bpo-39223: Support slices in constant folding

Lib/test/test_compile.py

@@ -648,6 +648,17 @@ def unused_code_at_end():
             'RETURN_VALUE',
             list(dis.get_instructions(unused_code_at_end))[-1].opname)
 
+    @support.cpython_only
+    def test_slice_folding(self):
+        def g():
+            "abcde"[2:4]
+        gc = g.__code__.co_consts
+
+        self.assertIn("cd", gc)
+        self.assertNotIn("abcde", gc)
+        self.assertNotIn(2, gc)
+        self.assertNotIn(4, gc)
+
     def test_dont_merge_constants(self):
         # Issue #25843: compile() must not merge constants which are equal
         # but have a different type.

Misc/NEWS.d/next/Core and Builtins/2020-01-05-18-52-56.bpo-39223.caMNwy.rst

@@ -0,0 +1,2 @@
+Support slices (e.g: ``"abcde"[2:4]``) in constant folding. Patch by
+Batuhan Taskaya.

Python/ast_opt.c

@@ -305,26 +305,46 @@ fold_tuple(expr_ty node, PyArena *arena, int optimize)
     return make_const(node, newval, arena);
 }
 
+#define ENSURE_CONSTANT(NODE) \
+    if ((NODE) != NULL && (NODE)->kind != Constant_kind) \
+        return 1;
+
+#define GET_CONSTANT(NODE) (((NODE) != NULL) ? (NODE)->v.Constant.value : NULL)
+
 static int
 fold_subscr(expr_ty node, PyArena *arena, int optimize)
 {
-    PyObject *newval;
-    expr_ty arg, idx;
+    PyObject *newval, *key;
+    expr_ty arg;
     slice_ty slice;
 
     arg = node->v.Subscript.value;
     slice = node->v.Subscript.slice;
-    if (node->v.Subscript.ctx != Load ||
-            arg->kind != Constant_kind ||
-            /* TODO: handle other types of slices */
-            slice->kind != Index_kind ||
-            slice->v.Index.value->kind != Constant_kind)
+    if (node->v.Subscript.ctx != Load)
     {
         return 1;
     }
+    ENSURE_CONSTANT(arg);
+    switch (slice->kind) {
+    case Index_kind:
+        ENSURE_CONSTANT(slice->v.Index.value);
+        key = GET_CONSTANT(slice->v.Index.value);
+        newval = PyObject_GetItem(GET_CONSTANT(arg), key);
+        break;
+    case Slice_kind:
+        ENSURE_CONSTANT(slice->v.Slice.lower);
+        ENSURE_CONSTANT(slice->v.Slice.upper);
+        ENSURE_CONSTANT(slice->v.Slice.step);
+        key = PySlice_New(GET_CONSTANT(slice->v.Slice.lower),
+                          GET_CONSTANT(slice->v.Slice.upper),
+                          GET_CONSTANT(slice->v.Slice.step));
+        newval = PyObject_GetItem(GET_CONSTANT(arg), key);
+        Py_DECREF(key);
+        break;
+    default:
+        return 1;
+    }
 
-    idx = slice->v.Index.value;
-    newval = PyObject_GetItem(arg->v.Constant.value, idx->v.Constant.value);
     return make_const(node, newval, arena);
 }