Fix marshal_exception not to use global state (#390)

traits_futures/exception_handling.py

@@ -14,12 +14,28 @@
 import traceback
 
 
-def marshal_exception(e):
+def marshal_exception(exception):
     """
     Turn exception details into something that can be safely
     transmitted across thread / process boundaries.
+
+    Parameters
+    ----------
+    exception : BaseException
+        The exception instance to be marshalled
+
+    Returns
+    -------
+    exception_type, exception_value, exception_traceback : str
+        Strings representing the exception type, value and
+        formatted traceback.
     """
-    exc_type = str(type(e))
-    exc_value = str(e)
-    formatted_traceback = str(traceback.format_exc())
-    return exc_type, exc_value, formatted_traceback
+    return (
+        str(type(exception)),
+        str(exception),
+        "".join(
+            traceback.format_exception(
+                type(exception), exception, exception.__traceback__
+            )
+        ),
+    )

traits_futures/tests/test_exception_handling.py

@@ -44,3 +44,21 @@ def test_marshal_exception_with_unicode_message(self):
         self.assertEqual(exc_type, str(ValueError))
         self.assertIn(message, exc_value)
         self.assertIn("test_marshal_exception", exc_traceback)
+
+    def test_marshal_exception_works_outside_except(self):
+        try:
+            raise RuntimeError("something went wrong")
+        except BaseException as exception:
+            stored_exception = exception
+
+        exc_type, exc_value, exc_traceback = marshal_exception(
+            stored_exception
+        )
+
+        self.assertIsInstance(exc_type, str)
+        self.assertIsInstance(exc_value, str)
+        self.assertIsInstance(exc_traceback, str)
+
+        self.assertEqual(exc_type, str(RuntimeError))
+        self.assertIn("something went wrong", exc_value)
+        self.assertIn("test_marshal_exception", exc_traceback)