Do not suppress exceptions in Output widget context

python/ipywidgets/ipywidgets/widgets/tests/test_widget_output.py

@@ -1,3 +1,5 @@
+import traceback
+import pytest
 import sys
 from unittest import TestCase
 from contextlib import contextmanager
@@ -33,7 +35,13 @@ def _mock_get_ipython(self, msg_id):
         # is still printed to screen
         def showtraceback(self_, exc_tuple, *args, **kwargs):
             etype, evalue, tb = exc_tuple
-            raise etype(evalue)
+            # Ipython notebook displays traceback and does not re-raise.
+            # This mock ipython object will mimic the behavior.
+            print("")
+            print("---------------------------------------------")
+            print("<mocked ipython Exception>")
+            traceback.print_exception(etype, evalue, tb)
+            print("---------------------------------------------")
 
         ipython = type(
             'mock_ipython',
@@ -64,6 +72,53 @@ def test_set_msg_id_when_capturing(self):
                 assert widget.msg_id == msg_id
             assert widget.msg_id == ''
 
+    def test_exception_when_capturing(self):
+        class CustomException(Exception):
+            pass
+
+        def _run_code(widget, msg):
+            with widget:
+                raise CustomException(msg)
+            raise AssertionError(
+                "This line should be never executed. "
+                "The output widget probably have suppressed the exception.")
+
+        # 1. without ipython (plain python)
+        no_ipython = lambda: None
+        with self._mocked_ipython(no_ipython, no_ipython):
+            # 1-1. without ipython
+            widget = widget_output.Output()
+            with pytest.raises(CustomException):
+                _run_code(widget, "NO ipython")
+
+            # 1-2. without ipython, catch_exception (no effect)
+            widget = widget_output.Output(catch_exception=True)
+            with pytest.raises(CustomException):
+                _run_code(widget, "NO ipython, catch_exception")
+
+        # 2. with ipython
+        msg_id = 'msg-id'
+        get_ipython = self._mock_get_ipython(msg_id)
+        clear_output = self._mock_clear_output()
+        with self._mocked_ipython(get_ipython, clear_output):
+            # 2-1. with ipython (throws outside)
+            # An exception should be thrown outside the capturing block.
+            widget = widget_output.Output(catch_exception=False)
+            with pytest.raises(CustomException):
+                _run_code(widget, "ipython + NO catch_exception")
+
+            # 2-2. with ipython + catch_exception (suppress exception)
+            widget = widget_output.Output(catch_exception=True)
+            with pytest.raises(AssertionError,
+                               match='.*should be never executed.*'):
+                _run_code(widget, "ipython + catch_exception")
+
+            # the default behavior: suppress exception (see #3417)
+            widget = widget_output.Output()
+            with pytest.raises(AssertionError,
+                               match='.*should be never executed.*'):
+                _run_code(widget, "ipython + catch_exception")
+
     def test_clear_output(self):
         msg_id = 'msg-id'
         get_ipython = self._mock_get_ipython(msg_id)

python/ipywidgets/ipywidgets/widgets/widget_output.py

@@ -14,7 +14,7 @@
 from .widget import register
 from .._version import __jupyter_widgets_output_version__
 
-from traitlets import Unicode, Dict
+from traitlets import Unicode, Dict, Bool
 from IPython.core.interactiveshell import InteractiveShell
 from IPython.display import clear_output
 from IPython import get_ipython
@@ -29,11 +29,13 @@ class Output(DOMWidget):
 
     You can then use the widget as a context manager: any output produced while in the
     context will be captured and displayed in the widget instead of the standard output
-    area.
+    area. Any exception from the context will be thrown outside by default, but if
+    catch_exception is set to True, the context manager will catch and suppress the
+    exceptions.
 
     You can also use the .capture() method to decorate a function or a method. Any output
     produced by the function will then go to the output widget. This is useful for
-    debugging widget callbacks, for example.
+    debugging widget callbacks, for example:
 
     Example::
         import ipywidgets as widgets
@@ -49,6 +51,11 @@ class Output(DOMWidget):
         @out.capture()
         def func():
             print('prints to output widget')
+
+    Parameters
+    ----------
+    catch_exception: {True,False}
+        Whether exceptions will be suppressed or not. Default is False.
     """
     _view_name = Unicode('OutputView').tag(sync=True)
     _model_name = Unicode('OutputModel').tag(sync=True)
@@ -60,6 +67,8 @@ def func():
     msg_id = Unicode('', help="Parent message id of messages to capture").tag(sync=True)
     outputs = TypedTuple(trait=Dict(), help="The output messages synced from the frontend.").tag(sync=True)
 
+    catch_exception = Bool(True, help="Whether to catch and suppress all exceptions.")
+
     __counter = 0
 
     def clear_output(self, *pargs, **kwargs):
@@ -113,7 +122,7 @@ def __enter__(self):
             kernel = ip.kernel
         elif self.comm is not None and self.comm.kernel is not None:
             kernel = self.comm.kernel
-        
+
         if kernel:
             parent = None
             if hasattr(kernel, "get_parent"):
@@ -147,9 +156,12 @@ def __exit__(self, etype, evalue, tb):
         self.__counter -= 1
         if self.__counter == 0:
             self.msg_id = ''
-        # suppress exceptions when in IPython, since they are shown above,
-        # otherwise let someone else handle it
-        return True if kernel else None
+
+        # suppress exceptions when in IPython and capture_exception is set,
+        # since they are shown above. Otherwise let the exception rethrown
+        # so that someone else handles it.
+        if kernel and self.catch_exception:
+            return True
 
     def _flush(self):
         """Flush stdout and stderr buffers."""