16.8. "logging.handlers" — Logging handlers
*******************************************

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


Important
^^^^^^^^^

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

* Basic Tutorial

* Advanced Tutorial

* Logging Cookbook

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

The following useful handlers are provided in the package. Note that
three of the handlers ("StreamHandler", "FileHandler" and
"NullHandler") are actually defined in the "logging" module itself,
but have been documented here along with the other handlers.


16.8.1. StreamHandler
=====================

The "StreamHandler" class, located in the core "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
"write()" and "flush()" methods).

class logging.StreamHandler(stream=None)

   Returns a new instance of the "StreamHandler" class. If *stream* is
   specified, the instance will use it for logging output; otherwise,
   *sys.stderr* will be used.

   emit(record)

      If a formatter is specified, it is used to format the record.
      The record is then written to the stream with a terminator. If
      exception information is present, it is formatted using
      "traceback.print_exception()" and appended to the stream.

   flush()

      Flushes the stream by calling its "flush()" method. Note that
      the "close()" method is inherited from "Handler" and so does no
      output, so an explicit "flush()" call may be needed at times.

Changed in version 3.2: The "StreamHandler" class now has a
"terminator" attribute, default value "'\n'", which is used as the
terminator when writing a formatted record to a stream. If you don’t
want this newline termination, you can set the handler instance’s
"terminator" attribute to the empty string. In earlier versions, the
terminator was hardcoded as "'\n'".


16.8.2. FileHandler
===================

The "FileHandler" class, located in the core "logging" package, sends
logging output to a disk file.  It inherits the output functionality
from "StreamHandler".

class logging.FileHandler(filename, mode='a', encoding=None, delay=False)

   Returns a new instance of the "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 "emit()". By default,
   the file grows indefinitely.

   close()

      Closes the file.

   emit(record)

      Outputs the record to the file.


16.8.3. NullHandler
===================

New in version 3.1.

The "NullHandler" class, located in the core "logging" package, does
not do any formatting or output. It is essentially a ‘no-op’ handler
for use by library developers.

class logging.NullHandler

   Returns a new instance of the "NullHandler" class.

   emit(record)

      This method does nothing.

   handle(record)

      This method does nothing.

   createLock()

      This method returns "None" for the lock, since there is no
      underlying I/O to which access needs to be serialized.

See Configuring Logging for a Library for more information on how to
use "NullHandler".


16.8.4. WatchedFileHandler
==========================

The "WatchedFileHandler" class, located in the "logging.handlers"
module, is a "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;
"stat()" always returns zero for this value.

class logging.handlers.WatchedFileHandler(filename, mode='a', encoding=None, delay=False)

   Returns a new instance of the "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
   "emit()".  By default, the file grows indefinitely.

   emit(record)

      Outputs the record to the file, but first checks to see if the
      file has changed.  If it has, the existing stream is flushed and
      closed and the file opened again, before outputting the record
      to the file.


16.8.5. BaseRotatingHandler
===========================

The "BaseRotatingHandler" class, located in the "logging.handlers"
module, is the base class for the rotating file handlers,
"RotatingFileHandler" and "TimedRotatingFileHandler". You should not
need to instantiate this class, but it has attributes and methods you
may need to override.

class logging.handlers.BaseRotatingHandler(filename, mode, encoding=None, delay=False)

   The parameters are as for "FileHandler". The attributes are:

   namer

      If this attribute is set to a callable, the
      "rotation_filename()" method delegates to this callable. The
      parameters passed to the callable are those passed to
      "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.

      New in version 3.3.

   rotator

      If this attribute is set to a callable, the "rotate()" method
      delegates to this callable.  The parameters passed to the
      callable are those passed to "rotate()".

      New in version 3.3.

   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.

      Parameters:
         **default_name** – The default name for the log file.

      New in version 3.3.

   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.

      Parameters:
         * **source** – The source filename. This is normally the
           base filename, e.g. ‘test.log’.

         * **dest** – The destination filename. This is normally
           what the source is rotated to, e.g. ‘test.log.1’.

      New in version 3.3.

The reason the attributes exist is to save you having to subclass -
you can use the same callables for instances of "RotatingFileHandler"
and "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 "emit()" call, i.e. via the
"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 Using a rotator and namer to customize log
rotation processing.


16.8.6. RotatingFileHandler
===========================

The "RotatingFileHandler" class, located in the "logging.handlers"
module, supports rotation of disk log files.

class logging.handlers.RotatingFileHandler(filename, mode='a', maxBytes=0, backupCount=0, encoding=None, delay=False)

   Returns a new instance of the "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
   "emit()".  By default, the file grows indefinitely.

