[Python-checkins] bpo-29587: Enable implicit exception chaining with gen.throw() (GH-19811)

[Chris Jerdonek]

https://github.com/python/cpython/commit/2514a632fb7d37be24c2059d0e286d35600f9795
commit: 2514a632fb7d37be24c2059d0e286d35600f9795
branch: master
author: Chris Jerdonek <chris.jerdonek at gmail.com>
committer: GitHub <noreply at github.com>
date: 2020-04-30T12:18:05-07:00
summary:

bpo-29587: Enable implicit exception chaining with gen.throw() (GH-19811)

Before this commit, if an exception was active inside a generator
when calling gen.throw(), then that exception was lost (i.e. there
was no implicit exception chaining).  This commit fixes that.

files:
A Misc/NEWS.d/next/Core and Builtins/2020-04-30-00-50-25.bpo-29587.oEwSq.rst
M Lib/test/test_generators.py
M Objects/genobject.c

diff --git a/Lib/test/test_generators.py b/Lib/test/test_generators.py
index 3e42bc6b69a81..4d96f44b15062 100644
--- a/Lib/test/test_generators.py
+++ b/Lib/test/test_generators.py
@@ -316,6 +316,23 @@ def g():
         self.assertEqual(cm.exception.value.value, 2)
 
 
+class GeneratorThrowTest(unittest.TestCase):
+
+    def test_exception_context_set(self):
+        def f():
+            try:
+                raise KeyError('a')
+            except Exception:
+                yield
+
+        gen = f()
+        gen.send(None)
+        with self.assertRaises(ValueError) as cm:
+            gen.throw(ValueError)
+        context = cm.exception.__context__
+        self.assertEqual((type(context), context.args), (KeyError, ('a',)))
+
+
 class YieldFromTests(unittest.TestCase):
     def test_generator_gi_yieldfrom(self):
         def a():
diff --git a/Misc/NEWS.d/next/Core and Builtins/2020-04-30-00-50-25.bpo-29587.oEwSq.rst b/Misc/NEWS.d/next/Core and Builtins/2020-04-30-00-50-25.bpo-29587.oEwSq.rst
new file mode 100644
index 0000000000000..f44aa360cc2ef
--- /dev/null
+++ b/Misc/NEWS.d/next/Core and Builtins/2020-04-30-00-50-25.bpo-29587.oEwSq.rst	
@@ -0,0 +1 @@
+Enable implicit exception chaining when calling :meth:`generator.throw`.
diff --git a/Objects/genobject.c b/Objects/genobject.c
index 6e36690b65148..289ed79aa28a6 100644
--- a/Objects/genobject.c
+++ b/Objects/genobject.c
@@ -512,6 +512,12 @@ _gen_throw(PyGenObject *gen, int close_on_genexit,
     }
 
     PyErr_Restore(typ, val, tb);
+    if (gen->gi_exc_state.exc_type) {
+        Py_INCREF(gen->gi_exc_state.exc_type);
+        Py_XINCREF(gen->gi_exc_state.exc_value);
+        Py_XINCREF(gen->gi_exc_state.exc_traceback);
+        _PyErr_ChainExceptions(gen->gi_exc_state.exc_type, gen->gi_exc_state.exc_value, gen->gi_exc_state.exc_traceback);
+    }
     return gen_send_ex(gen, Py_None, 1, 0);
 
 failed_throw: