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