Logging
=======

Flask uses standard Python :mod:`logging`. Messages about your Flask
application are logged with :meth:`app.logger <flask.Flask.logger>`,
which takes the same name as :attr:`app.name <flask.Flask.name>`. This
logger can also be used to log your own messages.

.. code-block:: python

    @app.route('/login', methods=['POST'])
    def login():
        user = get_user(request.form['username'])

        if user.check_password(request.form['password']):
            login_user(user)
            app.logger.info('%s logged in successfully', user.username)
            return redirect(url_for('index'))
        else:
            app.logger.info('%s failed to log in', user.username)
            abort(401)

If you don't configure logging, Python's default log level is usually
'warning'. Nothing below the configured level will be visible.


Basic Configuration
-------------------

When you want to configure logging for your project, you should do it as soon
as possible when the program starts. If :meth:`app.logger <flask.Flask.logger>`
is accessed before logging is configured, it will add a default handler. If
possible, configure logging before creating the application object.

This example uses :func:`~logging.config.dictConfig` to create a logging
configuration similar to Flask's default, except for all logs::

    from logging.config import dictConfig

    dictConfig({
        'version': 1,
        'formatters': {'default': {
            'format': '[%(asctime)s] %(levelname)s in %(module)s: %(message)s',
        }},
        'handlers': {'wsgi': {
            'class': 'logging.StreamHandler',
            'stream': 'ext://flask.logging.wsgi_errors_stream',
            'formatter': 'default'
        }},
        'root': {
            'level': 'INFO',
            'handlers': ['wsgi']
        }
    })

    app = Flask(__name__)


Default Configuration
`````````````````````

If you do not configure logging yourself, Flask will add a
:class:`~logging.StreamHandler` to :meth:`app.logger <flask.Flask.logger>`
automatically. During requests, it will write to the stream specified by the
WSGI server in ``environ['wsgi.errors']`` (which is usually
:data:`sys.stderr`). Outside a request, it will log to :data:`sys.stderr`.


Removing the Default Handler
````````````````````````````

If you configured logging after accessing
:meth:`app.logger <flask.Flask.logger>`, and need to remove the default
handler, you can import and remove it::

    from flask.logging import default_handler

    app.logger.removeHandler(default_handler)


Email Errors to Admins
----------------------

When running the application on a remote server for production, you probably
won't be looking at the log messages very often. The WSGI server will probably
send log messages to a file, and you'll only check that file if a user tells
you something went wrong.

To be proactive about discovering and fixing bugs, you can configure a
:class:`logging.handlers.SMTPHandler` to send an email when errors and higher
are logged. ::

    import logging
    from logging.handlers import SMTPHandler

    mail_handler = SMTPHandler(
        mailhost='127.0.0.1',
        fromaddr='server-error@example.com',
        toaddrs=['admin@example.com'],
        subject='Application Error'
    )
    mail_handler.setLevel(logging.ERROR)
    mail_handler.setFormatter(logging.Formatter(
        '[%(asctime)s] %(levelname)s in %(module)s: %(message)s'
    ))

    if not app.debug:
        app.logger.addHandler(mail_handler)

This requires that you have an SMTP server set up on the same server. See the
Python docs for more information about configuring the handler.


Injecting Request Information
-----------------------------

Seeing more information about the request, such as the IP address, may help
debugging some errors. You can subclass :class:`logging.Formatter` to inject
your own fields that can be used in messages. You can change the formatter for
Flask's default handler, the mail handler defined above, or any other
handler. ::

    from flask import has_request_context, request
    from flask.logging import default_handler

    class RequestFormatter(logging.Formatter):
        def format(self, record):
            if has_request_context():
                record.url = request.url
                record.remote_addr = request.remote_addr
            else:
                record.url = None
                record.remote_addr = None

            return super().format(record)

    formatter = RequestFormatter(
        '[%(asctime)s] %(remote_addr)s requested %(url)s\n'
        '%(levelname)s in %(module)s: %(message)s'
    )
    default_handler.setFormatter(formatter)
    mail_handler.setFormatter(formatter)


Other Libraries
---------------

Other libraries may use logging extensively, and you want to see relevant
messages from those logs too. The simplest way to do this is to add handlers
to the root logger instead of only the app logger. ::

    from flask.logging import default_handler

    root = logging.getLogger()
    root.addHandler(default_handler)
    root.addHandler(mail_handler)

Depending on your project, it may be more useful to configure each logger you
care about separately, instead of configuring only the root logger. ::

    for logger in (
        app.logger,
        logging.getLogger('sqlalchemy'),
        logging.getLogger('other_package'),
    ):
        logger.addHandler(default_handler)
        logger.addHandler(mail_handler)


Werkzeug
````````

Werkzeug logs basic request/response information to the ``'werkzeug'`` logger.
If the root logger has no handlers configured, Werkzeug adds a
:class:`~logging.StreamHandler` to its logger.


Flask Extensions
````````````````

Depending on the situation, an extension may choose to log to
:meth:`app.logger <flask.Flask.logger>` or its own named logger. Consult each
extension's documentation for details.