Viewing github.com/django/django-stable-4.1.x/docs/topics/logging.txt

```
.. _logging-explanation:
```
```
=======
```
```
Logging
```
```
=======
```
```
.. seealso::
```
```
    * :ref:`logging-how-to`
```

    * :ref:`Django logging reference <logging-ref>`

Python programmers will often use ``print()`` in their code as a quick and

convenient debugging tool. Using the logging framework is only a little more

effort than that, but it's much more elegant and flexible. As well as being

useful for debugging, logging can also provide you with more - and better

structured - information about the state and health of your application.

```
Overview
```
```
========
```

Django uses and extends Python's builtin :mod:`logging` module to perform

system logging. This module is discussed in detail in Python's own

documentation; this section provides a quick overview.

```
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.

```
.. _logging-security-implications:
```
```
Security implications
```
```
=====================
```

The logging system handles potentially sensitive information. For example, the

log record may contain information about a web request or a stack trace, while

some of the data you collect in your own loggers may also have security

implications. You need to be sure you know:

```
* what information is collected
```
```
* where it will subsequently be stored
```
```
* how it will be transferred
```
```
* who might have access to it.
```

To help control the collection of sensitive information, you can explicitly

designate certain sensitive information to be filtered out of error reports --

read more about how to :ref:`filter error reports <filtering-error-reports>`.

```
``AdminEmailHandler``
```
```
---------------------
```

The built-in :class:`~django.utils.log.AdminEmailHandler` deserves a mention in

the context of security. If its ``include_html`` option is enabled, the email

message it sends will contain a full traceback, with names and values of local

variables at each level of the stack, plus the values of your Django settings

(in other words, the same level of detail that is exposed in a web page when

```
:setting:`DEBUG` is ``True``).
```

It's generally not considered a good idea to send such potentially sensitive

information over email. Consider instead using one of the many third-party

services to which detailed logs can be sent to get the best of multiple worlds

-- the rich information of full tracebacks, clear management of who is notified

and has access to the information, and so on.

```
.. _configuring-logging:
```
```
Configuring logging
```
```
===================
```

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 describe 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 ``dictConfig`` default if the key is missing)

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.

To begin, here's a small configuration that will allow you to output all log

```
messages to the console:
```
```
.. code-block:: python
```
```
    :caption: ``settings.py``
```
```
    import os
```
```
    LOGGING = {
```
```
        'version': 1,
```

        'disable_existing_loggers': False,

```
        'handlers': {
```
```
            'console': {
```

                'class': 'logging.StreamHandler',

```
            },
```
```
        },
```
```
        'root': {
```
```
            'handlers': ['console'],
```
```
            'level': 'WARNING',
```
```
        },
```
```
    }
```

This configures the parent ``root`` logger to send messages with the

``WARNING`` level and higher to the console handler. By adjusting the level to

``INFO`` or ``DEBUG`` you can display more messages. This may be useful during

```
development.
```

Next we can add more fine-grained logging. Here's an example of how to make the

logging system print more messages from just the :ref:`django-logger` named

```
logger:
```
```
.. code-block:: python
```
```
    :caption: ``settings.py``
```
```
    import os
```
```
    LOGGING = {
```
```
        'version': 1,
```

        'disable_existing_loggers': False,

```
        'handlers': {
```
```
            'console': {
```

                'class': 'logging.StreamHandler',

```
            },
```
```
        },
```
```
        'root': {
```
```
            'handlers': ['console'],
```
```
            'level': 'WARNING',
```
```
        },
```
```
        'loggers': {
```
```
            'django': {
```

                'handlers': ['console'],

                'level': os.getenv('DJANGO_LOG_LEVEL', 'INFO'),

```
                'propagate': False,
```
```
            },
```
```
        },
```
```
    }
```

By default, this config sends messages from the ``django`` logger of level

``INFO`` or higher to the console. This is the same level as Django's default

logging config, except that the default config only displays log records when

``DEBUG=True``. Django does not log many such ``INFO`` level 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.

You don't have to log to the console. Here's a configuration which writes all

logging from the :ref:`django-logger` named logger to a local file:

```
.. code-block:: python
```
```
    :caption: ``settings.py``
```
```
    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.

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

```
.. code-block:: python
```
```
    :caption: ``settings.py``
```
```
    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 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:`~django.utils.log.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>`.
```

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.

Here's an example that disables Django's logging configuration and then

```
manually configures logging:
```
```
.. code-block:: python
```
```
    :caption: ``settings.py``
```
```
    LOGGING_CONFIG = None
```
```
    import logging.config
```
```
    logging.config.dictConfig(...)
```

Note that the default configuration process only calls

:setting:`LOGGING_CONFIG` once settings are fully-loaded. In contrast, manually

configuring the logging in your settings file will load your logging config

immediately. As such, your logging config must appear *after* any settings on

```
which it depends.
```