=======
Logging
=======

.. module:: django.utils.log
   :synopsis: Logging tools for Django applications

A quick logging primer
======================

Django uses Python's builtin :mod:`logging` module to perform system logging.
The usage of this module is discussed in detail in Python's own documentation.
However, if you've never used Python's logging framework (or even if you have),
here's a quick primer.

The cast of players
-------------------

A Python logging configuration consists of four parts:

* :ref:`topic-logging-parts-loggers`
* :ref:`topic-logging-parts-handlers`
* :ref:`topic-logging-parts-filters`
* :ref:`topic-logging-parts-formatters`

.. _topic-logging-parts-loggers:

Loggers
~~~~~~~

A logger is the entry point into the logging system. Each logger is
a named bucket to which messages can be written for processing.

A logger is configured to have a *log level*. This log level describes
the severity of the messages that the logger will handle. Python
defines the following log levels:

* ``DEBUG``: Low level system information for debugging purposes

* ``INFO``: General system information

* ``WARNING``: Information describing a minor problem that has
  occurred.

* ``ERROR``: Information describing a major problem that has
  occurred.

* ``CRITICAL``: Information describing a critical problem that has
  occurred.

Each message that is written to the logger is a *Log Record*. Each log
record also has a *log level* indicating the severity of that specific
message. A log record can also contain useful metadata that describes
the event that is being logged. This can include details such as a
stack trace or an error code.

When a message is given to the logger, the log level of the message is
compared to the log level of the logger. If the log level of the
message meets or exceeds the log level of the logger itself, the
message will undergo further processing. If it doesn't, the message
will be ignored.

Once a logger has determined that a message needs to be processed,
it is passed to a *Handler*.

.. _topic-logging-parts-handlers:

Handlers
~~~~~~~~

The handler is the engine that determines what happens to each message
in a logger. It describes a particular logging behavior, such as
writing a message to the screen, to a file, or to a network socket.

Like loggers, handlers also have a log level. If the log level of a
log record doesn't meet or exceed the level of the handler, the
handler will ignore the message.

A logger can have multiple handlers, and each handler can have a
different log level. In this way, it is possible to provide different
forms of notification depending on the importance of a message. For
example, you could install one handler that forwards ``ERROR`` and
``CRITICAL`` messages to a paging service, while a second handler
logs all messages (including ``ERROR`` and ``CRITICAL`` messages) to a
file for later analysis.

.. _topic-logging-parts-filters:

Filters
~~~~~~~

A filter is used to provide additional control over which log records
are passed from logger to handler.

By default, any log message that meets log level requirements will be
handled. However, by installing a filter, you can place additional
criteria on the logging process. For example, you could install a
filter that only allows ``ERROR`` messages from a particular source to
be emitted.

Filters can also be used to modify the logging record prior to being
emitted. For example, you could write a filter that downgrades
``ERROR`` log records to ``WARNING`` records if a particular set of
criteria are met.

Filters can be installed on loggers or on handlers; multiple filters
can be used in a chain to perform multiple filtering actions.

.. _topic-logging-parts-formatters:

Formatters
~~~~~~~~~~

Ultimately, a log record needs to be rendered as text. Formatters
describe the exact format of that text. A formatter usually consists
of a Python formatting string containing
:ref:`LogRecord attributes <python:logrecord-attributes>`; however,
you can also write custom formatters to implement specific formatting behavior.

Using logging
=============

Once you have configured your loggers, handlers, filters and
formatters, you need to place logging calls into your code. Using the
logging framework is very simple. Here's an example::

    # import the logging library
    import logging

    # Get an instance of a logger
    logger = logging.getLogger(__name__)

    def my_view(request, arg1, arg):
        ...
        if bad_mojo:
            # Log an error message
            logger.error('Something went wrong!')

And that's it! Every time the ``bad_mojo`` condition is activated, an
error log record will be written.

