Merge pull request #2753 from plotly/improve-layout-error

Edit two error messages to suggest that users check for trailing commas

dash/_validate.py

@@ -392,8 +392,11 @@ def validate_index(name, checks, index):
 def validate_layout_type(value):
     if not isinstance(value, (Component, patch_collections_abc("Callable"))):
         raise exceptions.NoLayoutException(
-            "Layout must be a dash component "
-            "or a function that returns a dash component."
+            """
+            Layout must be a single dash component
+            or a function that returns a dash component.
+            Cannot be a tuple (are there any trailing commas?)
+            """
         )
 
 

dash/dash-renderer/src/utils/TreeContainer.ts

@@ -61,8 +61,8 @@ export function validateComponent(componentDefinition: any) {
     if (type(componentDefinition) === 'Array') {
         throw new Error(
             'The children property of a component is a list of lists, instead ' +
-                'of just a list. ' +
-                'Check the component that has the following contents, ' +
+                'of just a list. This can sometimes be due to a trailing comma. ' +
+                'Check the component that has the following contents ' +
                 'and remove one of the levels of nesting: \n' +
                 JSON.stringify(componentDefinition, null, 2)
         );