Added a closed property to the Widget class. Modified __repr__ to i…

…dentify closed widgets.

Added a new class `Children` for the children trait of Box optimised for checking widgets
and quietly dropping widgets that are closed, and objects that aren't widgets. Widgets in Box.children
 are also removed when the widget is closed.
 Added tests test_gc_box & test_gc_box_advanced

python/ipywidgets/ipywidgets/widgets/tests/test_widget.py

@@ -15,6 +15,7 @@
 from .. import widget
 from ..widget import Widget
 from ..widget_button import Button
+from ..widget_box import VBox
 import copy
 
 import ipywidgets as ipw
@@ -148,3 +149,59 @@ def on_delete():
     del b
     gc.collect()
     assert deleted
+
+
+def test_gc_box():
+    # Test Box gc collected and children lifecycle managed.
+    deleted = False
+    b = VBox(children=[Button(description='button')])
+
+    def on_delete():
+        nonlocal deleted
+        deleted = True
+
+    weakref.finalize(b, on_delete)
+    del b
+    gc.collect()
+    assert deleted
+
+def test_gc_box_advanced():
+    # A more advanced test for:
+    # 1. A child widget is removed from the children when it is closed
+    # 2. The children are discarded when the widget is closed.
+
+    deleted = False
+
+    b = VBox(
+        children=[
+            Button(description="b0"),
+            Button(description="b1"),
+            Button(description="b2"),
+        ]
+    )
+
+    def on_delete():
+        nonlocal deleted
+        deleted = True
+
+    weakref.finalize(b, on_delete)
+
+    ids = [model_id for w in  b.children if (model_id:=w.model_id)  in widget._instances]
+    assert len(ids) == 3, 'Not all button comms were registered.'
+
+    # keep a strong ref to `b1`
+    b1 = b.children[1] 
+
+    # When a widget is closed it should be removed from the box.children.
+    b.children[0].close()
+    assert len(b.children) == 2, "b0 not removed."
+    # assert 'b0' in deleted
+
+    # When the ref to box is removed it should be deleted.
+    del b
+    assert deleted, "`b` should have been the only strong ref to the box."
+    # assert not b.children, '`children` should be removed when the widget is closed.'
+    assert not b1.closed, 'A removed widget should remain alive.'
+
+    # b2 shouldn't have any strong references so should be deleted.
+    assert ids[2] not in widget._instances, 'b2 should have been auto deleted.'

python/ipywidgets/ipywidgets/widgets/widget.py

@@ -51,6 +51,9 @@ def _widget_to_json(x, obj):
     elif isinstance(x, (list, tuple)):
         return [_widget_to_json(v, obj) for v in x]
     elif isinstance(x, Widget):
+        if x.closed:
+            msg = f"Widget is {x!r}"
+            raise RuntimeError(msg)
         return "IPY_MODEL_" + x.model_id
     else:
         return x
@@ -557,6 +560,14 @@ def model_id(self):
 
         If a Comm doesn't exist yet, a Comm will be created automagically."""
         return getattr(self.comm, "comm_id", None)
+
+    @property
+    def closed(self) -> bool:
+        """Returns True when comms is closed.
+        
+        There is no possibility to re-open once it is closed."""
+        # If comm is None it indicates the comm is closed and the widget is closed. 
+        return self._trait_values.get('comm', False) is None
 
     #-------------------------------------------------------------------------
     # Methods
@@ -706,7 +717,8 @@ def notify_change(self, change):
         super().notify_change(change)
 
     def __repr__(self):
-        return self._gen_repr_from_keys(self._repr_keys())
+        rep  =  self._gen_repr_from_keys(self._repr_keys())
+        return  'closed: ' + rep  if  self.closed else rep
 
     #-------------------------------------------------------------------------
     # Support methods

python/ipywidgets/ipywidgets/widgets/widget_box.py

@@ -7,13 +7,13 @@
 group other widgets together and control their
 relative layouts.
 """
+import weakref
 
 from .widget import register, widget_serialization, Widget
 from .domwidget import DOMWidget
 from .widget_core import CoreWidget
 from .docutils import doc_subst
-from .trait_types import TypedTuple
-from traitlets import Unicode, CaselessStrEnum, Instance
+from traitlets import Unicode, CaselessStrEnum, TraitType, observe
 
 
 _doc_snippets = {}
@@ -27,6 +27,12 @@
         which applies no pre-defined style.
 """
 
+class Children(TraitType[tuple[Widget],tuple[Widget]]):
+    default_value = ()
+
+    def validate(self, obj:'Box', value):
+        return tuple(v for v in value if isinstance(v, Widget) and not v.closed)
+
 
 @register
 @doc_subst(_doc_snippets)
@@ -52,17 +58,43 @@ class Box(DOMWidget, CoreWidget):
     # Child widgets in the container.
     # Using a tuple here to force reassignment to update the list.
     # When a proper notifying-list trait exists, use that instead.
-    children = TypedTuple(trait=Instance(Widget), help="List of widget children").tag(
+    children = Children(help="List of widget children").tag(
         sync=True, **widget_serialization)
 
     box_style = CaselessStrEnum(
         values=['success', 'info', 'warning', 'danger', ''], default_value='',
         help="""Use a predefined styling for the box.""").tag(sync=True)
 
     def __init__(self, children=(), **kwargs):
-        kwargs['children'] = children
+        if children:
+            kwargs['children'] = children
         super().__init__(**kwargs)
 
+    @observe('children')
+    def _box_observe_children(self, change):
+        # Monitor widgets for when the comm is closed.
+        handler = getattr(self, "_widget_children_comm_handler", None)
+        if not handler:    
+            ref = weakref.ref(self)
+            def handler(change):
+                self_ = ref()
+                if self_ and change['owner']:
+                    # Re-validation will discard all closed widgets.
+                    self_.children = self_.children
+
+            self._widget_children_comm_handler = handler      
+        if change['new']:
+            w:Widget
+            for w in set(change['new']).difference(change['old'] or ()):
+                w.observe(handler, names='comm')
+        if change['old']:
+            for w in set(change['old']).difference(change['new']):
+                try:
+                    w.unobserve(handler,  names='comm')
+                except ValueError:
+                    pass     
+
+
 @register
 @doc_subst(_doc_snippets)
 class VBox(Box):