A little more work on python logging

-- Changed the initialization code for WeaveStack so that a default log handler is only
installed if no other handler is in place at any point in the logger hierarchy. Now, if you
initialize the root logger prior to initializing WeaveStack, openweave log output will
naturally go to the root handler, whereas if you don’t, output will go to stdout by default.

Author: Jay Logue

src/device-manager/python/openweave/WeaveStack.py

@@ -145,7 +145,7 @@ def formatTime(self, record, datefmt=None):
 
 @_singleton
 class WeaveStack(object):
-    def __init__(self):
+    def __init__(self, installDefaultLogHandler=True):
         self.networkLock = Lock()
         self.completeEvent = Event()
         self._weaveStackLib = None
@@ -157,28 +157,49 @@ def __init__(self):
         # Locate and load the openweave shared library.
         self._loadLib()
 
-        # By default, configure the openweave library to log to a python logger object with the
-        # name 'openweave.WeaveStack'.  If this logger has not already been initialized by
-        # the application, automatically configure it to log to stdout. 
+        # Arrange to log output from the openweave library to a python logger object with the
+        # name 'openweave.WeaveStack'.  If desired, applications can override this behavior by
+        # setting self.logger to a different python logger object, or by calling setLogFunct()
+        # with their own logging function.
+        self.logger = logging.getLogger(__name__)
+        self.setLogFunct(self.defaultLogFunct)
+
+        # Determine if there are already handlers installed for the logger.  Python 3.5+
+        # has a method for this; on older versions the check has to be done manually.  
+        if hasattr(self.logger, 'hasHandlers'):
+            hasHandlers = self.logger.hasHandlers()
+        else:
+            hasHandlers = False
+            logger = self.logger
+            while logger is not None:
+                if len(logger.handlers) > 0:
+                    hasHandlers = True
+                    break
+                if not logger.propagate:
+                    break
+                logger = logger.parent
+
+        # If a logging handler has not already been initialized for 'openweave.WeaveStack',
+        # or any one of its parent loggers, automatically configure a handler to log to
+        # stdout.  This maintains compatibility with a number of applications which expect
+        # openweave log output to go to stdout by default.
         #
-        # Applications can override this behavior in any one the following ways:
-        #     - Setting self.logger to a different python logger object.
-        #     - Replacing the StreamHandler on self.logger with a different handler object.
-        #     - Setting a different Formatter object on the existing StreamHandler object.
-        #     - Reconfiguring the existing LogMessageFormatter object, e.g. to enable logging
-        #       of timestamps.
-        #     - Configuring openwave to call an application-specific logging function by
+        # This behavior can be overridden in a variety of ways:
+        #     - Initialize a different log handler before WeaveStack is initialized.
+        #     - Pass installDefaultLogHandler=False when initializing WeaveStack.
+        #     - Replace the StreamHandler on self.logger with a different handler object.
+        #     - Set a different Formatter object on the existing StreamHandler object.
+        #     - Reconfigure the existing WeaveLogFormatter object.
+        #     - Configure openweave to call an application-specific logging function by
         #       calling self.setLogFunct().
-        #     - Calling self.setLogFunct(None), which will configure the openweave library
+        #     - Call self.setLogFunct(None), which will configure the openweave library
         #       to log directly to stdout, bypassing python altogether.
         #
-        self.logger = logging.getLogger(__name__)
-        if len(self.logger.handlers) == 0:
+        if installDefaultLogHandler and not hasHandlers:
             logHandler = logging.StreamHandler(stream=sys.stdout)
             logHandler.setFormatter(WeaveLogFormatter())
             self.logger.addHandler(logHandler)
             self.logger.setLevel(logging.DEBUG)
-        self.setLogFunct(self.defaultLogFunct)
 
         def HandleComplete(appState, reqState):
             self.callbackRes = True