   You can use the *maxBytes* and *backupCount* values to allow the
   file to *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; if either of *maxBytes* or
   *backupCount* is zero, rollover never occurs.  If *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 "app.log", you would get
   "app.log", "app.log.1", "app.log.2", up to "app.log.5". The file
   being written to is always "app.log".  When this file is filled, it
   is closed and renamed to "app.log.1", and if files "app.log.1",
   "app.log.2", etc.  exist, then they are renamed to "app.log.2",
   "app.log.3" etc.  respectively.

   doRollover()

      Does a rollover, as described above.

   emit(record)

      Outputs the record to the file, catering for rollover as
      described previously.


16.8.7. TimedRotatingFileHandler
================================

The "TimedRotatingFileHandler" class, located in the
"logging.handlers" module, supports rotation of disk log files at
certain timed intervals.

class logging.handlers.TimedRotatingFileHandler(filename, when='h', interval=1, backupCount=0, encoding=None, delay=False, utc=False, atTime=None)

   Returns a new instance of the "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        |
   +==================+=========================+
   | "'S'"            | Seconds                 |
   +------------------+-------------------------+
   | "'M'"            | Minutes                 |
   +------------------+-------------------------+
   | "'H'"            | Hours                   |
   +------------------+-------------------------+
   | "'D'"            | Days                    |
   +------------------+-------------------------+
   | "'W0'-'W6'"      | Weekday (0=Monday)      |
   +------------------+-------------------------+
   | "'midnight'"     | Roll over at midnight   |
   +------------------+-------------------------+

   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 "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”.

   Changed in version 3.4: *atTime* parameter was added.

   doRollover()

      Does a rollover, as described above.

   emit(record)

      Outputs the record to the file, catering for rollover as
      described above.


16.8.8. SocketHandler
=====================

The "SocketHandler" class, located in the "logging.handlers" module,
sends logging output to a network socket. The base class uses a TCP
socket.

class logging.handlers.SocketHandler(host, port)

   Returns a new instance of the "SocketHandler" class intended to
   communicate with a remote machine whose address is given by *host*
   and *port*.

   Changed in version 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.

   close()

      Closes the socket.

   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 "LogRecord", use the "makeLogRecord()"
      function.

   handleError()

      Handles an error which has occurred during "emit()". The most
      likely cause is a lost connection. Closes the socket so that we
      can retry on the next event.

   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 ("socket.SOCK_STREAM").

   makePickle(record)

      Pickles the record’s attribute dictionary in binary format with
      a length prefix, and returns it ready for transmission across
      the socket.

      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.

   send(packet)

      Send a pickled string *packet* to the socket. This function
      allows for partial sends which can happen when the network is
      busy.

   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).


16.8.9. DatagramHandler
=======================

The "DatagramHandler" class, located in the "logging.handlers" module,
inherits from "SocketHandler" to support sending logging messages over
UDP sockets.

class logging.handlers.DatagramHandler(host, port)

   Returns a new instance of the "DatagramHandler" class intended to
   communicate with a remote machine whose address is given by *host*
   and *port*.

   Changed in version 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.

   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 "LogRecord", use the "makeLogRecord()"
      function.

   makeSocket()

      The factory method of "SocketHandler" is here overridden to
      create a UDP socket ("socket.SOCK_DGRAM").

   send(s)

      Send a pickled string to a socket.


16.8.10. SysLogHandler
======================

The "SysLogHandler" class, located in the "logging.handlers" module,
supports sending logging messages to a remote or local Unix syslog.

class logging.handlers.SysLogHandler(address=('localhost', SYSLOG_UDP_PORT), facility=LOG_USER, socktype=socket.SOCK_DGRAM)

   Returns a new instance of the "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, "LOG_USER" is used. The
   type of socket opened depends on the *socktype* argument, which
   defaults to "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 "socket.SOCK_STREAM".

   Note that if your server is not listening on UDP port 514,
   "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.

   Changed in version 3.2: *socktype* was added.

   close()

      Closes the socket to the remote host.

   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.

      Changed in version 3.2.1: (See: bpo-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.

      Changed in version 3.3: (See: bpo-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.

   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 "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      |
      +-----------------+-----------------+

   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’.


16.8.11. NTEventLogHandler
==========================

The "NTEventLogHandler" class, located in the "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 logging.handlers.NTEventLogHandler(appname, dllname=None, logtype='Application')

   Returns a new instance of the "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 - this is installed
   with the Win32 extensions and contains some basic placeholder
   message definitions. Note that use of these placeholders will make
   your event logs big, as the entire message source is held in the
   log. If you want slimmer logs, you have to pass in the name of your
   own .dll or .exe which contains the message definitions you want to
   use in the event log). The *logtype* is one of "'Application'",
   "'System'" or "'Security'", and defaults to "'Application'".

   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.

   emit(record)

      Determines the message ID, event category and event type, and
      then logs the message in the NT event log.

   getEventCategory(record)

      Returns the event category for the record. Override this if you
      want to specify your own categories. This version returns 0.

