Raise error when __copy__ method is used on base widget class;

Base widget class does not have a copy implementation and raises a NotImplementedError.
Test case added in test_widget.py
Fixes #2352

Author: marym

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

@@ -13,6 +13,7 @@
 from .. import widget
 from ..widget import Widget
 from ..widget_button import Button
+import copy
 
 
 def test_no_widget_view():
@@ -80,3 +81,11 @@ def test_compatibility():
     caller_path = inspect.stack(context=0)[1].filename
     assert all(x.filename == caller_path for x in record)
     assert len(record) == 6
+
+
+def test_widget_copy():
+    button = Button()
+    with pytest.raises(NotImplementedError):
+        copy.copy(button)
+    with pytest.raises(NotImplementedError):
+        copy.deepcopy(button)

python/ipywidgets/ipywidgets/widgets/widget.py

@@ -502,6 +502,12 @@ def __init__(self, **kwargs):
 
         Widget._call_widget_constructed(self)
         self.open()
+    
+    def __copy__(self):
+        raise NotImplementedError("Widgets cannot be copied; custom implementation required")
+
+    def __deepcopy__(self, memo):
+        raise NotImplementedError("Widgets cannot be copied; custom implementation required")
 
     def __del__(self):
         """Object disposal"""