QueueListener` instance for use with this
handler. Otherwise, it will be ``None``.
.. versionadded:: 3.12
.. _queue-listener:
QueueListener ^^^^^^^^^^^^^
.. versionadded:: 3.2
The :class:QueueListener class, located in the :mod:!logging.handlers
module, supports receiving logging messages from a queue, such as those
implemented in the :mod:queue or :mod:multiprocessing modules. The
messages are received from a queue in an internal thread and passed, on
the same thread, to one or more handlers for processing. While
:class:QueueListener is not itself a handler, it is documented here
because it works hand-in-hand with :class:QueueHandler.
Along with the :class:QueueHandler class, :class:QueueListener can be used
to let handlers do their work on a separate thread from the one which does the
logging. This is important in web applications and also other service
applications where threads servicing clients need to respond as quickly as
possible, while any potentially slow operations (such as sending an email via
:class:SMTPHandler) are done on a separate thread.
.. class:: QueueListener(queue, *handlers, respect_handler_level=False)
Returns a new instance of the :class:QueueListener class. The instance is
initialized with the queue to send messages to and a list of handlers which
will handle entries placed on the queue. The queue can be any queue-like
object; it's passed as-is to the :meth:dequeue method, which needs
to know how to get messages from it. The queue is not required to have the
task tracking API (though it's used if available), which means that you can
use :class:~queue.SimpleQueue instances for queue.
.. note:: If you are using :mod:multiprocessing, you should avoid using
:class:~queue.SimpleQueue and instead use :class:multiprocessing.Queue.
If respect_handler_level is True, a handler's level is respected
(compared with the level for the message) when deciding whether to pass
messages to that handler; otherwise, the behaviour is as in previous Python
versions - to always pass each message to each handler.
.. versionchanged:: 3.5
The respect_handler_level argument was added.
.. versionchanged:: 3.14
:class:QueueListener can now be used as a context manager via
:keyword:with. When entering the context, the listener is started. When
exiting the context, the listener is stopped.
:meth:~contextmanager.__enter__ returns the
:class:QueueListener object.
.. method:: dequeue(block)
Dequeues a record and return it, optionally blocking.
The base implementation uses ``get()``. You may want to override this
method if you want to use timeouts or work with custom queue
implementations.
.. method:: prepare(record)
Prepare a record for handling.
This implementation just returns the passed-in record. You may want to
override this method if you need to do any custom marshalling or
manipulation of the record before passing it to the handlers.
.. method:: handle(record)
Handle a record.
This just loops through the handlers offering them the record
to handle. The actual object passed to the handlers is that which
is returned from :meth:`prepare`.
.. method:: start()
Starts the listener.
This starts up a background thread to monitor the queue for
LogRecords to process.
.. versionchanged:: 3.14
Raises :exc:`RuntimeError` if called and the listener is already
running.
.. method:: stop()
Stops the listener.
This asks the thread to terminate, and then waits for it to do so.
Note that if you don't call this before your application exits, there
may be some records still left on the queue, which won't be processed.
.. method:: enqueue_sentinel()
Writes a sentinel to the queue to tell the listener to quit. This
implementation uses ``put_nowait()``. You may want to override this
method if you want to use timeouts or work with custom queue
implementations.
.. versionadded:: 3.3
.. seealso::
Module :mod:logging
API reference for the logging module.
Module :mod:logging.config
Configuration API for the logging module.