   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 "__init__()" to
      a dictionary which contains mappings for "DEBUG", "INFO",
      "WARNING", "ERROR" and "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.

   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
      "win32service.pyd".


16.8.12. SMTPHandler
====================

The "SMTPHandler" class, located in the "logging.handlers" module,
supports sending logging messages to an email address via SMTP.

class logging.handlers.SMTPHandler(mailhost, fromaddr, toaddrs, subject, credentials=None, secure=None, timeout=1.0)

   Returns a new instance of the "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 "smtplib.SMTP.starttls()" method.)

   A timeout can be specified for communication with the SMTP server
   using the *timeout* argument.

   New in version 3.3: The *timeout* argument was added.

   emit(record)

      Formats the record and sends it to the specified addressees.

   getSubject(record)

      If you want to specify a subject line which is record-dependent,
      override this method.


16.8.13. MemoryHandler
======================

The "MemoryHandler" class, located in the "logging.handlers" module,
supports buffering of logging records in memory, periodically flushing
them to a *target* handler. Flushing occurs whenever the buffer is
full, or when an event of a certain severity or greater is seen.

"MemoryHandler" is a subclass of the more general "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 "shouldFlush()" to see if the buffer should be flushed.  If it
should, then "flush()" is expected to do the flushing.

class logging.handlers.BufferingHandler(capacity)

   Initializes the handler with a buffer of the specified capacity.

   emit(record)

      Appends the record to the buffer. If "shouldFlush()" returns
      true, calls "flush()" to process the buffer.

   flush()

      You can override this to implement custom flushing behavior.
      This version just zaps the buffer to empty.

   shouldFlush(record)

      Returns true if the buffer is up to capacity. This method can be
      overridden to implement custom flushing strategies.

class logging.handlers.MemoryHandler(capacity, flushLevel=ERROR, target=None)

   Returns a new instance of the "MemoryHandler" class. The instance
   is initialized with a buffer size of *capacity*. If *flushLevel* is
   not specified, "ERROR" is used. If no *target* is specified, the
   target will need to be set using "setTarget()" before this handler
   does anything useful.

   close()

      Calls "flush()", sets the target to "None" and clears the
      buffer.

   flush()

      For a "MemoryHandler", flushing means just sending the buffered
      records to the target, if there is one. The buffer is also
      cleared when this happens. Override if you want different
      behavior.

   setTarget(target)

      Sets the target handler for this handler.

   shouldFlush(record)

      Checks for buffer full or a record at the *flushLevel* or
      higher.


16.8.14. HTTPHandler
====================

The "HTTPHandler" class, located in the "logging.handlers" module,
supports sending logging messages to a Web server, using either "GET"
or "POST" semantics.

class logging.handlers.HTTPHandler(host, url, method='GET', secure=False, credentials=None, context=None)

   Returns a new instance of the "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 "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.

   Changed in version 3.5: The *context* parameter was added.

   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 "LogRecord" is to be sent to the web
      server, or if more specific customization of what’s sent to the
      server is required.

   emit(record)

      Sends the record to the Web server as a URL-encoded dictionary.
      The "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
     "setFormatter()" to specify a "Formatter" for a "HTTPHandler" has
     no effect. Instead of calling "format()", this handler calls
     "mapLogRecord()" and then "urllib.parse.urlencode()" to encode
     the dictionary in a form suitable for sending to a Web server.


16.8.15. QueueHandler
=====================

New in version 3.2.

The "QueueHandler" class, located in the "logging.handlers" module,
supports sending logging messages to a queue, such as those
implemented in the "queue" or "multiprocessing" modules.

Along with the "QueueListener" 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 "SMTPHandler") are done on a separate thread.

class logging.handlers.QueueHandler(queue)

   Returns a new instance of the "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 "enqueue()" method,
   which needs to know how to send messages to it.

   emit(record)

      Enqueues the result of preparing the LogRecord.

   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
      and arguments, and removes unpickleable items from the record
      in-place.

      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.

   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.


16.8.16. QueueListener
======================

New in version 3.2.

The "QueueListener" class, located in the "logging.handlers" module,
supports receiving logging messages from a queue, such as those
implemented in the "queue" or "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
"QueueListener" is not itself a handler, it is documented here because
it works hand-in-hand with "QueueHandler".

Along with the "QueueHandler" 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 "SMTPHandler") are done on a separate thread.

class logging.handlers.QueueListener(queue, *handlers, respect_handler_level=False)

   Returns a new instance of the "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 "dequeue()"
   method, which needs to know how to get messages from it. 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.

   Changed in version 3.5: The "respect_handler_levels" argument was
   added.

   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.

   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.

   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 "prepare()".

   start()

      Starts the listener.

      This starts up a background thread to monitor the queue for
      LogRecords to process.

   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.

   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.

      New in version 3.3.

See also:

  Module "logging"
     API reference for the logging module.

  Module "logging.config"
     Configuration API for the logging module.
