Merge pull request realpython#512 from tanyaschlusser/master

Logging section

Author: sigmavirus24

docs/contents.rst.inc

@@ -32,6 +32,7 @@ This part of the guide focuses on best practices for writing Python code.
    writing/reading
    writing/documentation
    writing/tests
+   writing/logging
    writing/gotchas
    writing/license
 

docs/writing/logging.rst

@@ -0,0 +1,203 @@
+Logging
+=======
+
+The :mod:`logging` module has been a part of Python's Standard Library since
+version 2.3.  It is succinctly described in :pep:`282`.  The documentation
+is notoriously hard to read, except for the `basic logging tutorial`_.
+
+Logging serves two purposes:
+
+- **Diagnostic logging** records events related to the application's
+  operation. If a user calls in to report an error, for example, the logs
+  can be searched for context.
+- **Audit logging** records events for business analysis. A user's
+  transactions can be extracted and combined with other user details for
+  reports or to optimize a business goal.
+
+
+... or Print?
+-------------
+
+The only time that ``print`` is a better option than logging is when
+the goal is to display a help statement for a command line application.
+Other reasons why logging is better than ``print``:
+
+- The `log record`_, which is created with every logging event, contains
+  readily available diagnostic information such as the file name,
+  full path, function, and line number of the logging event.
+- Events logged in included modules are automatically accessible via the
+  root logger
+  to your application's logging stream, unless you filter them out.
+- Logging can be selectively silenced by using the method
+  :meth:`logging.Logger.setLevel` or disabled by setting the attribute
+  :attr:`logging.Logger.disabled` to ``True``.
+
+
+Logging in a Library
+--------------------
+
+Notes for `configuring logging for a library`_ are in the 
+`logging tutorial`_.  Because the *user*, not the library, should
+dictate what happens when a logging event occurs, one admonition bears
+repeating:
+
+.. note::
+    It is strongly advised that you do not add any handlers other than
+    NullHandler to your library’s loggers.  
+
+
+Best practice when instantiating loggers in a library is to only create them
+using the ``__name__`` global variable: the :mod:`logging` module creates a
+hierarchy of loggers using dot notation, so using ``__name__`` ensures
+no name collisions.
+
+Here is an example of best practice from the `requests source`_ -- place
+this in your ``__init__.py``
+
+.. code-block:: python
+
+    # Set default logging handler to avoid "No handler found" warnings.
+    import logging
+    try:  # Python 2.7+
+        from logging import NullHandler
+    except ImportError:
+        class NullHandler(logging.Handler):
+            def emit(self, record):
+                pass
+
+    logging.getLogger(__name__).addHandler(NullHandler())
+
+
+
+Logging in an Application
+-------------------------
+
+The `twelve factor app <http://12factor.net>`_, an authoritative reference
+for good practice in application development, contains a section on
+`logging best practice <http://12factor.net/logs>`_. It emphatically
+advocates for treating log events as an event stream, and for
+sending that event stream to standard output to be handled by the
+application environment.
+
+
+There are at least three ways to configure a logger:
+
+- Using an INI-formatted file:
+    - **Pro**: possible to update configuration while running
+      using the function :func:`logging.config.listen` to listen
+      on a socket.
+    - **Con**: less control (*e.g.* custom subclassed filters or loggers)
+      than possible when configuring a logger in code.
+- Using a dictionary or a JSON-formatted file:
+    - **Pro**: in addition to updating while running, it is possible to
+      load from a file using the :mod:`json` module, in the standard
+      library since Python 2.6.
+    - **Con**: less control than when configuring a logger in code.
+- Using code:
+    - **Pro**: complete control over the configuration.
+    - **Con**: modifications require a change to source code.
+
+
+Example Configuration via an INI File
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Let us say the file is named ``logging_config.ini``.
+More details for the file format are in the `logging configuration`_
+section of the `logging tutorial`_.
+
+.. code-block:: ini
+
+    [loggers]
+    keys=root
+    
+    [handlers]
+    keys=stream_handler
+    
+    [formatters]
+    keys=formatter
+    
+    [logger_root]
+    level=DEBUG
+    handlers=stream_handler
+    
+    [handler_stream_handler]
+    class=StreamHandler
+    level=DEBUG
+    formatter=formatter
+    args=(sys.stderr,)
+    
+    [formatter_formatter]
+    format=%(asctime)s %(name)-12s %(levelname)-8s %(message)s
+
+
+Then use :meth:`logging.config.fileConfig` in the code:
+
+.. code-block:: python
+
+    import logging
+    from logging.config import fileConfig
+
+    fileConfig('logging_config.txt')
+    logger = logging.getLogger()
+    logger.debug('often makes a very good meal of %s', 'visiting tourists')
+    
+
+Example Configuration via a Dictionary
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+As of Python 2.7, you can use a dictionary with configuration details.
+:pep:`319` contains a list of the mandatory and optional elements in
+the configuration dictionary.
+
+.. code-block:: python
+
+    import logging
+    from logging.config import dictConfig
+
+    logging_config = dict(
+        version = 1,
+        formatters = {
+            'f': {'format':
+                  '%(asctime)s %(name)-12s %(levelname)-8s %(message)s'}
+            },
+        handlers = {
+            'h': {'class': 'logging.StreamHandler',
+                  'formatter': 'f',
+                  'level': logging.DEBUG}
+            },
+        loggers = {
+            root : {'handlers': ['h'],
+                    'level': logging.DEBUG}
+            }
+    )
+
+    dictConfig(logging_config)
+
+    logger = logging.getLogger()
+    logger.debug('often makes a very good meal of %s', 'visiting tourists')
+
+
+Example Configuration Directly in Code
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. code-block:: python
+
+    import logging
+
+    logger = logging.getLogger()
+    handler = logging.StreamHandler()
+    formatter = logging.Formatter(
+            '%(asctime)s %(name)-12s %(levelname)-8s %(message)s')
+    handler.setFormatter(formatter)
+    logger.addHandler(handler)
+    logger.setLevel(logging.DEBUG)
+
+    logger.debug('often makes a very good meal of %s', 'visiting tourists')
+
+
+.. _basic logging tutorial: http://docs.python.org/howto/logging.html#logging-basic-tutorial
+.. _logging configuration: https://docs.python.org/howto/logging.html#configuring-logging
+.. _logging tutorial: http://docs.python.org/howto/logging.html
+.. _configuring logging for a library: https://docs.python.org/howto/logging.html#configuring-logging-for-a-library
+.. _log record: https://docs.python.org/library/logging.html#logrecord-attributes
+.. _requests source: https://github.com/kennethreitz/requests