Naming loggers
--------------

The call to :func:`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 its 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.debug()``
* ``logger.info()``
* ``logger.warning()``
* ``logger.error()``
* ``logger.critical()``

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.

.. _configuring-logging:

Configuring logging
===================

Of course, it isn't enough to just put logging calls into your code.
You also need to configure the loggers, handlers, filters and
formatters to ensure that logging output is output in a useful way.

Python's logging library provides several techniques to configure
logging, ranging from a programmatic interface to configuration files.
By default, Django uses the :ref:`dictConfig format
<logging-config-dictschema>`.

In order to configure logging, you use :setting:`LOGGING` to define a
dictionary of logging settings. These settings describes the loggers,
handlers, filters and formatters that you want in your logging setup,
and the log levels and other properties that you want those components
to have.

By default, the :setting:`LOGGING` setting is merged with :ref:`Django's
default logging configuration <default-logging-configuration>` using the
following scheme.

If the ``disable_existing_loggers`` key in the :setting:`LOGGING` dictConfig is
set to ``True`` (which is the default) then all loggers from the default
configuration will be disabled. Disabled loggers are not the same as removed;
the logger will still exist, but will silently discard anything logged to it,
not even propagating entries to a parent logger. Thus you should be very
careful using ``'disable_existing_loggers': True``; it's probably not what you
want. Instead, you can set ``disable_existing_loggers`` to ``False`` and
redefine some or all of the default loggers; or you can set
:setting:`LOGGING_CONFIG` to ``None`` and :ref:`handle logging config yourself
<disabling-logging-configuration>`.

Logging is configured as part of the general Django ``setup()`` function.
Therefore, you can be certain that loggers are always ready for use in your
project code.

Examples
--------

The full documentation for :ref:`dictConfig format <logging-config-dictschema>`
is the best source of information about logging configuration dictionaries.
However, to give you a taste of what is possible, here are several examples.

First, here's a simple configuration which writes all logging from the
:ref:`django-logger` logger to a local file::

    LOGGING = {
        'version': 1,
        'disable_existing_loggers': False,
        'handlers': {
            'file': {
                'level': 'DEBUG',
                'class': 'logging.FileHandler',
                'filename': '/path/to/django/debug.log',
            },
        },
        'loggers': {
            'django': {
                'handlers': ['file'],
                'level': 'DEBUG',
                'propagate': True,
            },
        },
    }

If you use this example, be sure to change the ``'filename'`` path to a
location that's writable by the user that's running the Django application.

Second, here's an example of how to make the logging system print Django's
logging to the console. It may be useful during local development.

By default, this config only sends messages of level ``INFO`` or higher to the
console (same as Django's default logging config, except that the default only
displays log records when ``DEBUG=True``). Django does not log many such
messages. With this config, however, you can also set the environment variable
``DJANGO_LOG_LEVEL=DEBUG`` to see all of Django's debug logging which is very
verbose as it includes all database queries::

    import os

    LOGGING = {
        'version': 1,
        'disable_existing_loggers': False,
        'handlers': {
            'console': {
                'class': 'logging.StreamHandler',
            },
        },
        'loggers': {
            'django': {
                'handlers': ['console'],
                'level': os.getenv('DJANGO_LOG_LEVEL', 'INFO'),
            },
        },
    }

Finally, here's an example of a fairly complex logging setup::

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

* 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 :ref:`formatter-objects`.

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

* Defines two filters:

  * ``project.logging.SpecialFilter``, using the alias ``special``. If this
    filter required additional arguments, 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
    ``SpecialFilter``.

  * ``django.utils.log.RequireDebugTrue``, which passes on records when
    :setting:`DEBUG` is ``True``.

* Defines two handlers:

  * ``console``, a :class:`~logging.StreamHandler`, which prints any ``INFO``
    (or higher) message to ``sys.stderr``. This handler uses the ``simple``
    output format.

