Added HTTP exception trapping. This should fix #294

CHANGES

@@ -14,6 +14,8 @@ Relase date to be decided, codename to be chosen.
 - Empty session cookies are now deleted properly automatically.
 - View functions can now opt out of getting the automatic
   OPTIONS implementation.
+- HTTP exceptions and Bad Request Key Errors can now be trapped so that they
+  show up normally in the traceback.
 
 Version 0.7.3
 -------------

docs/config.rst

@@ -81,6 +81,23 @@ The following configuration values are used internally by Flask:
                                   reject incoming requests with a
                                   content length greater than this by
                                   returning a 413 status code.
+``TRAP_HTTP_EXCEPTIONS``          If this is set to ``True`` Flask will
+                                  not execute the error handlers of HTTP
+                                  exceptions but instead treat the
+                                  exception like any other and bubble it
+                                  through the exception stack.  This is
+                                  helpful for hairy debugging situations
+                                  where you have to find out where an HTTP
+                                  exception is coming from.
+``TRAP_BAD_REQUEST_KEY_ERRORS``   Werkzeug's internal data structures that
+                                  deal with request specific data will
+                                  raise special key errors that are also
+                                  bad request exceptions.  By default
+                                  these will be converted into 400
+                                  responses which however can make
+                                  debugging some issues harder.  If this
+                                  config is set to ``True`` you will get
+                                  a regular traceback instead.
 ================================= =========================================
 
 .. admonition:: More on ``SERVER_NAME``
@@ -114,6 +131,9 @@ The following configuration values are used internally by Flask:
 .. versionadded:: 0.7
    ``PROPAGATE_EXCEPTIONS``, ``PRESERVE_CONTEXT_ON_EXCEPTION``
 
+.. versionadded:: 0.8
+   ``TRAP_BAD_REQUEST_KEY_ERRORS``, ``TRAP_HTTP_EXCEPTIONS``
+
 Configuring from Files
 ----------------------
 

flask/app.py

@@ -19,7 +19,7 @@ from itertools import chain
 from werkzeug.datastructures import ImmutableDict
 from werkzeug.routing import Map, Rule
 from werkzeug.exceptions import HTTPException, InternalServerError, \
-     MethodNotAllowed
+     MethodNotAllowed, BadRequest
 
 from .helpers import _PackageBoundObject, url_for, get_flashed_messages, \
     locked_cached_property, _tojson_filter, _endpoint_from_view_func
@@ -197,7 +197,9 @@ class Flask(_PackageBoundObject):
         'USE_X_SENDFILE':                       False,
         'LOGGER_NAME':                          None,
         'SERVER_NAME':                          None,
-        'MAX_CONTENT_LENGTH':                   None
+        'MAX_CONTENT_LENGTH':                   None,
+        'TRAP_BAD_REQUEST_KEY_ERRORS':          False,
+        'TRAP_HTTP_EXCEPTIONS':                 False
     })
 
     #: The rule object to use for URL rules created.  This is used by
@@ -983,6 +985,24 @@ class Flask(_PackageBoundObject):
             return e
         return handler(e)
 
+    def trap_http_exception(self, e):
+        """Checks if an HTTP exception should be trapped or not.  By default
+        this will return `False` for all exceptions except for a bad request
+        key error if ``TRAP_BAD_REQUEST_KEY_ERRORS`` is set to `True`.  It
+        also returns `True` if ``TRAP_HTTP_EXCEPTIONS`` is set to `True`.
+
+        This is called for all HTTP exceptions raised by a view function.
+        If it returns `True` for any exception the error handler for this
+        exception is not called and it shows up as regular exception in the
+        traceback.  This is helpful for debugging implicitly raised HTTP
+        exceptions.
+        """
+        if self.config['TRAP_HTTP_EXCEPTIONS']:
+            return True
+        if self.config['TRAP_BAD_REQUEST_KEY_ERRORS']:
+            return isinstance(e, BadRequest) and isinstance(e, LookupError)
+        return False
+
     def handle_user_exception(self, e):
         """This method is called whenever an exception occurs that should be
         handled.  A special case are
@@ -993,14 +1013,16 @@ class Flask(_PackageBoundObject):
 
         .. versionadded:: 0.7
         """
-        # ensure not to trash sys.exc_info() at that point in case someone
-        # wants the traceback preserved in handle_http_exception.
-        if isinstance(e, HTTPException):
-            return self.handle_http_exception(e)
-
         exc_type, exc_value, tb = sys.exc_info()
         assert exc_value is e
 
+        # ensure not to trash sys.exc_info() at that point in case someone
+        # wants the traceback preserved in handle_http_exception.  Of course
+        # we cannot prevent users from trashing it themselves in a custom
+        # trap_http_exception method so that's their fault then.
+        if isinstance(e, HTTPException) and not self.trap_http_exception(e):
+            return self.handle_http_exception(e)
+
         blueprint_handlers = ()
         handlers = self.error_handler_spec.get(request.blueprint)
         if handlers is not None:

tests/flask_tests.py

@@ -23,7 +23,7 @@ from contextlib import contextmanager
 from functools import update_wrapper
 from datetime import datetime
 from werkzeug import parse_date, parse_options_header
-from werkzeug.exceptions import NotFound
+from werkzeug.exceptions import NotFound, BadRequest
 from werkzeug.http import parse_set_header
 from jinja2 import TemplateNotFound
 from cStringIO import StringIO
@@ -592,6 +592,40 @@ class BasicFunctionalityTestCase(unittest.TestCase):
         c = app.test_client()
         assert c.get('/').data == '42'
 
+    def test_trapping_of_bad_request_key_errors(self):
+        app = flask.Flask(__name__)
+        app.testing = True
+        @app.route('/fail')
+        def fail():
+            flask.request.form['missing_key']
+        c = app.test_client()
+        assert c.get('/fail').status_code == 400
+
+        app.config['TRAP_BAD_REQUEST_KEY_ERRORS'] = True
+        c = app.test_client()
+        try:
+            c.get('/fail')
+        except KeyError, e:
+            assert isinstance(e, BadRequest)
+        else:
+            self.fail('Expected exception')
+
+    def test_trapping_of_all_http_exceptions(self):
+        app = flask.Flask(__name__)
+        app.testing = True
+        app.config['TRAP_HTTP_EXCEPTIONS'] = True
+        @app.route('/fail')
+        def fail():
+            flask.abort(404)
+
+        c = app.test_client()
+        try:
+            c.get('/fail')
+        except NotFound, e:
+            pass
+        else:
+            self.fail('Expected exception')
+
     def test_teardown_on_pop(self):
         buffer = []
         app = flask.Flask(__name__)