bpo-42635: Mark JUMP_ABSOLUTE at end of 'for' loop as artificial to avoid spurious line events. (GH-23761)

Lib/test/test_sys_settrace.py

@@ -646,6 +646,32 @@ class TraceTestCase(unittest.TestCase):
              (6, 'line'),
              (6, 'return')])
 
+    def test_nested_loops(self):
+
+        def func():
+            for i in range(2):
+                for j in range(2):
+                    a = i + j
+            return a == 1
+
+        self.run_and_compare(func,
+            [(0, 'call'),
+             (1, 'line'),
+             (2, 'line'),
+             (3, 'line'),
+             (2, 'line'),
+             (3, 'line'),
+             (2, 'line'),
+             (1, 'line'),
+             (2, 'line'),
+             (3, 'line'),
+             (2, 'line'),
+             (3, 'line'),
+             (2, 'line'),
+             (1, 'line'),
+             (4, 'line'),
+             (4, 'return')])
+
 
 class SkipLineEventsTraceTestCase(TraceTestCase):
     """Repeat the trace tests, but with per-line events skipped"""

Python/compile.c

@@ -2773,6 +2773,8 @@ compiler_for(struct compiler *c, stmt_ty s)
     compiler_use_next_block(c, body);
     VISIT(c, expr, s->v.For.target);
     VISIT_SEQ(c, stmt, s->v.For.body);
+    /* Mark jump as artificial */
+    c->u->u_lineno = -1;
     ADDOP_JUMP(c, JUMP_ABSOLUTE, start);
     compiler_use_next_block(c, cleanup);