  * ``mail_admins``, an :class:`AdminEmailHandler`, which emails any ``ERROR``
    (or higher) message to the site :setting:`ADMINS`. This handler uses the
    ``special`` filter.

* Configures three loggers:

  * ``django``, which passes all messages to the ``console`` 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 :setting:`LOGGING_CONFIG` setting defines the callable that will
be used to configure Django's loggers. By default, it points at
Python's :func:`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 :setting:`LOGGING` will
be provided as the value of that argument when logging is configured.

.. _disabling-logging-configuration:

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
:setting:`LOGGING_CONFIG` to ``None``. This will disable the
configuration process for :ref:`Django's default logging
<default-logging-configuration>`. Here's an example that disables Django's
logging configuration and then manually configures logging:

.. snippet::
    :filename: settings.py

    LOGGING_CONFIG = None

    import logging.config
    logging.config.dictConfig(...)

Setting :setting:`LOGGING_CONFIG` to ``None`` only means that the automatic
configuration process is disabled, not logging itself. If you disable the
configuration process, Django will still make logging calls, falling back to
whatever default logging behavior is defined.

Django's logging extensions
===========================

Django provides a number of utilities to handle the unique
requirements of logging in Web server environment.

Loggers
-------

Django provides several built-in loggers.

.. _django-logger:

``django``
~~~~~~~~~~

The catch-all logger for messages in the  ``django`` hierarchy. No messages are
posted using this name but instead using one of the loggers below.

.. _django-request-logger:

``django.request``
~~~~~~~~~~~~~~~~~~

Log messages related to the handling of requests. 5XX responses are
raised as ``ERROR`` messages; 4XX responses are raised as ``WARNING``
messages.

Messages to this logger have the following extra context:

* ``status_code``: The HTTP response code associated with the
  request.

* ``request``: The request object that generated the logging
  message.

.. _django-server-logger:

``django.server``
~~~~~~~~~~~~~~~~~

Log messages related to the handling of requests received by the server invoked
by the :djadmin:`runserver` command. HTTP 5XX responses are logged as ``ERROR``
messages, 4XX responses are logged as ``WARNING`` messages, and everything else
is logged as ``INFO``.

Messages to this logger have the following extra context:

* ``status_code``: The HTTP response code associated with the request.

* ``request``: The request object that generated the logging message.

.. _django-template-logger:

``django.template``
~~~~~~~~~~~~~~~~~~~

Log messages related to the rendering of templates.

* Missing context variables are logged as ``DEBUG`` messages.

.. _django-db-logger:

``django.db.backends``
~~~~~~~~~~~~~~~~~~~~~~

Messages relating to the interaction of code with the database. For example,
every application-level SQL statement executed by a request is logged at the
``DEBUG`` level to this logger.

Messages to this logger have the following extra context:

* ``duration``: The time taken to execute the SQL statement.
* ``sql``: The SQL statement that was executed.
* ``params``: The parameters that were used in the SQL call.

For performance reasons, SQL logging is only enabled when
``settings.DEBUG`` is set to ``True``, regardless of the logging
level or handlers that are installed.

This logging does not include framework-level initialization (e.g.
``SET TIMEZONE``) or transaction management queries (e.g. ``BEGIN``,
``COMMIT``, and ``ROLLBACK``). Turn on query logging in your database if you
wish to view all database queries.

.. _django-security-logger:

``django.security.*``
~~~~~~~~~~~~~~~~~~~~~~

The security loggers will receive messages on any occurrence of
:exc:`~django.core.exceptions.SuspiciousOperation` and other security-related
errors. There is a sub-logger for each subtype of security error, including all
``SuspiciousOperation``\s. The level of the log event depends on where the
exception is handled.  Most occurrences are logged as a warning, while
any ``SuspiciousOperation`` that reaches the WSGI handler will be logged as an
error. For example, when an HTTP ``Host`` header is included in a request from
a client that does not match :setting:`ALLOWED_HOSTS`, Django will return a 400
response, and an error message will be logged to the
``django.security.DisallowedHost`` logger.

