Clear exceptions when pushing a context

Fixes #882

CHANGES

@@ -12,6 +12,8 @@ Version 0.10.2
 - Raise an :exc:`AttributeError` in :func:`flask.helpers.find_package` with a
   useful message explaining why it is raised when a PEP 302 import hook is used
   without an `is_package()` method.
+- Fixed an issue causing exceptions raised before entering a request or app
+  context to be passed to teardown handlers.
 
 Version 0.10.1
 --------------

flask/ctx.py

@@ -163,6 +163,8 @@ class AppContext(object):
     def push(self):
         """Binds the app context to the current context."""
         self._refcnt += 1
+        if hasattr(sys, 'exc_clear'):
+            sys.exc_clear()
         _app_ctx_stack.push(self)
         appcontext_pushed.send(self.app)
 
@@ -312,6 +314,9 @@ class RequestContext(object):
         else:
             self._implicit_app_ctx_stack.append(None)
 
+        if hasattr(sys, 'exc_clear'):
+            sys.exc_clear()
+
         _request_ctx_stack.push(self)
 
         # Open the session at the moment that the request context is

flask/testsuite/appctx.py

@@ -63,6 +63,23 @@ class AppContextTestCase(FlaskTestCase):
 
         self.assert_equal(cleanup_stuff, [None])
 
+    def test_app_tearing_down_with_previous_exception(self):
+        cleanup_stuff = []
+        app = flask.Flask(__name__)
+        @app.teardown_appcontext
+        def cleanup(exception):
+            cleanup_stuff.append(exception)
+
+        try:
+            raise Exception('dummy')
+        except Exception:
+            pass
+
+        with app.app_context():
+            pass
+
+        self.assert_equal(cleanup_stuff, [None])
+
     def test_custom_app_ctx_globals_class(self):
         class CustomRequestGlobals(object):
             def __init__(self):

flask/testsuite/reqctx.py

@@ -33,6 +33,22 @@ class RequestContextTestCase(FlaskTestCase):
         ctx.pop()
         self.assert_equal(buffer, [None])
 
+    def test_teardown_with_previous_exception(self):
+        buffer = []
+        app = flask.Flask(__name__)
+        @app.teardown_request
+        def end_of_request(exception):
+            buffer.append(exception)
+
+        try:
+            raise Exception('dummy')
+        except Exception:
+            pass
+
+        with app.test_request_context():
+            self.assert_equal(buffer, [])
+        self.assert_equal(buffer, [None])
+
     def test_proper_test_request_context(self):
         app = flask.Flask(__name__)
         app.config.update(