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292 lines
12 KiB
292 lines
12 KiB
# Easy to use system logging for Python's logging module.
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#
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# Author: Peter Odding <peter@peterodding.com>
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# Last Change: December 10, 2020
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# URL: https://coloredlogs.readthedocs.io
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"""
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Easy to use UNIX system logging for Python's :mod:`logging` module.
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Admittedly system logging has little to do with colored terminal output, however:
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- The `coloredlogs` package is my attempt to do Python logging right and system
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logging is an important part of that equation.
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- I've seen a surprising number of quirks and mistakes in system logging done
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in Python, for example including ``%(asctime)s`` in a format string (the
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system logging daemon is responsible for adding timestamps and thus you end
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up with duplicate timestamps that make the logs awful to read :-).
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- The ``%(programname)s`` filter originated in my system logging code and I
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wanted it in `coloredlogs` so the step to include this module wasn't that big.
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- As a bonus this Python module now has a test suite and proper documentation.
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So there :-P. Go take a look at :func:`enable_system_logging()`.
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"""
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# Standard library modules.
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import logging
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import logging.handlers
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import os
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import socket
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import sys
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# External dependencies.
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from humanfriendly import coerce_boolean
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from humanfriendly.compat import on_macos, on_windows
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# Modules included in our package.
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from coloredlogs import (
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DEFAULT_LOG_LEVEL,
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ProgramNameFilter,
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adjust_level,
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find_program_name,
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level_to_number,
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replace_handler,
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)
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LOG_DEVICE_MACOSX = '/var/run/syslog'
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"""The pathname of the log device on Mac OS X (a string)."""
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LOG_DEVICE_UNIX = '/dev/log'
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"""The pathname of the log device on Linux and most other UNIX systems (a string)."""
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DEFAULT_LOG_FORMAT = '%(programname)s[%(process)d]: %(levelname)s %(message)s'
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"""
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The default format for log messages sent to the system log (a string).
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The ``%(programname)s`` format requires :class:`~coloredlogs.ProgramNameFilter`
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but :func:`enable_system_logging()` takes care of this for you.
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The ``name[pid]:`` construct (specifically the colon) in the format allows
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rsyslogd_ to extract the ``$programname`` from each log message, which in turn
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allows configuration files in ``/etc/rsyslog.d/*.conf`` to filter these log
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messages to a separate log file (if the need arises).
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.. _rsyslogd: https://en.wikipedia.org/wiki/Rsyslog
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"""
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# Initialize a logger for this module.
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logger = logging.getLogger(__name__)
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class SystemLogging(object):
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"""Context manager to enable system logging."""
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def __init__(self, *args, **kw):
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"""
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Initialize a :class:`SystemLogging` object.
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:param args: Positional arguments to :func:`enable_system_logging()`.
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:param kw: Keyword arguments to :func:`enable_system_logging()`.
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"""
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self.args = args
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self.kw = kw
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self.handler = None
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def __enter__(self):
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"""Enable system logging when entering the context."""
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if self.handler is None:
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self.handler = enable_system_logging(*self.args, **self.kw)
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return self.handler
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def __exit__(self, exc_type=None, exc_value=None, traceback=None):
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"""
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Disable system logging when leaving the context.
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.. note:: If an exception is being handled when we leave the context a
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warning message including traceback is logged *before* system
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logging is disabled.
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"""
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if self.handler is not None:
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if exc_type is not None:
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logger.warning("Disabling system logging due to unhandled exception!", exc_info=True)
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(self.kw.get('logger') or logging.getLogger()).removeHandler(self.handler)
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self.handler = None
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def enable_system_logging(programname=None, fmt=None, logger=None, reconfigure=True, **kw):
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"""
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Redirect :mod:`logging` messages to the system log (e.g. ``/var/log/syslog``).
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:param programname: The program name to embed in log messages (a string, defaults
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to the result of :func:`~coloredlogs.find_program_name()`).
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:param fmt: The log format for system log messages (a string, defaults to
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:data:`DEFAULT_LOG_FORMAT`).
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:param logger: The logger to which the :class:`~logging.handlers.SysLogHandler`
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should be connected (defaults to the root logger).
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:param level: The logging level for the :class:`~logging.handlers.SysLogHandler`
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(defaults to :data:`.DEFAULT_LOG_LEVEL`). This value is coerced
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using :func:`~coloredlogs.level_to_number()`.
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:param reconfigure: If :data:`True` (the default) multiple calls to
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:func:`enable_system_logging()` will each override
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the previous configuration.
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:param kw: Refer to :func:`connect_to_syslog()`.
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:returns: A :class:`~logging.handlers.SysLogHandler` object or
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:data:`None`. If an existing handler is found and `reconfigure`
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is :data:`False` the existing handler object is returned. If the
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connection to the system logging daemon fails :data:`None` is
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returned.
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As of release 15.0 this function uses :func:`is_syslog_supported()` to
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check whether system logging is supported and appropriate before it's
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enabled.
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.. note:: When the logger's effective level is too restrictive it is
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relaxed (refer to `notes about log levels`_ for details).
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"""
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# Check whether system logging is supported / appropriate.
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if not is_syslog_supported():
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return None
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# Provide defaults for omitted arguments.
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programname = programname or find_program_name()
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logger = logger or logging.getLogger()
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fmt = fmt or DEFAULT_LOG_FORMAT
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level = level_to_number(kw.get('level', DEFAULT_LOG_LEVEL))
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# Check whether system logging is already enabled.