These log events will reach the ``django`` logger by default, which mails error
events to admins when ``DEBUG=False``. Requests resulting in a 400 response due
to a ``SuspiciousOperation`` will not be logged to the ``django.request``
logger, but only to the ``django.security`` logger.

To silence a particular type of ``SuspiciousOperation``, you can override that
specific logger following this example:

.. code-block:: python

    'handlers': {
        'null': {
            'class': 'logging.NullHandler',
        },
    },
    'loggers': {
        'django.security.DisallowedHost': {
            'handlers': ['null'],
            'propagate': False,
        },
    },

Other ``django.security`` loggers not based on ``SuspiciousOperation`` are:

* ``django.security.csrf``: For :ref:`CSRF failures <csrf-rejected-requests>`.

``django.db.backends.schema``
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Logs the SQL queries that are executed during schema changes to the database by
the :doc:`migrations framework </topics/migrations>`. Note that it won't log the
queries executed by :class:`~django.db.migrations.operations.RunPython`.
Messages to this logger have ``params`` and ``sql`` in their extra context (but
unlike ``django.db.backends``, not duration). The values have the same meaning
as explained in :ref:`django-db-logger`.

Handlers
--------

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

.. class:: AdminEmailHandler(include_html=False, email_backend=None)

    This handler sends an email to the site :setting:`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. The email subject will
    include the phrase "internal IP" if the client's IP address is in the
    :setting:`INTERNAL_IPS` setting; if not, it will include "EXTERNAL IP".

    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 :setting:`DEBUG` were ``True``. To set this value in your
    configuration, include it in the handler definition for
    ``django.utils.log.AdminEmailHandler``, like this:

    .. code-block:: python

        '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
    :ref:`Filtering error reports<filtering-error-reports>`.

    By setting the ``email_backend`` argument of ``AdminEmailHandler``, the
    :ref:`email backend <topic-email-backends>` that is being used by the
    handler can be overridden, like this:

    .. code-block:: python

        'handlers': {
            'mail_admins': {
                'level': 'ERROR',
                'class': 'django.utils.log.AdminEmailHandler',
                'email_backend': 'django.core.mail.backends.filebased.EmailBackend',
            }
        },

    By default, an instance of the email backend specified in
    :setting:`EMAIL_BACKEND` will be used.

    .. method:: send_mail(subject, message, *args, **kwargs)

        Sends emails to admin users. To customize this behavior, you can
        subclass the :class:`~django.utils.log.AdminEmailHandler` class and
        override this method.

.. _Sentry: https://pypi.python.org/pypi/sentry

Filters
-------

Django provides some 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 :exc:`~django.http.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:

    .. code-block:: python

        '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 :setting:`LOGGING`
    configuration to ensure that the :class:`AdminEmailHandler` only sends
    error emails to admins when :setting:`DEBUG` is ``False``:

    .. code-block:: python

        '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 :class:`RequireDebugFalse`, except that records are
    passed only when :setting:`DEBUG` is ``True``.

.. _default-logging-configuration:

Django's default logging configuration
======================================

By default, Django configures the following logging:

When :setting:`DEBUG` is ``True``:

* The ``django`` logger sends messages in the ``django`` hierarchy (except
  ``django.server``) at the ``INFO`` level or higher to the console.

When :setting:`DEBUG` is ``False``:

* The ``django`` logger sends messages in the ``django`` hierarchy (except
  ``django.server``)  with ``ERROR`` or ``CRITICAL`` level to
  :class:`AdminEmailHandler`.

Independent of the value of :setting:`DEBUG`:

* The :ref:`django-server-logger` logger sends messages at the ``INFO`` level
  or higher to the console.

See also :ref:`Configuring logging <configuring-logging>` to learn how you can
complement or replace this default logging configuration.
