Added cookbook entry on logging filter configuration using dictConfig().

Doc/howto/logging-cookbook.rst

@@ -1694,6 +1694,9 @@ When the above script is run, it prints::
 Note that the order of items might be different according to the version of
 Python used.
 
+
+.. _custom-handlers:
+
 .. currentmodule:: logging.config
 
 Customizing handlers with :func:`dictConfig`
@@ -1948,3 +1951,87 @@ handler. So the only slightly unusual thing which might trip you up is that the
 parentheses go around the format string and the arguments, not just the format
 string. That’s because the __ notation is just syntax sugar for a constructor
 call to one of the ``XXXMessage`` classes shown above.
+
+
+.. _filters-dictconfig:
+
+.. currentmodule:: logging.config
+
+Configuring filters with :func:`dictConfig`
+-------------------------------------------
+
+You *can* configure filters using :func:`~logging.config.dictConfig`, though it
+might not be obvious at first glance how to do it (hence this recipe). Since
+:class:`~logging.Filter` is the only filter class included in the standard
+library, and it is unlikely to cater to many requirements (it's only there as a
+base class), you will typically need to define your own :class:`~logging.Filter`
+subclass with an overridden :meth:`~logging.Filter.filter` method. To do this,
+specify the ``()`` key in the configuration dictionary for the filter,
+specifying a callable which will be used to create the filter (a class is the
+most obvious, but you can provide any callable which returns a
+:class:`~logging.Filter` instance). Here is a complete example::
+
+    import logging
+    import logging.config
+    import sys
+
+    class MyFilter(logging.Filter):
+        def __init__(self, param=None):
+            self.param = param
+
+        def filter(self, record):
+            if self.param is None:
+                allow = True
+            else:
+                allow = self.param not in record.msg
+            if allow:
+                record.msg = 'changed: ' + record.msg
+            return allow
+
+    LOGGING = {
+        'version': 1,
+        'filters': {
+            'myfilter': {
+                '()': MyFilter,
+                'param': 'noshow',
+            }
+        },
+        'handlers': {
+            'console': {
+                'class': 'logging.StreamHandler',
+                'filters': ['myfilter']
+            }
+        },
+        'root': {
+            'level': 'DEBUG',
+            'handlers': ['console']
+        },
+    }
+
+    if __name__ == '__main__':
+        logging.config.dictConfig(LOGGING)
+        logging.debug('hello')
+        logging.debug('hello - noshow')
+
+This example shows how you can pass configuration data to the callable which
+constructs the instance, in the form of keyword parameters. When run, the above
+script will print::
+
+    changed: hello
+
+which shows that the filter is working as configured.
+
+A couple of extra points to note:
+
+* If you can't refer to the callable directly in the configuration (e.g. if it
+  lives in a different module, and you can't import it directly where the
+  configuration dictionary is), you can use the form ``ext://...`` as described
+  in :ref:`logging-config-dict-externalobj`. For example, you could have used
+  the text ``'ext://__main__.MyFilter'`` instead of ``MyFilter`` in the above
+  example.
+
+* As well as for filters, this technique can also be used to configure custom
+  handlers and formatters. See :ref:`logging-config-dict-userdef` for more
+  information on how logging supports using user-defined objects in its
+  configuration, and see the other cookbook recipe :ref:`custom-handlers` above.
+