15.9. "logging.handlers" — Logging handlers
*******************************************


Important
^^^^^^^^^

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

* Basic Tutorial

* Advanced Tutorial

* Logging Cookbook

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

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

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.


15.9.1. 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 newline
      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.


15.9.2. 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.

   Changed in version 2.6: *delay* was added.

   close()

      Closes the file.

   emit(record)

      Outputs the record to the file.


15.9.3. NullHandler
===================

New in version 2.7.

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".


15.9.4. WatchedFileHandler
==========================

New in version 2.6.

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[, encoding[, delay]]])

   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.


15.9.5. 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=0)

   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.

   Changed in version 2.6: *delay* was added.

   doRollover()

      Does a rollover, as described above.

   emit(record)

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


15.9.6. 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)

   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()".

   Changed in version 2.6: *delay* and *utc* were added.

   doRollover()

      Does a rollover, as described above.

   emit(record)

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


15.9.7. 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*.

   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).


15.9.8. 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*.

   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.


15.9.9. 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 2.7: *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.

   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’.


15.9.10. 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".


15.9.11. 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)

   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.)

   Changed in version 2.6: *credentials* was added.

   Changed in version 2.7: *secure* 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.


15.9.12. 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.


15.9.13. 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')

   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.

   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.urlencode()" to encode the
     dictionary in a form suitable for sending to a Web server.

See also:

  Module "logging"
     API reference for the logging module.

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