Issue #4486: When an exception has an explicit cause, do not print its implicit context too.

pitrou · pitrou · commit 7b0d4a238f2a · 2009-11-28T16:12:28.000Z
diff --git a/Lib/test/test_traceback.py b/Lib/test/test_traceback.py
@@ -253,6 +253,26 @@ def outer_raise():
         self.check_zero_div(blocks[0])
         self.assertTrue('inner_raise() # Marker' in blocks[2])
 
+    def test_cause_and_context(self):
+        # When both a cause and a context are set, only the cause should be
+        # displayed and the context should be muted.
+        def inner_raise():
+            try:
+                self.zero_div()
+            except ZeroDivisionError as _e:
+                e = _e
+            try:
+                xyzzy
+            except NameError:
+                raise KeyError from e
+        def outer_raise():
+            inner_raise() # Marker
+        blocks = boundaries.split(self.get_report(outer_raise))
+        self.assertEquals(len(blocks), 3)
+        self.assertEquals(blocks[1], cause_message)
+        self.check_zero_div(blocks[0])
+        self.assert_('inner_raise() # Marker' in blocks[2])
+
     def test_cause_recursive(self):
         def inner_raise():
             try:
diff --git a/Lib/traceback.py b/Lib/traceback.py
@@ -120,13 +120,14 @@ def _iter_chain(exc, custom_tb=None, seen=None):
     seen.add(exc)
     its = []
     cause = exc.__cause__
-    context = exc.__context__
     if cause is not None and cause not in seen:
         its.append(_iter_chain(cause, None, seen))
         its.append([(_cause_message, None)])
-    if context is not None and context is not cause and context not in seen:
-        its.append(_iter_chain(context, None, seen))
-        its.append([(_context_message, None)])
+    else:
+        context = exc.__context__
+        if context is not None and context not in seen:
+            its.append(_iter_chain(context, None, seen))
+            its.append([(_context_message, None)])
     its.append([(exc, custom_tb or exc.__traceback__)])
     # itertools.chain is in an extension module and may be unavailable
     for it in its:
diff --git a/Misc/NEWS b/Misc/NEWS
@@ -140,6 +140,10 @@ C-API
 Library
 -------
 
+- Issue #4486: When an exception has an explicit cause, do not print its
+  implicit context too.  This affects the `traceback` module as well as
+  built-in exception printing.
+
 - Issue #1515: Enable use of deepcopy() with instance methods.  Patch by
   Robert Collins.
 
diff --git a/Python/pythonrun.c b/Python/pythonrun.c
@@ -1576,7 +1576,7 @@ print_exception_recursive(PyObject *f, PyObject *value, PyObject *seen)
 						cause_message, f);
 				}
 			}
-			if (context) {
+			else if (context) {
 				res = PySet_Contains(seen, context);
 				if (res == -1)
 					PyErr_Clear();

Original file line number	Diff line number	Diff line change
`@@ -1576,7 +1576,7 @@ print_exception_recursive(PyObject f, PyObject value, PyObject *seen)`
`1576`	`1576`	`cause_message, f);`
`1577`	`1577`	`}`
`1578`	`1578`	`}`
`1579`		`- if (context) {`
	`1579`	`+ else if (context) {`
`1580`	`1580`	`res = PySet_Contains(seen, context);`
`1581`	`1581`	`if (res == -1)`
`1582`	`1582`	`PyErr_Clear();`