Raise ValueError when a watcher cannot be unwatched instead of logging a warning

param/parameterized.py

@@ -3568,12 +3568,17 @@ def _register_watcher(self_, action, watcher, what='value'):
                     watchers[parameter_name] = {}
                 if what not in watchers[parameter_name]:
                     watchers[parameter_name][what] = []
-                getattr(watchers[parameter_name][what], action)(watcher)
+                method = getattr(watchers[parameter_name][what], action)
             else:
                 watchers = self_[parameter_name].watchers
                 if what not in watchers:
                     watchers[what] = []
-                getattr(watchers[what], action)(watcher)
+                method = getattr(watchers[what], action)
+            try:
+                method(watcher)
+            except ValueError:
+                # raised when method is 'remove' and watcher is not in the list.
+                raise ValueError(f"Watcher '{watcher}' has already been removed or was never added.")
 
     def watch(
         self_,
@@ -3724,10 +3729,7 @@ def unwatch(self_, watcher: Watcher) -> None:
 
         >>> instance.a = 20  # No output
         """
-        try:
-            self_._register_watcher('remove', watcher, what=watcher.what)
-        except Exception:
-            self_.warning(f'No such watcher {str(watcher)} to remove.')
+        self_._register_watcher('remove', watcher, what=watcher.what)
 
     def watch_values(
         self_,

tests/testwatch.py

@@ -8,7 +8,7 @@
 
 from param.parameterized import Skip, discard_events
 
-from .utils import MockLoggingHandler, warnings_as_excepts
+from .utils import MockLoggingHandler
 
 
 class Accumulator:
@@ -250,15 +250,8 @@ def accumulator(change):
         obj = SimpleWatchExample()
         watcher = obj.param.watch(accumulator, 'a')
         obj.param.unwatch(watcher)
-        with warnings_as_excepts(match='No such watcher'):
+        with pytest.raises(ValueError, match="has already been removed or was never added"):
             obj.param.unwatch(watcher)
-        try:
-            param.parameterized.warnings_as_exceptions = False
-            obj.param.unwatch(watcher)
-            self.log_handler.assertEndsWith('WARNING',
-                                ' to remove.')
-        finally:
-            param.parameterized.warnings_as_exceptions = True
 
     def test_simple_batched_watch_setattr(self):
 
@@ -720,6 +713,14 @@ def __init__(self, **params):
         P()
 
 
+    def test_watch_raises_bad_parameter(self):
+        obj = SimpleWatchExample()
+        with pytest.raises(
+            ValueError,
+            match="does_not_exist parameter was not found in list of parameters of class SimpleWatchExample"
+        ):
+            obj.param.watch(lambda e: print(e), 'does_not_exist')
+
 
 class TestWatchMethod(unittest.TestCase):