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handler, logger = replace_handler(logger, match_syslog_handler, reconfigure)
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# Make sure reconfiguration is allowed or not relevant.
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if not (handler and not reconfigure):
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# Create a system logging handler.
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handler = connect_to_syslog(**kw)
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# Make sure the handler was successfully created.
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if handler:
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# Enable the use of %(programname)s.
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ProgramNameFilter.install(handler=handler, fmt=fmt, programname=programname)
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# Connect the formatter, handler and logger.
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handler.setFormatter(logging.Formatter(fmt))
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logger.addHandler(handler)
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# Adjust the level of the selected logger.
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adjust_level(logger, level)
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return handler
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def connect_to_syslog(address=None, facility=None, level=None):
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"""
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Create a :class:`~logging.handlers.SysLogHandler`.
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:param address: The device file or network address of the system logging
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daemon (a string or tuple, defaults to the result of
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:func:`find_syslog_address()`).
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:param facility: Refer to :class:`~logging.handlers.SysLogHandler`.
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Defaults to ``LOG_USER``.
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:param level: The logging level for the :class:`~logging.handlers.SysLogHandler`
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(defaults to :data:`.DEFAULT_LOG_LEVEL`). This value is coerced
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using :func:`~coloredlogs.level_to_number()`.
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:returns: A :class:`~logging.handlers.SysLogHandler` object or :data:`None` (if the
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system logging daemon is unavailable).
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The process of connecting to the system logging daemon goes as follows:
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- The following two socket types are tried (in decreasing preference):
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1. :data:`~socket.SOCK_RAW` avoids truncation of log messages but may
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not be supported.
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2. :data:`~socket.SOCK_STREAM` (TCP) supports longer messages than the
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default (which is UDP).
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"""
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if not address:
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address = find_syslog_address()
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if facility is None:
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facility = logging.handlers.SysLogHandler.LOG_USER
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if level is None:
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level = DEFAULT_LOG_LEVEL
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for socktype in socket.SOCK_RAW, socket.SOCK_STREAM, None:
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kw = dict(facility=facility, address=address)
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if socktype is not None:
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kw['socktype'] = socktype
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try:
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handler = logging.handlers.SysLogHandler(**kw)
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except IOError:
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# IOError is a superclass of socket.error which can be raised if the system
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# logging daemon is unavailable.
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pass
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else:
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handler.setLevel(level_to_number(level))
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return handler
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def find_syslog_address():
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"""
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Find the most suitable destination for system log messages.
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:returns: The pathname of a log device (a string) or an address/port tuple as
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supported by :class:`~logging.handlers.SysLogHandler`.
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On Mac OS X this prefers :data:`LOG_DEVICE_MACOSX`, after that :data:`LOG_DEVICE_UNIX`
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is checked for existence. If both of these device files don't exist the default used
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by :class:`~logging.handlers.SysLogHandler` is returned.
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"""
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if sys.platform == 'darwin' and os.path.exists(LOG_DEVICE_MACOSX):
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return LOG_DEVICE_MACOSX
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elif os.path.exists(LOG_DEVICE_UNIX):
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return LOG_DEVICE_UNIX
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else:
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return 'localhost', logging.handlers.SYSLOG_UDP_PORT
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def is_syslog_supported():
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"""
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Determine whether system logging is supported.
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:returns:
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:data:`True` if system logging is supported and can be enabled,
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:data:`False` if system logging is not supported or there are good
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reasons for not enabling it.
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The decision making process here is as follows:
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Override
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If the environment variable ``$COLOREDLOGS_SYSLOG`` is set it is evaluated
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using :func:`~humanfriendly.coerce_boolean()` and the resulting value
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overrides the platform detection discussed below, this allows users to
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override the decision making process if they disagree / know better.
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Linux / UNIX
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On systems that are not Windows or MacOS (see below) we assume UNIX which
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means either syslog is available or sending a bunch of UDP packets to
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nowhere won't hurt anyone...
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Microsoft Windows
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Over the years I've had multiple reports of :pypi:`coloredlogs` spewing
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extremely verbose errno 10057 warning messages to the console (once for
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each log message I suppose) so I now assume it a default that
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"syslog-style system logging" is not generally available on Windows.
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Apple MacOS
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There's cPython issue `#38780`_ which seems to result in a fatal exception
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when the Python interpreter shuts down. This is (way) worse than not
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having system logging enabled. The error message mentioned in `#38780`_
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has actually been following me around for years now, see for example:
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- https://github.com/xolox/python-rotate-backups/issues/9 mentions Docker
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images implying Linux, so not strictly the same as `#38780`_.
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- https://github.com/xolox/python-npm-accel/issues/4 is definitely related
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to `#38780`_ and is what eventually prompted me to add the
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:func:`is_syslog_supported()` logic.
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.. _#38780: https://bugs.python.org/issue38780
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"""
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override = os.environ.get("COLOREDLOGS_SYSLOG")
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if override is not None:
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return coerce_boolean(override)
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else:
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return not (on_windows() or on_macos())
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def match_syslog_handler(handler):
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"""
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Identify system logging handlers.
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:param handler: The :class:`~logging.Handler` class to check.
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:returns: :data:`True` if the handler is a
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:class:`~logging.handlers.SysLogHandler`,
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:data:`False` otherwise.
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This function can be used as a callback for :func:`.find_handler()`.
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"""
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return isinstance(handler, logging.handlers.SysLogHandler)
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