fix prevent_initial_call as positional arg to callback

Author: alexcjohnson

dash/dependencies.py

@@ -156,7 +156,7 @@ def handle_callback_args(args, kwargs):
     """Split args into outputs, inputs and states"""
     prevent_initial_call = kwargs.get("prevent_initial_call", None)
     if prevent_initial_call is None and args and isinstance(args[-1], bool):
-        prevent_initial_call = args.pop()
+        args, prevent_initial_call = args[:-1], args[-1]
 
     # flatten args, to support the older syntax where outputs, inputs, and states
     # each needed to be in their own list

tests/integration/callbacks/test_prevent_initial.py

@@ -333,3 +333,25 @@ def test_cbpi003_multi_outputs(flavor, dash_duo):
     dash_duo.wait_for_text_to_equal("#c", "BlueCheese")
     dash_duo.wait_for_text_to_equal("#b", "Cheese")
     dash_duo.wait_for_text_to_equal("#a", "Blue")
+
+
+def test_cbpi004_positional_arg(dash_duo):
+    app = dash.Dash(__name__)
+    app.layout = html.Div([html.Button("click", id="btn"), html.Div(id="out")])
+
+    @app.callback(
+        Output("out", "children"),
+        Input("btn", "n_clicks"),
+        True
+    )
+    def f(n):
+        return n
+
+    dash_duo.start_server(app)
+    dash_duo._wait_for_callbacks()
+
+    dash_duo.wait_for_text_to_equal("#out", "")
+
+    dash_duo.find_element("#btn").click()
+
+    dash_duo.wait_for_text_to_equal("#out", "1")