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pythongh-98731: Improvements to the logging documentation (pythonGH-1…
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Co-authored-by: Vinay Sajip <vinay_sajip@yahoo.co.uk>
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118 changes: 41 additions & 77 deletions Doc/howto/logging.rst
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Expand Up @@ -25,10 +25,12 @@ or *severity*.
When to use logging
^^^^^^^^^^^^^^^^^^^

Logging provides a set of convenience functions for simple logging usage. These
are :func:`debug`, :func:`info`, :func:`warning`, :func:`error` and
:func:`critical`. To determine when to use logging, see the table below, which
states, for each of a set of common tasks, the best tool to use for it.
You can access logging functionality by creating a logger via ``logger =
getLogger(__name__)``, and then calling the logger's :meth:`~Logger.debug`,
:meth:`~Logger.info`, :meth:`~Logger.warning`, :meth:`~Logger.error` and
:meth:`~Logger.critical` methods. To determine when to use logging, and to see
which logger methods to use when, see the table below. It states, for each of a
set of common tasks, the best tool to use for that task.

+-------------------------------------+--------------------------------------+
| Task you want to perform | The best tool for the task |
Expand All @@ -37,8 +39,8 @@ states, for each of a set of common tasks, the best tool to use for it.
| usage of a command line script or | |
| program | |
+-------------------------------------+--------------------------------------+
| Report events that occur during | :func:`logging.info` (or |
| normal operation of a program (e.g. | :func:`logging.debug` for very |
| Report events that occur during | A logger's :meth:`~Logger.info` (or |
| normal operation of a program (e.g. | :meth:`~Logger.debug` method for very|
| for status monitoring or fault | detailed output for diagnostic |
| investigation) | purposes) |
+-------------------------------------+--------------------------------------+
Expand All @@ -47,22 +49,23 @@ states, for each of a set of common tasks, the best tool to use for it.
| | the client application should be |
| | modified to eliminate the warning |
| | |
| | :func:`logging.warning` if there is |
| | nothing the client application can do|
| | about the situation, but the event |
| | should still be noted |
| | A logger's :meth:`~Logger.warning` |
| | method if there is nothing the client|
| | application can do about the |
| | situation, but the event should still|
| | be noted |
+-------------------------------------+--------------------------------------+
| Report an error regarding a | Raise an exception |
| particular runtime event | |
+-------------------------------------+--------------------------------------+
| Report suppression of an error | :func:`logging.error`, |
| without raising an exception (e.g. | :func:`logging.exception` or |
| error handler in a long-running | :func:`logging.critical` as |
| Report suppression of an error | A logger's :meth:`~Logger.error`, |
| without raising an exception (e.g. | :meth:`~Logger.exception` or |
| error handler in a long-running | :meth:`~Logger.critical` method as |
| server process) | appropriate for the specific error |
| | and application domain |
+-------------------------------------+--------------------------------------+

The logging functions are named after the level or severity of the events
The logger methods are named after the level or severity of the events
they are used to track. The standard levels and their applicability are
described below (in increasing order of severity):

Expand Down Expand Up @@ -115,12 +118,18 @@ If you type these lines into a script and run it, you'll see:
WARNING:root:Watch out!
printed out on the console. The ``INFO`` message doesn't appear because the
default level is ``WARNING``. The printed message includes the indication of
the level and the description of the event provided in the logging call, i.e.
'Watch out!'. Don't worry about the 'root' part for now: it will be explained
later. The actual output can be formatted quite flexibly if you need that;
formatting options will also be explained later.

default level is ``WARNING``. The printed message includes the indication of the
level and the description of the event provided in the logging call, i.e.
'Watch out!'. The actual output can be formatted quite flexibly if you need
that; formatting options will also be explained later.

Notice that in this example, we use functions directly on the ``logging``
module, like ``logging.debug``, rather than creating a logger and calling
functions on it. These functions operation on the root logger, but can be useful
as they will call :func:`~logging.basicConfig` for you if it has not been called yet, like in
this example. In larger programs you'll usually want to control the logging
configuration explicitly however - so for that reason as well as others, it's
better to create loggers and call their methods.

Logging to a file
^^^^^^^^^^^^^^^^^
Expand All @@ -130,11 +139,12 @@ look at that next. Be sure to try the following in a newly started Python
interpreter, and don't just continue from the session described above::

import logging
logger = logging.getLogger(__name__)
logging.basicConfig(filename='example.log', encoding='utf-8', level=logging.DEBUG)
logging.debug('This message should go to the log file')
logging.info('So should this')
logging.warning('And this, too')
logging.error('And non-ASCII stuff, too, like Øresund and Malmö')
logger.debug('This message should go to the log file')
logger.info('So should this')
logger.warning('And this, too')
logger.error('And non-ASCII stuff, too, like Øresund and Malmö')

.. versionchanged:: 3.9
The *encoding* argument was added. In earlier Python versions, or if not
Expand All @@ -148,10 +158,10 @@ messages:

.. code-block:: none
DEBUG:root:This message should go to the log file
INFO:root:So should this
WARNING:root:And this, too
ERROR:root:And non-ASCII stuff, too, like Øresund and Malmö
DEBUG:__main__:This message should go to the log file
INFO:__main__:So should this
WARNING:__main__:And this, too
ERROR:__main__:And non-ASCII stuff, too, like Øresund and Malmö
This example also shows how you can set the logging level which acts as the
threshold for tracking. In this case, because we set the threshold to
Expand Down Expand Up @@ -180,11 +190,9 @@ following example::
raise ValueError('Invalid log level: %s' % loglevel)
logging.basicConfig(level=numeric_level, ...)

