Updated the logging documentation for recent changes.

docs/errorhandling.rst

@@ -32,18 +32,16 @@ Error Mails
 -----------
 
 If the application runs in production mode (which it will do on your
-server) you won't see any log messages by default.  Why is that?  Flask
+server) you might not see any log messages.  The reason for that is that
-tries to be a zero-configuration framework.  Where should it drop the logs
+Flask by default will just report to the WSGI error stream or stderr
-for you if there is no configuration?  Guessing is not a good idea because
+(depending on what's available).  Where this ends up is sometimes hard to
-chances are, the place it guessed is not the place where the user has
+find.  Often it's in your webserver's log files.
-permission to create a logfile.  Also, for most small applications nobody
+
-will look at the logs anyways.
+I can pretty much promise you however that if you only use a logfile for
-
+the application errors you will never look at it except for debugging an
-In fact, I promise you right now that if you configure a logfile for the
+issue when a user reported it for you.  What you probably want instead is
-application errors you will never look at it except for debugging an issue
+a mail the second the exception happened.  Then you get an alert and you
-when a user reported it for you.  What you want instead is a mail the
+can do something about it.
-second the exception happened.  Then you get an alert and you can do
-something about it.
 
 Flask uses the Python builtin logging system, and it can actually send
 you mails for errors which is probably what you want.  Here is how you can
@@ -81,9 +79,10 @@ Logging to a File
 
 Even if you get mails, you probably also want to log warnings.  It's a
 good idea to keep as much information around that might be required to
-debug a problem.  Please note that Flask itself will not issue any
+debug a problem.  By default as of Flask 1.0, errors are logged to your
-warnings in the core system, so it's your responsibility to warn in the
+webserver's log automatically.  Warnings however are not.  Please note
-code if something seems odd.
+that Flask itself will not issue any warnings in the core system, so it's
+your responsibility to warn in the code if something seems odd.
 
 There are a couple of handlers provided by the logging system out of the
 box but not all of them are useful for basic error logging.  The most

flask/logging.py

@@ -11,8 +11,12 @@
 
 from __future__ import absolute_import
 
+import sys
+
+from werkzeug.local import LocalProxy
 from logging import getLogger, StreamHandler, Formatter, getLoggerClass, \
      DEBUG, ERROR
+from .globals import _request_ctx_stack
 
 
 PROD_LOG_FORMAT = '[%(asctime)s] %(levelname)s in %(module)s: %(message)s'
@@ -24,7 +28,19 @@ DEBUG_LOG_FORMAT = (
 )
 
 
-def should_log_for(app, mode):
+@LocalProxy
+def _proxy_stream():
+    """Finds the most appropriate error stream for the application.  If a
+    WSGI request is in flight we log to wsgi.errors, otherwise this resolves
+    to sys.stderr.
+    """
+    ctx = _request_ctx_stack.top
+    if ctx is not None:
+        return ctx.request.environ['wsgi.errors']
+    return sys.stderr
+
+
+def _should_log_for(app, mode):
     policy = app.config['LOGGER_HANDLER_POLICY']
     if policy == mode or policy == 'always':
         return True
@@ -48,19 +64,19 @@ def create_logger(app):
 
     class DebugHandler(StreamHandler):
         def emit(self, record):
-            if app.debug and should_log_for(app, 'debug'):
+            if app.debug and _should_log_for(app, 'debug'):
                 StreamHandler.emit(self, record)
 
     class ProductionHandler(StreamHandler):
         def emit(self, record):
-            if not app.debug and should_log_for(app, 'production'):
+            if not app.debug and _should_log_for(app, 'production'):
                 StreamHandler.emit(self, record)
 
     debug_handler = DebugHandler()
     debug_handler.setLevel(DEBUG)
     debug_handler.setFormatter(Formatter(DEBUG_LOG_FORMAT))
 
-    prod_handler = ProductionHandler()
+    prod_handler = ProductionHandler(_proxy_stream)
     prod_handler.setLevel(ERROR)
     prod_handler.setFormatter(Formatter(PROD_LOG_FORMAT))