Logging¶

The cast of players¶

A Python logging configuration consists of four parts:

• Loggers
• Handlers
• Filters
• Formatters

Naming loggers¶

The call to logging.getLogger() obtains (creating, if necessary) an instance of a logger. The logger instance is identified by a name. This name is used to identify the logger for configuration purposes.

By convention, the logger name is usually __name__, the name of the python module that contains the logger. This allows you to filter and handle logging calls on a per-module basis. However, if you have some other way of organizing your logging messages, you can provide any dot-separated name to identify your logger:

# Get an instance of a specific named logger
logger = logging.getLogger('project.interesting.stuff')

The dotted paths of logger names define a hierarchy. The project.interesting logger is considered to be a parent of the project.interesting.stuff logger; the project logger is a parent of the project.interesting logger.

Why is the hierarchy important? Well, because loggers can be set to propagate their logging calls to their parents. In this way, you can define a single set of handlers at the root of a logger tree, and capture all logging calls in the subtree of loggers. A logging handler defined in the project namespace will catch all logging messages issued on the project.interesting and project.interesting.stuff loggers.

This propagation can be controlled on a per-logger basis. If you don’t want a particular logger to propagate to it’s parents, you can turn off this behavior.

Making logging calls¶

The logger instance contains an entry method for each of the default log levels:

• logger.critical()
• logger.error()
• logger.warning()
• logger.info()
• logger.debug()

There are two other logging calls available:

• logger.log(): Manually emits a logging message with a specific log level.
• logger.exception(): Creates an ERROR level logging message wrapping the current exception stack frame.

An example¶

The full documentation for dictConfig format is the best source of information about logging configuration dictionaries. However, to give you a taste of what is possible, here is an example of a fairly complex logging setup, configured using logging.config.dictConfig():

LOGGING = {
    'version': 1,
    'disable_existing_loggers': True,
    'formatters': {
        'verbose': {
            'format': '%(levelname)s %(asctime)s %(module)s %(process)d %(thread)d %(message)s'
        },
        'simple': {
            'format': '%(levelname)s %(message)s'
        },
    },
    'filters': {
        'special': {
            '()': 'project.logging.SpecialFilter',
            'foo': 'bar',
        }
    },
    'handlers': {
        'null': {
            'level': 'DEBUG',
            'class': 'django.utils.log.NullHandler',
        },
        'console':{
            'level': 'DEBUG',
            'class': 'logging.StreamHandler',
            'formatter': 'simple'
        },
        'mail_admins': {
            'level': 'ERROR',
            'class': 'django.utils.log.AdminEmailHandler',
            'filters': ['special']
        }
    },
    'loggers': {
        'django': {
            'handlers': ['null'],
            'propagate': True,
            'level': 'INFO',
        },
        'django.request': {
            'handlers': ['mail_admins'],
            'level': 'ERROR',
            'propagate': False,
        },
        'myproject.custom': {
            'handlers': ['console', 'mail_admins'],
            'level': 'INFO',
            'filters': ['special']
        }
    }
}

This logging configuration does the following things:

• Identifies the configuration as being in ‘dictConfig version 1’ format. At present, this is the only dictConfig format version.

• Disables all existing logging configurations.

• Defines two formatters:

  • simple, that just outputs the log level name (e.g., DEBUG) and the log message.

    The format string is a normal Python formatting string describing the details that are to be output on each logging line. The full list of detail that can be output can be found in the formatter documentation.

  • verbose, that outputs the log level name, the log message, plus the time, process, thread and module that generate the log message.

• Defines one filter – project.logging.SpecialFilter, using the alias special. If this filter required additional arguments at time of construction, they can be provided as additional keys in the filter configuration dictionary. In this case, the argument foo will be given a value of bar when instantiating the SpecialFilter.

• Defines three handlers:

  • null, a NullHandler, which will pass any DEBUG (or higher) message to /dev/null.
  • console, a StreamHandler, which will print any DEBUG (or higher) message to stderr. This handler uses the simple output format.
  • mail_admins, an AdminEmailHandler, which will email any ERROR (or higher) message to the site admins. This handler uses the special filter.

