Issue #20041: Fixed TypeError when frame.f_trace is set to None.

Patch by Xavier de Gaye.

Lib/test/test_sys_settrace.py

@@ -388,6 +388,15 @@ class TraceTestCase(unittest.TestCase):
              (257, 'line'),
              (257, 'return')])
 
+    def test_17_none_f_trace(self):
+        # Issue 20041: fix TypeError when f_trace is set to None.
+        def func():
+            sys._getframe().f_trace = None
+            lineno = 2
+        self.run_and_compare(func,
+            [(0, 'call'),
+             (1, 'line')])
+
 
 class RaisingTraceFuncTestCase(unittest.TestCase):
     def setUp(self):

Misc/NEWS

@@ -10,6 +10,9 @@ Release date: tba
 Core and Builtins
 -----------------
 
+- Issue #20041: Fixed TypeError when frame.f_trace is set to None.
+  Patch by Xavier de Gaye.
+
 - Issue #26168: Fixed possible refleaks in failing Py_BuildValue() with the "N"
   format unit.
 

Objects/frameobject.c

@@ -349,15 +349,13 @@ frame_gettrace(PyFrameObject *f, void *closure)
 static int
 frame_settrace(PyFrameObject *f, PyObject* v, void *closure)
 {
-    PyObject* old_value;
-
     /* We rely on f_lineno being accurate when f_trace is set. */
     f->f_lineno = PyFrame_GetLineNumber(f);
 
-    old_value = f->f_trace;
+    if (v == Py_None)
+        v = NULL;
     Py_XINCREF(v);
-    f->f_trace = v;
-    Py_XDECREF(old_value);
+    Py_XSETREF(f->f_trace, v);
 
     return 0;
 }