Added a PROPAGATE_EXCEPTIONS flag

CHANGES

@@ -23,6 +23,10 @@ Release date to be announced, codename to be selected
   1.0 the old behaviour will continue to work but issue dependency
   warnings.
 - fixed a problem for Flask to run on jython.
+- added a `PROPAGATE_EXCEPTIONS` configuration variable that can be
+  used to flip the setting of exception propagation which previously
+  was linked to `DEBUG` alone and is now linked to either `DEBUG` or
+  `TESTING`.
 
 Version 0.6.1
 -------------

docs/config.rst

@@ -54,6 +54,11 @@ The following configuration values are used internally by Flask:
 =============================== =========================================
 ``DEBUG``                       enable/disable debug mode
 ``TESTING``                     enable/disable testing mode
+``PROPAGATE_EXCEPTIONS``        explicitly enable or disable the
+                                propagation of exceptions.  If not set or
+                                explicitly set to `None` this is
+                                implicitly true if either `TESTING` or
+                                `DEBUG` is true.
 ``SECRET_KEY``                  the secret key
 ``SESSION_COOKIE_NAME``         the name of the session cookie
 ``PERMANENT_SESSION_LIFETIME``  the lifetime of a permanent session as
@@ -96,6 +101,9 @@ The following configuration values are used internally by Flask:
 .. versionadded:: 0.6
    ``MAX_CONTENT_LENGTH``
 
+.. versionadded:: 0.7
+   ``PROPAGATE_EXCEPTIONS``
+
 Configuring from Files
 ----------------------
 

flask/app.py

@@ -189,6 +189,7 @@ class Flask(_PackageBoundObject):
     default_config = ImmutableDict({
         'DEBUG':                                False,
         'TESTING':                              False,
+        'PROPAGATE_EXCEPTIONS':                 None,
         'SECRET_KEY':                           None,
         'SESSION_COOKIE_NAME':                  'session',
         'PERMANENT_SESSION_LIFETIME':           timedelta(days=31),
@@ -303,6 +304,18 @@ class Flask(_PackageBoundObject):
         self.jinja_env = self.create_jinja_environment()
         self.init_jinja_globals()
 
+    @property
+    def propagate_exceptions(self):
+        """Returns the value of the `PROPAGATE_EXCEPTIONS` configuration
+        value in case it's set, otherwise a sensible default is returned.
+
+        .. versionadded:: 0.7
+        """
+        rv = self.config['PROPAGATE_EXCEPTIONS']
+        if rv is not None:
+            return rv
+        return self.testing or self.debug
+
     @property
     def logger(self):
         """A :class:`logging.Logger` object for this application.  The
@@ -682,7 +695,7 @@ class Flask(_PackageBoundObject):
         """
         got_request_exception.send(self, exception=e)
         handler = self.error_handlers.get(500)
-        if self.debug:
+        if self.propagate_exceptions:
             raise
         self.logger.exception('Exception on %s [%s]' % (
             request.path,

tests/flask_tests.py

@@ -15,6 +15,7 @@ import re
 import sys
 import flask
 import unittest
+from threading import Thread
 from logging import StreamHandler
 from contextlib import contextmanager
 from datetime import datetime
@@ -547,7 +548,6 @@ class BasicFunctionalityTestCase(unittest.TestCase):
                 "No ValueError exception should have been raised \"%s\"" % e
             )
 
-
     def test_test_app_proper_environ(self):
         app = flask.Flask(__name__)
         app.config.update(
@@ -619,6 +619,33 @@ class BasicFunctionalityTestCase(unittest.TestCase):
                 "No ValueError exception should have been raised \"%s\"" % e
             )
 
+    def test_exception_propagation(self):
+        def apprunner(configkey):
+            app = flask.Flask(__name__)
+            @app.route('/')
+            def index():
+                1/0
+            c = app.test_client()
+            if config_key is not None:
+                app.config[config_key] = True
+                try:
+                    resp = c.get('/')
+                except Exception:
+                    pass
+                else:
+                    self.fail('expected exception')
+            else:
+                assert c.get('/').status_code == 500
+
+        # we have to run this test in an isolated thread because if the
+        # debug flag is set to true and an exception happens the context is
+        # not torn down.  This causes other tests that run after this fail
+        # when they expect no exception on the stack.
+        for config_key in 'TESTING', 'PROPAGATE_EXCEPTIONS', 'DEBUG', None:
+            t = Thread(target=apprunner, args=(config_key,))
+            t.start()
+            t.join()
+
 
 class JSONTestCase(unittest.TestCase):