• Configures three loggers:

  • django, which passes all messages at INFO or higher to the null handler.
  • django.request, which passes all ERROR messages to the mail_admins handler. In addition, this logger is marked to not propagate messages. This means that log messages written to django.request will not be handled by the django logger.
  • myproject.custom, which passes all messages at INFO or higher that also pass the special filter to two handlers – the console, and mail_admins. This means that all INFO level messages (or higher) will be printed to the console; ERROR and CRITICAL messages will also be output via email.

Custom logging configuration¶

If you don’t want to use Python’s dictConfig format to configure your logger, you can specify your own configuration scheme.

The LOGGING_CONFIG setting defines the callable that will be used to configure Django’s loggers. By default, it points at Python’s logging.config.dictConfig() function. However, if you want to use a different configuration process, you can use any other callable that takes a single argument. The contents of LOGGING will be provided as the value of that argument when logging is configured.

Disabling logging configuration¶

If you don’t want to configure logging at all (or you want to manually configure logging using your own approach), you can set LOGGING_CONFIG to None. This will disable the configuration process.

Loggers¶

Django provides three built-in loggers.

Handlers¶

Django provides one log handler in addition to those provided by the Python logging module.

class AdminEmailHandler([include_html=False])¶

This handler sends an email to the site admins for each log message it receives.

If the log record contains a request attribute, the full details of the request will be included in the email.

If the log record contains stack trace information, that stack trace will be included in the email.

The include_html argument of AdminEmailHandler is used to control whether the traceback email includes an HTML attachment containing the full content of the debug Web page that would have been produced if DEBUG were True. To set this value in your configuration, include it in the handler definition for django.utils.log.AdminEmailHandler, like this:

'handlers': {
    'mail_admins': {
        'level': 'ERROR',
        'class': 'django.utils.log.AdminEmailHandler',
        'include_html': True,
    }
},

Note that this HTML version of the email contains a full traceback, with names and values of local variables at each level of the stack, plus the values of your Django settings. This information is potentially very sensitive, and you may not want to send it over email. Consider using something such as Sentry to get the best of both worlds – the rich information of full tracebacks plus the security of not sending the information over email. You may also explicitly designate certain sensitive information to be filtered out of error reports – learn more on Filtering error reports.

Filters¶

Django provides two log filters in addition to those provided by the Python logging module.

class CallbackFilter(callback)¶

This filter accepts a callback function (which should accept a single argument, the record to be logged), and calls it for each record that passes through the filter. Handling of that record will not proceed if the callback returns False.

For instance, to filter out UnreadablePostError (raised when a user cancels an upload) from the admin emails, you would create a filter function:

from django.http import UnreadablePostError

def skip_unreadable_post(record):
    if record.exc_info:
        exc_type, exc_value = record.exc_info[:2]
        if isinstance(exc_value, UnreadablePostError):
            return False
    return True

and then add it to your logging config:

'filters': {
    'skip_unreadable_posts': {
        '()': 'django.utils.log.CallbackFilter',
        'callback': skip_unreadable_post,
    }
},
'handlers': {
    'mail_admins': {
        'level': 'ERROR',
        'filters': ['skip_unreadable_posts'],
        'class': 'django.utils.log.AdminEmailHandler'
    }
},

class RequireDebugFalse¶

This filter will only pass on records when settings.DEBUG is False.

This filter is used as follows in the default LOGGING configuration to ensure that the AdminEmailHandler only sends error emails to admins when DEBUG is False:

'filters': {
     'require_debug_false': {
         '()': 'django.utils.log.RequireDebugFalse',
     }
 },
 'handlers': {
     'mail_admins': {
         'level': 'ERROR',
         'filters': ['require_debug_false'],
         'class': 'django.utils.log.AdminEmailHandler'
     }
 },

class RequireDebugTrue¶

This filter is similar to RequireDebugFalse, except that records are passed only when DEBUG is True.