Add #379 again and make related test rework

Original PR: https://github.com/channelcat/sanic/pull/379

sanic/handlers.py

@@ -16,9 +16,12 @@ from sanic.response import text, html
 
 class ErrorHandler:
     handlers = None
+    cached_handlers = None
+    _missing = object()
 
     def __init__(self):
-        self.handlers = {}
+        self.handlers = []
+        self.cached_handlers = {}
         self.debug = False
 
     def _render_traceback_html(self, exception, request):
@@ -37,7 +40,18 @@ class ErrorHandler:
             path=request.path)
 
     def add(self, exception, handler):
-        self.handlers[exception] = handler
+        self.handlers.append((exception, handler))
+
+    def lookup(self, exception):
+        handler = self.cached_handlers.get(exception, self._missing)
+        if handler is self._missing:
+            for exception_class, handler in self.handlers:
+                if isinstance(exception, exception_class):
+                    self.cached_handlers[type(exception)] = handler
+                    return handler
+            self.cached_handlers[type(exception)] = None
+            handler = None
+        return handler
 
     def response(self, request, exception):
         """Fetches and executes an exception handler and returns a response
@@ -47,9 +61,13 @@ class ErrorHandler:
         :param exception: Exception to handle
         :return: Response object
         """
-        handler = self.handlers.get(type(exception), self.default)
+        handler = self.lookup(exception)
+        response = None
         try:
-            response = handler(request=request, exception=exception)
+            if handler:
+                response = handler(request=request, exception=exception)
+            if response is None:
+                response = self.default(request=request, exception=exception)
         except Exception:
             self.log(format_exc())
             if self.debug:

tests/test_exceptions_handler.py

@@ -1,6 +1,7 @@
 from sanic import Sanic
 from sanic.response import text
 from sanic.exceptions import InvalidUsage, ServerError, NotFound
+from sanic.handlers import ErrorHandler
 from bs4 import BeautifulSoup
 
 exception_handler_app = Sanic('test_exception_handler')
@@ -30,7 +31,7 @@ def handler_4(request):
 @exception_handler_app.route('/5')
 def handler_5(request):
     class CustomServerError(ServerError):
-        status_code=200
+        pass
     raise CustomServerError('Custom server error')
 
 
@@ -81,3 +82,44 @@ def test_html_traceback_output_in_debug_mode():
 def test_inherited_exception_handler():
     request, response = exception_handler_app.test_client.get('/5')
     assert response.status == 200
+
+
+def test_exception_handler_lookup():
+    class CustomError(Exception):
+        pass
+
+    class CustomServerError(ServerError):
+        pass
+
+    def custom_error_handler():
+        pass
+
+    def server_error_handler():
+        pass
+
+    def import_error_handler():
+        pass
+
+    try:
+        ModuleNotFoundError
+    except:
+        class ModuleNotFoundError(ImportError):
+            pass
+
+    handler = ErrorHandler()
+    handler.add(ImportError, import_error_handler)
+    handler.add(CustomError, custom_error_handler)
+    handler.add(ServerError, server_error_handler)
+
+    assert handler.lookup(ImportError()) == import_error_handler
+    assert handler.lookup(ModuleNotFoundError()) == import_error_handler
+    assert handler.lookup(CustomError()) == custom_error_handler
+    assert handler.lookup(ServerError('Error')) == server_error_handler
+    assert handler.lookup(CustomServerError('Error')) == server_error_handler
+
+    # once again to ensure there is no caching bug
+    assert handler.lookup(ImportError()) == import_error_handler
+    assert handler.lookup(ModuleNotFoundError()) == import_error_handler
+    assert handler.lookup(CustomError()) == custom_error_handler
+    assert handler.lookup(ServerError('Error')) == server_error_handler
+    assert handler.lookup(CustomServerError('Error')) == server_error_handler