The call to :func:`basicConfig` should come *before* any calls to
:func:`debug`, :func:`info`, etc. Otherwise, those functions will call
:func:`basicConfig` for you with the default options. As it's intended as a
one-off simple configuration facility, only the first call will actually do
anything: subsequent calls are effectively no-ops.
The call to :func:`basicConfig` should come *before* any calls to a logger's
methods such as :meth:`~Logger.debug`, :meth:`~Logger.info`, etc. Otherwise,
that logging event may not be handled in the desired manner.

If you run the above script several times, the messages from successive runs
are appended to the file *example.log*. If you want each run to start afresh,
Expand All @@ -197,50 +205,6 @@ The output will be the same as before, but the log file is no longer appended
to, so the messages from earlier runs are lost.


Logging from multiple modules
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

If your program consists of multiple modules, here's an example of how you
could organize logging in it::

# myapp.py
import logging
import mylib

def main():
logging.basicConfig(filename='myapp.log', level=logging.INFO)
logging.info('Started')
mylib.do_something()
logging.info('Finished')

if __name__ == '__main__':
main()

::

# mylib.py
import logging

def do_something():
logging.info('Doing something')

If you run *myapp.py*, you should see this in *myapp.log*:

.. code-block:: none
INFO:root:Started
INFO:root:Doing something
INFO:root:Finished
which is hopefully what you were expecting to see. You can generalize this to
multiple modules, using the pattern in *mylib.py*. Note that for this simple
usage pattern, you won't know, by looking in the log file, *where* in your
application your messages came from, apart from looking at the event
description. If you want to track the location of your messages, you'll need
to refer to the documentation beyond the tutorial level -- see
:ref:`logging-advanced-tutorial`.


Logging variable data
^^^^^^^^^^^^^^^^^^^^^

Expand Down
152 changes: 67 additions & 85 deletions Doc/library/logging.rst
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Expand Up @@ -30,13 +30,53 @@ is that all Python modules can participate in logging, so your application log
can include your own messages integrated with messages from third-party
modules.

The simplest example:
Here's a simple example of idiomatic usage: ::

# myapp.py
import logging
import mylib
logger = logging.getLogger(__name__)

def main():
logging.basicConfig(filename='myapp.log', level=logging.INFO)
logger.info('Started')
mylib.do_something()
logger.info('Finished')

if __name__ == '__main__':
main()

::

# mylib.py
import logging
logger = logging.getLogger(__name__)

def do_something():
logger.info('Doing something')

If you run *myapp.py*, you should see this in *myapp.log*:

.. code-block:: none
>>> import logging
>>> logging.warning('Watch out!')
WARNING:root:Watch out!
INFO:__main__:Started
INFO:mylib:Doing something
INFO:__main__:Finished
The key features of this idiomatic usage is that the majority of code is simply
creating a module level logger with ``getLogger(__name__)``, and using that
logger to do any needed logging. This is concise while allowing downstream code
fine grained control if needed. Logged messages to the module-level logger get
forwarded up to handlers of loggers in higher-level modules, all the way up to
the root logger; for this reason this approach is known as hierarchical logging.

For logging to be useful, it needs to be configured: setting the levels and
destinations for each logger, potentially changing how specific modules log,
often based on command-line arguments or application configuration. In most
cases, like the one above, only the root logger needs to be so configured, since
all the lower level loggers at module level eventually forward their messages to
its handlers. :func:`~logging.basicConfig` provides a quick way to configure
the root logger that handles many use cases.

The module provides a lot of functionality and flexibility. If you are
unfamiliar with logging, the best way to get to grips with it is to view the
Expand Down Expand Up @@ -1151,89 +1191,31 @@ functions.

.. function:: debug(msg, *args, **kwargs)

Logs a message with level :const:`DEBUG` on the root logger. The *msg* is the
message format string, and the *args* are the arguments which are merged into
*msg* using the string formatting operator. (Note that this means that you can
use keywords in the format string, together with a single dictionary argument.)

There are three keyword arguments in *kwargs* which are inspected: *exc_info*
which, if it does not evaluate as false, causes exception information to be
added to the logging message. If an exception tuple (in the format returned by
:func:`sys.exc_info`) or an exception instance is provided, it is used;
otherwise, :func:`sys.exc_info` is called to get the exception information.

