Add warnings when function without dependencies is passed to ParamFun…

…ction (#3545)

* Add warnings when function without dependencies is passed to ParamFunction/ParamMethod

* Simplify warning

* Remove ParamMethod warning

panel/depends.py

@@ -207,6 +207,7 @@ async def wrapped(*wargs, **wkwargs):
         def wrapped(*wargs, **wkwargs):
             combined_args, combined_kwargs = combine_arguments(wargs, wkwargs)
             return function(*combined_args, **combined_kwargs)
+    wrapped.__bound_function__ = function
     return wrapped
 
 __all__ = ["bind", "depends"]

panel/param.py

@@ -745,17 +745,16 @@ def _validate_object(self):
         if not dependencies or not dependencies.get('watch'):
             return
         fn_type = 'method' if type(self) is ParamMethod else 'function'
-        self.param.warning(f"The {fn_type} supplied for Panel to display "
-                           "was declared with `watch=True`, which will "
-                           f"cause the {fn_type} to be called twice for "
-                           "any change in a dependent Parameter. "
-                           "`watch` should be False when Panel is "
-                           "responsible for displaying the result "
-                           f"of the {fn_type} call, while `watch=True` "
-                           f"should be reserved for {fn_type}s that work "
-                           "via side-effects, e.g. by modifying internal  "
-                           "state of a class or global state in an "
-                           "application's namespace.")
+        self.param.warning(
+            f"The {fn_type} supplied for Panel to display was declared "
+            f"with `watch=True`, which will cause the {fn_type} to be "
+            "called twice for any change in a dependent Parameter. "
+            "`watch` should be False when Panel is responsible for "
+            f"displaying the result of the {fn_type} call, while "
+            f"`watch=True` should be reserved for {fn_type}s that work "
+            "via side-effects, e.g. by modifying internal state of a "
+            "class or global state in an application's namespace."
+        )
 
     #----------------------------------------------------------------
     # Callback API
@@ -878,6 +877,19 @@ class ParamFunction(ParamMethod):
     def _link_object_params(self):
         deps = getattr(self.object, '_dinfo', {})
         dep_params = list(deps.get('dependencies', [])) + list(deps.get('kw', {}).values())
+        if not dep_params:
+            fn = getattr(self.object, '__bound_function__', self.object)
+            fn_name = getattr(fn, '__name__', repr(self.object))
+            self.param.warning(
+                f"The function {fn_name!r} does not have any dependencies "
+                "and will never update. Are you sure you did not intend "
+                "to depend on or bind a parameter or widget to this function? "
+                "If not simply call the function before passing it to Panel. "
+                "Otherwise, when passing a parameter as an argument, "
+                "ensure you pass at least one parameter and reference the "
+                "actual parameter object not the current value, i.e. use "
+                "object.param.parameter not object.parameter."
+            )
         grouped = defaultdict(list)
         for dep in dep_params:
             grouped[id(dep.owner)].append(dep)