bpo-24340: Fixed a "leak" in the computation of the stack size effect

Lib/test/test_compile.py

@@ -708,6 +708,19 @@ def test_func_and(self):
         code += "   x and x\n" * self.N
         self.check_stack_size(code)
 
+    def test_try_except(self):
+        # See issue #24340.
+        source = "try: x\nexcept E: y\n"
+        code = compile(source, "<foo>", "exec")
+        code2 = compile(source * self.N, "<foo>", "exec")
+        self.assertEqual(code2.co_stacksize, code.co_stacksize)
+
+    def test_try_finally(self):
+        source = "try: x\nfinally: y\n"
+        code = compile(source, "<foo>", "exec")
+        code2 = compile(source * self.N, "<foo>", "exec")
+        self.assertEqual(code2.co_stacksize, code.co_stacksize)
+
 
 if __name__ == "__main__":
     unittest.main()

Misc/NEWS

@@ -10,6 +10,9 @@ What's New in Python 3.7.0 alpha 1?
 Core and Builtins
 -----------------
 
+- bpo-24340: Fixed a "leak" in the computation of the stack size effect for
+  try/except and try/finally.
+
 - bpo-25324: Tokens needed for parsing in Python moved to C. ``COMMENT``,
   ``NL`` and ``ENCODING``. This way the tokens and tok_names in the token
   module don't get changed when you import the tokenize module.

Python/compile.c

@@ -946,9 +946,9 @@ PyCompile_OpcodeStackEffect(int opcode, int oparg)
         case POP_BLOCK:
             return 0;
         case POP_EXCEPT:
-            return 0;  /* -3 except if bad bytecode */
+            return -3; /* the previous exception state */
         case END_FINALLY:
-            return -1; /* or -2 or -3 if exception occurred */
+            return -6; /* see SETUP_FINALLY */
 
         case STORE_NAME:
             return -1;
@@ -4850,11 +4850,12 @@ stackdepth_walk(struct compiler *c, basicblock *b, int depth, int maxdepth)
             if (instr->i_opcode == FOR_ITER) {
                 target_depth = depth-2;
             }
-            else if (instr->i_opcode == SETUP_FINALLY ||
-                     instr->i_opcode == SETUP_EXCEPT) {
-                target_depth = depth+3;
-                if (target_depth > maxdepth)
-                    maxdepth = target_depth;
+            else if (instr->i_opcode == SETUP_EXCEPT) {
+                depth = depth - 6;
+            }
+            else if (instr->i_opcode == SETUP_FINALLY) {
+                depth = depth - 1; /* for None */
+                continue; /* should pass to finally handler in any case  */
             }
             else if (instr->i_opcode == JUMP_IF_TRUE_OR_POP ||
                      instr->i_opcode == JUMP_IF_FALSE_OR_POP)