objlist: Implement support for arbitrary (3-arg) slices.

py/obj.h

@@ -579,6 +579,7 @@ bool mp_seq_cmp_bytes(int op, const byte *data1, uint len1, const byte *data2, u
 bool mp_seq_cmp_objs(int op, const mp_obj_t *items1, uint len1, const mp_obj_t *items2, uint len2);
 mp_obj_t mp_seq_index_obj(const mp_obj_t *items, uint len, uint n_args, const mp_obj_t *args);
 mp_obj_t mp_seq_count_obj(const mp_obj_t *items, uint len, mp_obj_t value);
+mp_obj_t mp_seq_extract_slice(uint len, const mp_obj_t *seq, mp_bound_slice_t *indexes);
 // Helper to clear stale pointers from allocated, but unused memory, to preclude GC problems
 #define mp_seq_clear(start, len, alloc_len, item_sz) memset((byte*)(start) + (len) * (item_sz), 0, ((alloc_len) - (len)) * (item_sz))
 #define mp_seq_replace_slice_no_grow(dest, dest_len, beg, end, slice, slice_len, item_t) \

py/objarray.c

@@ -178,7 +178,8 @@ STATIC mp_obj_t array_subscr(mp_obj_t self_in, mp_obj_t index_in, mp_obj_t value
             }
             mp_bound_slice_t slice;
             if (!mp_seq_get_fast_slice_indexes(o->len, index_in, &slice)) {
-                assert(0);
+                nlr_raise(mp_obj_new_exception_msg(&mp_type_NotImplementedError,
+                    "Only slices with step=1 (aka None) are supported"));
             }
             mp_obj_array_t *res = array_new(o->typecode, slice.stop - slice.start);
             int sz = mp_binary_get_size('@', o->typecode, NULL);

py/objlist.c

@@ -178,7 +178,7 @@ STATIC mp_obj_t list_subscr(mp_obj_t self_in, mp_obj_t index, mp_obj_t value) {
         if (MP_OBJ_IS_TYPE(index, &mp_type_slice)) {
             mp_bound_slice_t slice;
             if (!mp_seq_get_fast_slice_indexes(self->len, index, &slice)) {
-                assert(0);
+                return mp_seq_extract_slice(self->len, self->items, &slice);
             }
             mp_obj_list_t *res = list_new(slice.stop - slice.start);
             mp_seq_copy(res->items, self->items + slice.start, res->len, mp_obj_t);

py/objstr.c

@@ -353,7 +353,8 @@ STATIC mp_obj_t str_subscr(mp_obj_t self_in, mp_obj_t index, mp_obj_t value) {
         if (MP_OBJ_IS_TYPE(index, &mp_type_slice)) {
             mp_bound_slice_t slice;
             if (!mp_seq_get_fast_slice_indexes(self_len, index, &slice)) {
-                assert(0);
+                nlr_raise(mp_obj_new_exception_msg(&mp_type_NotImplementedError,
+                    "Only slices with step=1 (aka None) are supported"));
             }
             return str_new(type, self_data + slice.start, slice.stop - slice.start);
         }

py/objtuple.c

@@ -165,7 +165,8 @@ mp_obj_t mp_obj_tuple_subscr(mp_obj_t self_in, mp_obj_t index, mp_obj_t value) {
         if (MP_OBJ_IS_TYPE(index, &mp_type_slice)) {
             mp_bound_slice_t slice;
             if (!mp_seq_get_fast_slice_indexes(self->len, index, &slice)) {
-                assert(0);
+                nlr_raise(mp_obj_new_exception_msg(&mp_type_NotImplementedError,
+                    "Only slices with step=1 (aka None) are supported"));
             }
             mp_obj_tuple_t *res = mp_obj_new_tuple(slice.stop - slice.start, NULL);
             mp_seq_copy(res->items, self->items + slice.start, res->len, mp_obj_t);

py/sequence.c

@@ -95,8 +95,6 @@ bool mp_seq_get_fast_slice_indexes(machine_uint_t len, mp_obj_t slice, mp_bound_
     indexes->stop = stop;
 
     if (ostep != mp_const_none && ostep != MP_OBJ_NEW_SMALL_INT(1)) {
-        nlr_raise(mp_obj_new_exception_msg(&mp_type_NotImplementedError,
-            "Only slices with step=1 (aka None) are supported"));
         indexes->step = MP_OBJ_SMALL_INT_VALUE(ostep);
         return false;
     }
@@ -104,6 +102,27 @@ bool mp_seq_get_fast_slice_indexes(machine_uint_t len, mp_obj_t slice, mp_bound_
     return true;
 }
 
+mp_obj_t mp_seq_extract_slice(uint len, const mp_obj_t *seq, mp_bound_slice_t *indexes) {
+    machine_int_t start = indexes->start, stop = indexes->stop;
+    machine_int_t step = indexes->step;
+
+    mp_obj_t res = mp_obj_new_list(0, NULL);
+
+    if (step < 0) {
+        stop--;
+        while (start <= stop) {
+            mp_obj_list_append(res, seq[stop]);
+            stop += step;
+        }
+    } else {
+        while (start < stop) {
+            mp_obj_list_append(res, seq[start]);
+            start += step;
+        }
+    }
+    return res;
+}
+
 // Special-case comparison function for sequences of bytes
 // Don't pass MP_BINARY_OP_NOT_EQUAL here
 bool mp_seq_cmp_bytes(int op, const byte *data1, uint len1, const byte *data2, uint len2) {

tests/basics/list_slice_3arg.py

+x = list(range(10))
+print(x[::-1])
+print(x[::2])
+print(x[::-2])