py/objslice: Make slice attributes (start/stop/step) readable.

Configurable with MICROPY_PY_BUILTINS_SLICE_ATTRS.  Disabled by default.

py/mpconfig.h

@@ -463,6 +463,12 @@ typedef double mp_float_t;
 #define MICROPY_PY_BUILTINS_SLICE (1)
 #endif
 
+// Whether to support slice attribute read access,
+// i.e. slice.start, slice.stop, slice.step
+#ifndef MICROPY_PY_BUILTINS_SLICE_ATTRS
+#define MICROPY_PY_BUILTINS_SLICE_ATTRS (0)
+#endif
+
 // Whether to support frozenset object
 #ifndef MICROPY_PY_BUILTINS_FROZENSET
 #define MICROPY_PY_BUILTINS_FROZENSET (0)

py/objslice.c

@@ -57,10 +57,30 @@ STATIC void slice_print(const mp_print_t *print, mp_obj_t o_in, mp_print_kind_t
     mp_print_str(print, ")");
 }
 
+#if MICROPY_PY_BUILTINS_SLICE_ATTRS
+STATIC void slice_attr(mp_obj_t self_in, qstr attr, mp_obj_t *dest) {
+    if (dest[0] != MP_OBJ_NULL) {
+        // not load attribute
+        return;
+    }
+    mp_obj_slice_t *self = self_in;
+    if (attr == MP_QSTR_start) {
+        dest[0] = self->start;
+    } else if (attr == MP_QSTR_stop) {
+        dest[0] = self->stop;
+    } else if (attr == MP_QSTR_step) {
+        dest[0] = self->step;
+    }
+}
+#endif
+
 const mp_obj_type_t mp_type_slice = {
     { &mp_type_type },
     .name = MP_QSTR_slice,
     .print = slice_print,
+#if MICROPY_PY_BUILTINS_SLICE_ATTRS
+	.attr = slice_attr,
+#endif
 };
 
 mp_obj_t mp_obj_new_slice(mp_obj_t ostart, mp_obj_t ostop, mp_obj_t ostep) {

stmhal/mpconfigport.h

@@ -64,6 +64,7 @@
 #define MICROPY_PY_BUILTINS_EXECFILE (1)
 #define MICROPY_PY_MICROPYTHON_MEM_INFO (1)
 #define MICROPY_PY_ARRAY_SLICE_ASSIGN (1)
+#define MICROPY_PY_BUILTINS_SLICE_ATTRS (1)
 #define MICROPY_PY_SYS_EXIT         (1)
 #define MICROPY_PY_SYS_MAXSIZE      (1)
 #define MICROPY_PY_SYS_STDFILES     (1)

tests/basics/slice_attrs.py

@@ -0,0 +1,16 @@
+# test builtin slice attributes access
+
+# print slice attributes
+class A:
+    def __getitem__(self, idx):
+        print(idx.start, idx.stop, idx.step)
+
+try:
+    t = A()[1:2]
+except:
+    import sys
+    print("SKIP")
+    sys.exit()
+
+
+A()[1:2:3]

unix/mpconfigport.h

@@ -73,6 +73,7 @@
 #define MICROPY_PY_MICROPYTHON_MEM_INFO (1)
 #define MICROPY_PY_ALL_SPECIAL_METHODS (1)
 #define MICROPY_PY_ARRAY_SLICE_ASSIGN (1)
+#define MICROPY_PY_BUILTINS_SLICE_ATTRS (1)
 #define MICROPY_PY_SYS_EXIT         (1)
 #if defined(__APPLE__) && defined(__MACH__)
     #define MICROPY_PY_SYS_PLATFORM  "darwin"