Let exception handler handle inherited exceptions

Original sanic exception handler only could handle exact matching
exceptions. New `lookup` method will provide ability to looking up
their ancestors without additional cost of performance.

sanic/exceptions.py

@@ -139,9 +139,12 @@ class PayloadTooLarge(SanicException):
 
 class Handler:
     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):
@@ -160,7 +163,18 @@ class Handler:
             uri=request.url)
 
     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):
         """
@@ -170,9 +184,12 @@ class Handler:
         :param exception: Exception to handle
         :return: Response object
         """
-        handler = self.handlers.get(type(exception), self.default)
+        handler = self.lookup(exception)
         try:
-            response = handler(request=request, exception=exception)
+            response = handler and handler(
+                request=request, exception=exception)
+            if response is None:
+                response = self.default(request=request, exception=exception)
         except:
             log.error(format_exc())
             if self.debug:

tests/test_exceptions_handler.py

@@ -28,6 +28,13 @@ def handler_4(request):
     return text(foo)
 
 
+@exception_handler_app.route('/5')
+def handler_5(request):
+    class CustomServerError(ServerError):
+        pass
+    raise CustomServerError('Custom server error')
+
+
 @exception_handler_app.exception(NotFound, ServerError)
 def handler_exception(request, exception):
     return text("OK")
@@ -71,3 +78,8 @@ def test_html_traceback_output_in_debug_mode():
     assert (
         "NameError: name 'bar' "
         "is not defined while handling uri /4") == summary_text
+
+
+def test_inherited_exception_handler():
+    request, response = sanic_endpoint_test(exception_handler_app, uri='/5')
+    assert response.status == 200