more targeted error message on out-of-order callback args

Author: alexcjohnson

Diff for: dash/_validate.py

@@ -7,6 +7,10 @@
 
 
 def validate_callback(output, inputs, state, extra_args, types):
+    is_multi = isinstance(output, (list, tuple))
+
+    outputs = output if is_multi else [output]
+
     Input, Output, State = types
     if extra_args:
         if not isinstance(extra_args[0], (Output, Input, State)):
@@ -24,17 +28,16 @@ def validate_callback(output, inputs, state, extra_args, types):
         raise exceptions.IncorrectTypeException(
             """
             In a callback definition, you must provide all Outputs first,
-            "then all Inputs, then all States. Found this out of order:
+            then all Inputs, then all States. After this item:
+            {}
+            we found this item next:
             {}
             """.format(
-                repr(extra_args)
+                repr((outputs + inputs + state)[-1]),
+                repr(extra_args[0])
             )
         )
 
-    is_multi = isinstance(output, (list, tuple))
-
-    outputs = output if is_multi else [output]
-
     for args in [outputs, inputs, state]:
         for arg in args:
             validate_callback_arg(arg)

Diff for: tests/integration/callbacks/test_validation.py

@@ -24,14 +24,29 @@ def test_cbva001_callback_dep_types():
         ]
     )
 
-    with pytest.raises(IncorrectTypeException):
+    with pytest.raises(IncorrectTypeException) as err:
 
         @app.callback([[Output("out1", "children")]], [Input("in1", "children")])
         def f(i):
             return i
 
         pytest.fail("extra output nesting")
 
+    assert "must be `Output`, `Input`, or `State`" in err.value.args[0]
+    assert "[<Output `out1.children`>]" in err.value.args[0]
+
+    with pytest.raises(IncorrectTypeException) as err:
+
+        @app.callback(Input("in1", "children"), Output("out1", "children"))
+        def f2(i):
+            return i
+
+        pytest.fail("out-of-order args")
+
+    assert "Outputs first,\nthen all Inputs, then all States." in err.value.args[0]
+    assert "<Input `in1.children`>" in err.value.args[0]
+    assert "<Output `out1.children`>" in err.value.args[0]
+
     # all OK with tuples
     @app.callback(
         (Output("out1", "children"),),