diff -r 1ff29b57628e Lib/test/test_exceptions.py
--- a/Lib/test/test_exceptions.py	Wed Dec 02 14:37:35 2015 +0100
+++ b/Lib/test/test_exceptions.py	Wed Dec 02 13:31:14 2015 -0500
@@ -625,6 +625,29 @@
 obj = wr()
 self.assertTrue(obj is None, "%s" % obj)
 
+ def testExceptionChainedContext(self):
+ class Err1(Exception): pass
+ class Err2(Exception): pass
+
+ try:
+ try:
+ raise Err1
+ except Err1 as ex:
+ ex.__context__ = ex
+ raise Err2
+ except Err2 as ex:
+ self.assertIs(type(ex.__context__), Err1)
+
+ try:
+ try:
+ raise Err1
+ except Err1 as ex:
+ ex.__context__ = Exception()
+ ex.__context__.__context__ = ex
+ raise Err2
+ except Err2 as ex:
+ self.assertIs(type(ex.__context__), Err1)
+
 def test_exception_target_in_nested_scope(self):
 # issue 4617: This used to raise a SyntaxError
 # "can not delete variable 'e' referenced in nested scope"
diff -r 1ff29b57628e Objects/exceptions.c
--- a/Objects/exceptions.c	Wed Dec 02 14:37:35 2015 +0100
+++ b/Objects/exceptions.c	Wed Dec 02 13:31:14 2015 -0500
@@ -347,6 +347,21 @@
 /* Steals a reference to context */
 void
 PyException_SetContext(PyObject *self, PyObject *context) {
+ PyObject *next_in_context;
+
+ if (context != NULL) {
+ if (context == self) {
+ return;
+ }
+ next_in_context = context;
+ while ((next_in_context = PyException_GetContext(next_in_context))) {
+ Py_DECREF(next_in_context);
+ if (next_in_context == self) {
+ return;
+ }
+ }
+ }
+
 PyObject *old_context = ((PyBaseExceptionObject *)self)->context;
 ((PyBaseExceptionObject *)self)->context = context;
 Py_XDECREF(old_context);
</div><div class="naked_ctrl">
<form action="/index.cgi/contrast" method="get" name="gate">
<p><a href="http://altstyle.alfasado.net">AltStyle</a> によって変換されたページ <a href="https://bugs.python.org/file41212/Issue25782.patch">(-&gt;オリジナル)</a>
/ <label>アドレス: <input type="text" name="naked_post_url" value="https://bugs.python.org/file41212/Issue25782.patch" size="22" /></label> <label>モード: <select name="naked_post_mode">
<option value="default">デフォルト</option>
<option value="speech">音声ブラウザ</option>
<option value="ruby">ルビ付き</option>
<option value="contrast" selected="selected">配色反転</option>
<option value="larger-text">文字拡大</option>
<option value="mobile">モバイル</option>
</select>
<input type="submit" value="表示" />
</p>
</form>
</div>