Merge pull request #1448 from almarklein/forgot-decorator

Improve error message

Author: alexcjohnson

CHANGELOG.md

@@ -8,6 +8,9 @@ This project adheres to [Semantic Versioning](https://semver.org/).
 ### Fixed
 - [#1434](https://github.com/plotly/dash/pull/1434) Fix [#1432](https://github.com/plotly/dash/issues/1432) for Julia to import non-core component packages without possible errors.
 
+### Changed
+- [#1448](https://github.com/plotly/dash/pull/1448) Provide a hint in the callback error when the user forgot to make `app.callback(...)` a decorator.
+
 ## [1.16.3] - 2020-10-07
 ### Fixed
 - [#1426](https://github.com/plotly/dash/pull/1426) Fix a regression caused by `flask-compress==1.6.0` causing performance degradation on server requests

dash/dash.py

@@ -1068,7 +1068,11 @@ def dispatch(self):
 
         args = inputs_to_vals(inputs + state)
 
-        func = self.callback_map[output]["callback"]
+        try:
+            func = self.callback_map[output]["callback"]
+        except KeyError:
+            msg = "Callback function not found for output '{}', perhaps you forgot to prepend the '@'?"
+            raise KeyError(msg.format(output))
         response.set_data(func(*args, outputs_list=outputs_list))
         return response