objslice: Support arbitrary objects start, stop, and step.

Older int-only encoding is not expressive enough to support arbitrary slice
assignment operations.

py/obj.h

@@ -517,7 +517,7 @@ mp_map_t *mp_obj_dict_get_map(mp_obj_t self_in);
 void mp_obj_set_store(mp_obj_t self_in, mp_obj_t item);
 
 // slice
-void mp_obj_slice_get(mp_obj_t self_in, machine_int_t *start, machine_int_t *stop, machine_int_t *step);
+void mp_obj_slice_get(mp_obj_t self_in, mp_obj_t *start, mp_obj_t *stop, mp_obj_t *step);
 
 // array
 uint mp_obj_array_len(mp_obj_t self_in);

py/objslice.c

@@ -62,13 +62,20 @@ const mp_obj_ellipsis_t mp_const_ellipsis_obj = {{&mp_type_ellipsis}};
 // CPython supports 3rd arg (step), plus args can be arbitrary Python objects.
 typedef struct _mp_obj_slice_t {
     mp_obj_base_t base;
-    machine_int_t start;
-    machine_int_t stop;
+    mp_obj_t start;
+    mp_obj_t stop;
+    mp_obj_t step;
 } mp_obj_slice_t;
 
 void slice_print(void (*print)(void *env, const char *fmt, ...), void *env, mp_obj_t o_in, mp_print_kind_t kind) {
     mp_obj_slice_t *o = o_in;
-    print(env, "slice(" INT_FMT ", " INT_FMT ")", o->start, o->stop);
+    print(env, "slice(");
+    mp_obj_print_helper(print, env, o->start, PRINT_REPR);
+    print(env, ", ");
+    mp_obj_print_helper(print, env, o->stop, PRINT_REPR);
+    print(env, ", ");
+    mp_obj_print_helper(print, env, o->step, PRINT_REPR);
+    print(env, ")");
 }
 
 const mp_obj_type_t mp_type_slice = {
@@ -77,39 +84,21 @@ const mp_obj_type_t mp_type_slice = {
     .print = slice_print,
 };
 
-// TODO: Make sure to handle "empty" values, which are signified by None in CPython
 mp_obj_t mp_obj_new_slice(mp_obj_t ostart, mp_obj_t ostop, mp_obj_t ostep) {
-    assert(ostep == NULL);
-    machine_int_t start = 0, stop = 0;
-    if (ostart != mp_const_none) {
-        start = mp_obj_get_int(ostart);
-    }
-    if (ostop != mp_const_none) {
-        stop = mp_obj_get_int(ostop);
-        if (stop == 0) {
-            // [x:0] is a special case - in our slice object, stop = 0 means
-            // "end of sequence". Fortunately, [x:0] is an empty seqence for
-            // any x (including negative). [x:x] is also always empty sequence.
-            // but x also can be 0. But note that b""[x:x] is b"" for any x (i.e.
-            // no IndexError, at least in Python 3.3.3). So, we just use -1's to
-            // signify that. -1 is catchy "special" number in case someone will
-            // try to print [x:0] slice ever.
-            start = stop = -1;
-        }
-    }
-    mp_obj_slice_t *o = m_new(mp_obj_slice_t, 1);
+    mp_obj_slice_t *o = m_new_obj(mp_obj_slice_t);
     o->base.type = &mp_type_slice;
-    o->start = start;
-    o->stop = stop;
-    return (mp_obj_t)o;
+    o->start = ostart;
+    o->stop = ostop;
+    o->step = ostep;
+    return o;
 }
 
-void mp_obj_slice_get(mp_obj_t self_in, machine_int_t *start, machine_int_t *stop, machine_int_t *step) {
+void mp_obj_slice_get(mp_obj_t self_in, mp_obj_t *start, mp_obj_t *stop, mp_obj_t *step) {
     assert(MP_OBJ_IS_TYPE(self_in, &mp_type_slice));
     mp_obj_slice_t *self = self_in;
     *start = self->start;
     *stop = self->stop;
-    *step = 1;
+    *step = self->step;
 }
 
 #endif

py/objstr.c

@@ -355,6 +355,9 @@ STATIC mp_obj_t str_subscr(mp_obj_t self_in, mp_obj_t index, mp_obj_t value) {
             if (!mp_seq_get_fast_slice_indexes(self_len, index, &start, &stop)) {
                 assert(0);
             }
+            if (start >= stop) {
+                return MP_OBJ_NEW_QSTR(MP_QSTR_);
+            }
             return str_new(type, self_data + start, stop - start);
         }
 #endif

py/sequence.c

@@ -52,12 +52,24 @@ void mp_seq_multiply(const void *items, uint item_sz, uint len, uint times, void
 }
 
 bool mp_seq_get_fast_slice_indexes(machine_uint_t len, mp_obj_t slice, machine_uint_t *begin, machine_uint_t *end) {
-    machine_int_t start, stop, step;
-    mp_obj_slice_get(slice, &start, &stop, &step);
-    if (step != 1) {
+    mp_obj_t ostart, ostop, ostep;
+    machine_int_t start, stop;
+    mp_obj_slice_get(slice, &ostart, &ostop, &ostep);
+    if (ostep != mp_const_none && ostep != MP_OBJ_NEW_SMALL_INT(1)) {
         return false;
     }
 
+    if (ostart == mp_const_none) {
+        start = 0;
+    } else {
+        start = MP_OBJ_SMALL_INT_VALUE(ostart);
+    }
+    if (ostop == mp_const_none) {
+        stop = len;
+    } else {
+        stop = MP_OBJ_SMALL_INT_VALUE(ostop);
+    }
+
     // Unlike subscription, out-of-bounds slice indexes are never error
     if (start < 0) {
         start = len + start;
@@ -67,7 +79,7 @@ bool mp_seq_get_fast_slice_indexes(machine_uint_t len, mp_obj_t slice, machine_u
     } else if (start > len) {
         start = len;
     }
-    if (stop <= 0) {
+    if (stop < 0) {
         stop = len + stop;
         // CPython returns empty sequence in such case
         if (stop < 0) {

py/vm.c

@@ -768,12 +768,12 @@ unwind_jump:
                     if (unum == 2) {
                         obj2 = POP();
                         obj1 = TOP();
-                        SET_TOP(mp_obj_new_slice(obj1, obj2, NULL));
+                        SET_TOP(mp_obj_new_slice(obj1, obj2, mp_const_none));
                     } else {
-                        obj1 = mp_obj_new_exception_msg(&mp_type_NotImplementedError, "3-argument slice is not supported");
-                        nlr_pop();
-                        fastn[0] = obj1;
-                        return MP_VM_RETURN_EXCEPTION;
+                        mp_obj_t obj3 = POP();
+                        obj2 = POP();
+                        obj1 = TOP();
+                        SET_TOP(mp_obj_new_slice(obj1, obj2, obj3));
                     }
                     DISPATCH();
 #endif