[doc] Update logging cookbook with an example of custom handling of levels. (GH-98290)

Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>

Doc/howto/logging-cookbook.rst

@@ -276,6 +276,211 @@ choose a different directory name for the log - just ensure that the directory e
 and that you have the permissions to create and update files in it.
 
 
+.. _custom-level-handling:
+
+Custom handling of levels
+-------------------------
+
+Sometimes, you might want to do something slightly different from the standard
+handling of levels in handlers, where all levels above a threshold get
+processed by a handler. To do this, you need to use filters. Let's look at a
+scenario where you want to arrange things as follows:
+
+* Send messages of severity ``INFO`` and ``WARNING`` to ``sys.stdout``
+* Send messages of severity ``ERROR`` and above to ``sys.stderr``
+* Send messages of severity ``DEBUG`` and above to file ``app.log``
+
+Suppose you configure logging with the following JSON:
+
+.. code-block:: json
+
+    {
+        "version": 1,
+        "disable_existing_loggers": false,
+        "formatters": {
+            "simple": {
+                "format": "%(levelname)-8s - %(message)s"
+            }
+        },
+        "handlers": {
+            "stdout": {
+                "class": "logging.StreamHandler",
+                "level": "INFO",
+                "formatter": "simple",
+                "stream": "ext://sys.stdout",
+            },
+            "stderr": {
+                "class": "logging.StreamHandler",
+                "level": "ERROR",
+                "formatter": "simple",
+                "stream": "ext://sys.stderr"
+            },
+            "file": {
+                "class": "logging.FileHandler",
+                "formatter": "simple",
+                "filename": "app.log",
+                "mode": "w"
+            }
+        },
+        "root": {
+            "level": "DEBUG",
+            "handlers": [
+                "stderr",
+                "stdout",
+                "file"
+            ]
+        }
+    }
+
+This configuration does *almost* what we want, except that ``sys.stdout`` would
+show messages of severity ``ERROR`` and above as well as ``INFO`` and
+``WARNING`` messages. To prevent this, we can set up a filter which excludes
+those messages and add it to the relevant handler. This can be configured by
+adding a ``filters`` section parallel to ``formatters`` and ``handlers``:
+
+.. code-block:: json
+
+    "filters": {
+        "warnings_and_below": {
+            "()" : "__main__.filter_maker",
+            "level": "WARNING"
+        }
+    }
+
+and changing the section on the ``stdout`` handler to add it:
+
+.. code-block:: json
+
+    "stdout": {
+        "class": "logging.StreamHandler",
+        "level": "INFO",
+        "formatter": "simple",
+        "stream": "ext://sys.stdout",
+        "filters": ["warnings_and_below"]
+    }
+
+A filter is just a function, so we can define the ``filter_maker`` (a factory
+function) as follows:
+
+.. code-block:: python
+
+    def filter_maker(level):
+        level = getattr(logging, level)
+
+        def filter(record):
+            return record.levelno <= level
+
+        return filter
+
+This converts the string argument passed in to a numeric level, and returns a
+function which only returns ``True`` if the level of the passed in record is
+at or below the specified level. Note that in this example I have defined the
+``filter_maker`` in a test script ``main.py`` that I run from the command line,
+so its module will be ``__main__`` - hence the ``__main__.filter_maker`` in the
+filter configuration. You will need to change that if you define it in a
+different module.
+
+With the filter added, we can run ``main.py``, which in full is:
+
+.. code-block:: python
+
+    import json
+    import logging
+    import logging.config
+
+    CONFIG = '''
+    {
+        "version": 1,
+        "disable_existing_loggers": false,
+        "formatters": {
+            "simple": {
+                "format": "%(levelname)-8s - %(message)s"
+            }
+        },
+        "filters": {
+            "warnings_and_below": {
+                "()" : "__main__.filter_maker",
+                "level": "WARNING"
+            }
+        },
+        "handlers": {
+            "stdout": {
+                "class": "logging.StreamHandler",
+                "level": "INFO",
+                "formatter": "simple",
+                "stream": "ext://sys.stdout",
+                "filters": ["warnings_and_below"]
+            },
+            "stderr": {
+                "class": "logging.StreamHandler",
+                "level": "ERROR",
+                "formatter": "simple",
+                "stream": "ext://sys.stderr"
+            },
+            "file": {
+                "class": "logging.FileHandler",
+                "formatter": "simple",
+                "filename": "app.log",
+                "mode": "w"
+            }
+        },
+        "root": {
+            "level": "DEBUG",
+            "handlers": [
+                "stderr",
+                "stdout",
+                "file"
+            ]
+        }
+    }
+    '''
+
+    def filter_maker(level):
+        level = getattr(logging, level)
+
+        def filter(record):
+            return record.levelno <= level
+
+        return filter
+
+    logging.config.dictConfig(json.loads(CONFIG))
+    logging.debug('A DEBUG message')
+    logging.info('An INFO message')
+    logging.warning('A WARNING message')
+    logging.error('An ERROR message')
+    logging.critical('A CRITICAL message')
+
+And after running it like this:
+
+.. code-block:: shell
+
+    python main.py 2>stderr.log >stdout.log
+
+We can see the results are as expected:
+
+.. code-block:: shell
+
+    $ more *.log
+    ::::::::::::::
+    app.log
+    ::::::::::::::
+    DEBUG    - A DEBUG message
+    INFO     - An INFO message
+    WARNING  - A WARNING message
+    ERROR    - An ERROR message
+    CRITICAL - A CRITICAL message
+    ::::::::::::::
+    stderr.log
+    ::::::::::::::
+    ERROR    - An ERROR message
+    CRITICAL - A CRITICAL message
+    ::::::::::::::
+    stdout.log
+    ::::::::::::::
+    INFO     - An INFO message
+    WARNING  - A WARNING message
+
+
 Configuration server example
 ----------------------------
 
@@ -3503,7 +3708,7 @@ instance). Then, you'd get this kind of result:
     WARNING:demo:Bar
     >>>
 
-Of course, these above examples show output according to the format used by
+Of course, the examples above show output according to the format used by
 :func:`~logging.basicConfig`, but you can use a different formatter when you
 configure logging.
 
@@ -3517,7 +3722,6 @@ need to do or deal with, it is worth mentioning some usage patterns which are
 *unhelpful*, and which should therefore be avoided in most cases. The following
 sections are in no particular order.
 
-
 Opening the same log file multiple times
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
@@ -3566,7 +3770,6 @@ that in other languages such as Java and C#, loggers are often static class
 attributes. However, this pattern doesn't make sense in Python, where the
 module (and not the class) is the unit of software decomposition.
 
-
 Adding handlers other than :class:`NullHandler` to a logger in a library
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
@@ -3575,7 +3778,6 @@ responsibility of the application developer, not the library developer. If you
 are maintaining a library, ensure that you don't add handlers to any of your
 loggers other than a :class:`~logging.NullHandler` instance.
 
-
 Creating a lot of loggers
 ^^^^^^^^^^^^^^^^^^^^^^^^^