Updated information on logging contextual information.

Doc/library/logging.rst

@@ -1317,6 +1317,10 @@ level of granularity you want to use in logging an application, it could
 be hard to manage if the number of :class:`Logger` instances becomes
 effectively unbounded.
 
+
+Using LoggerAdapters to impart contextual information
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
 An easy way in which you can pass contextual information to be output along
 with logging event information is to use the :class:`LoggerAdapter` class.
 This class is designed to look like a :class:`Logger`, so that you can call
@@ -1421,6 +1425,78 @@ When this script is run, the output should look something like this::
    2008-01-18 14:49:54,033 d.e.f WARNING  IP: 127.0.0.1       User: jim      A message at WARNING level with 2 parameters
 
 
+Using Filters to impart contextual information
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+You can also add contextual information to log output using a user-defined
+:class:`Filter`. ``Filter`` instances are allowed to modify the ``LogRecords``
+passed to them, including adding additional attributes which can then be output
+using a suitable format string, or if needed a custom :class:`Formatter`.
+
+For example in a web application, the request being processed (or at least,
+the interesting parts of it) can be stored in a threadlocal
+(:class:`threading.local`) variable, and then accessed from a ``Filter`` to
+add, say, information from the request - say, the remote IP address and remote
+user's username - to the ``LogRecord``, using the attribute names 'ip' and
+'user' as in the ``LoggerAdapter`` example above. In that case, the same format
+string can be used to get similar output to that shown above. Here's an example
+script::
+
+    import logging
+    from random import choice
+
+    class ContextFilter(logging.Filter):
+        """
+        This is a filter which injects contextual information into the log.
+
+        Rather than use actual contextual information, we just use random
+        data in this demo.
+        """
+
+        USERS = ['jim', 'fred', 'sheila']
+        IPS = ['123.231.231.123', '127.0.0.1', '192.168.0.1']
+
+        def filter(self, record):
+
+            record.ip = choice(ContextFilter.IPS)
+            record.user = choice(ContextFilter.USERS)
+            return True
+
+    if __name__ == "__main__":
+       levels = (logging.DEBUG, logging.INFO, logging.WARNING, logging.ERROR, logging.CRITICAL)
+       a1 = logging.LoggerAdapter(logging.getLogger("a.b.c"),
+                                  { "ip" : "123.231.231.123", "user" : "sheila" })
+       logging.basicConfig(level=logging.DEBUG,
+                           format="%(asctime)-15s %(name)-5s %(levelname)-8s IP: %(ip)-15s User: %(user)-8s %(message)s")
+       a1 = logging.getLogger("a.b.c")
+       a2 = logging.getLogger("d.e.f")
+
+       f = ContextFilter()
+       a1.addFilter(f)
+       a2.addFilter(f)
+       a1.debug("A debug message")
+       a1.info("An info message with %s", "some parameters")
+       for x in range(10):
+           lvl = choice(levels)
+           lvlname = logging.getLevelName(lvl)
+           a2.log(lvl, "A message at %s level with %d %s", lvlname, 2, "parameters")
+
+which, when run, produces something like::
+
+    2010-09-06 22:38:15,292 a.b.c DEBUG    IP: 123.231.231.123 User: fred     A debug message
+    2010-09-06 22:38:15,300 a.b.c INFO     IP: 192.168.0.1     User: sheila   An info message with some parameters
+    2010-09-06 22:38:15,300 d.e.f CRITICAL IP: 127.0.0.1       User: sheila   A message at CRITICAL level with 2 parameters
+    2010-09-06 22:38:15,300 d.e.f ERROR    IP: 127.0.0.1       User: jim      A message at ERROR level with 2 parameters
+    2010-09-06 22:38:15,300 d.e.f DEBUG    IP: 127.0.0.1       User: sheila   A message at DEBUG level with 2 parameters
+    2010-09-06 22:38:15,300 d.e.f ERROR    IP: 123.231.231.123 User: fred     A message at ERROR level with 2 parameters
+    2010-09-06 22:38:15,300 d.e.f CRITICAL IP: 192.168.0.1     User: jim      A message at CRITICAL level with 2 parameters
+    2010-09-06 22:38:15,300 d.e.f CRITICAL IP: 127.0.0.1       User: sheila   A message at CRITICAL level with 2 parameters
+    2010-09-06 22:38:15,300 d.e.f DEBUG    IP: 192.168.0.1     User: jim      A message at DEBUG level with 2 parameters
+    2010-09-06 22:38:15,301 d.e.f ERROR    IP: 127.0.0.1       User: sheila   A message at ERROR level with 2 parameters
+    2010-09-06 22:38:15,301 d.e.f DEBUG    IP: 123.231.231.123 User: fred     A message at DEBUG level with 2 parameters
+    2010-09-06 22:38:15,301 d.e.f INFO     IP: 123.231.231.123 User: fred     A message at INFO level with 2 parameters
+
+
 .. _multiple-processes:
 
 Logging to a single file from multiple processes