The second optional keyword argument is *stack_info*, which defaults to
``False``. If true, stack information is added to the logging
message, including the actual logging call. Note that this is not the same
stack information as that displayed through specifying *exc_info*: The
former is stack frames from the bottom of the stack up to the logging call
in the current thread, whereas the latter is information about stack frames
which have been unwound, following an exception, while searching for
exception handlers.

You can specify *stack_info* independently of *exc_info*, e.g. to just show
how you got to a certain point in your code, even when no exceptions were
raised. The stack frames are printed following a header line which says:

.. code-block:: none
This is a convenience function that calls :meth:`Logger.debug`, on the root
logger. The handling of the arguments is in every way identical
to what is described in that method.

Stack (most recent call last):
The only difference is that if the root logger has no handlers, then
:func:`basicConfig` is called, prior to calling ``debug`` on the root logger.

This mimics the ``Traceback (most recent call last):`` which is used when
displaying exception frames.
For very short scripts or quick demonstrations of ``logging`` facilities,
``debug`` and the other module-level functions may be convenient. However,
most programs will want to carefully and explicitly control the logging
configuration, and should therefore prefer creating a module-level logger and
calling :meth:`Logger.debug` (or other level-specific methods) on it, as
described at the beginnning of this documentation.

The third optional keyword argument is *extra* which can be used to pass a
dictionary which is used to populate the __dict__ of the LogRecord created for
the logging event with user-defined attributes. These custom attributes can then
be used as you like. For example, they could be incorporated into logged
messages. For example::

FORMAT = '%(asctime)s %(clientip)-15s %(user)-8s %(message)s'
logging.basicConfig(format=FORMAT)
d = {'clientip': '192.168.0.1', 'user': 'fbloggs'}
logging.warning('Protocol problem: %s', 'connection reset', extra=d)

would print something like:

.. code-block:: none
2006-02-08 22:20:02,165 192.168.0.1 fbloggs Protocol problem: connection reset
The keys in the dictionary passed in *extra* should not clash with the keys used
by the logging system. (See the :class:`Formatter` documentation for more
information on which keys are used by the logging system.)

If you choose to use these attributes in logged messages, you need to exercise
some care. In the above example, for instance, the :class:`Formatter` has been
set up with a format string which expects 'clientip' and 'user' in the attribute
dictionary of the LogRecord. If these are missing, the message will not be
logged because a string formatting exception will occur. So in this case, you
always need to pass the *extra* dictionary with these keys.

While this might be annoying, this feature is intended for use in specialized
circumstances, such as multi-threaded servers where the same code executes in
many contexts, and interesting conditions which arise are dependent on this
context (such as remote client IP address and authenticated user name, in the
above example). In such circumstances, it is likely that specialized
:class:`Formatter`\ s would be used with particular :class:`Handler`\ s.

This function (as well as :func:`info`, :func:`warning`, :func:`error` and
:func:`critical`) will call :func:`basicConfig` if the root logger doesn't
have any handler attached.

.. versionchanged:: 3.2
The *stack_info* parameter was added.

.. function:: info(msg, *args, **kwargs)

Logs a message with level :const:`INFO` on the root logger. The arguments are
interpreted as for :func:`debug`.
Logs a message with level :const:`INFO` on the root logger. The arguments and behavior
are otherwise the same as for :func:`debug`.


.. function:: warning(msg, *args, **kwargs)

Logs a message with level :const:`WARNING` on the root logger. The arguments
are interpreted as for :func:`debug`.
Logs a message with level :const:`WARNING` on the root logger. The arguments and behavior
are otherwise the same as for :func:`debug`.

.. note:: There is an obsolete function ``warn`` which is functionally
identical to ``warning``. As ``warn`` is deprecated, please do not use
Expand All @@ -1246,26 +1228,26 @@ functions.

.. function:: error(msg, *args, **kwargs)

Logs a message with level :const:`ERROR` on the root logger. The arguments are
interpreted as for :func:`debug`.
Logs a message with level :const:`ERROR` on the root logger. The arguments and behavior
are otherwise the same as for :func:`debug`.


.. function:: critical(msg, *args, **kwargs)

Logs a message with level :const:`CRITICAL` on the root logger. The arguments
are interpreted as for :func:`debug`.
Logs a message with level :const:`CRITICAL` on the root logger. The arguments and behavior
are otherwise the same as for :func:`debug`.


.. function:: exception(msg, *args, **kwargs)

Logs a message with level :const:`ERROR` on the root logger. The arguments are
interpreted as for :func:`debug`. Exception info is added to the logging
Logs a message with level :const:`ERROR` on the root logger. The arguments and behavior
are otherwise the same as for :func:`debug`. Exception info is added to the logging
message. This function should only be called from an exception handler.

.. function:: log(level, msg, *args, **kwargs)

Logs a message with level *level* on the root logger. The other arguments are
interpreted as for :func:`debug`.
Logs a message with level *level* on the root logger. The arguments and behavior
are otherwise the same as for :func:`debug`.

.. function:: disable(level=CRITICAL)

Expand Down

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