py/objslice: Add support for indices() method on slice objects.
Instances of the slice class are passed to __getitem__() on objects when
the user indexes them with a slice. In practice the majority of the time
(other than passing it on untouched) is to work out what the slice means in
the context of an array dimension of a particular length. Since Python 2.3
there has been a method on the slice class, indices(), that takes a
dimension length and returns the real start, stop and step, accounting for
missing or negative values in the slice spec. This commit implements such
a indices() method on the slice class.
It is configurable at compile-time via MICROPY_PY_BUILTINS_SLICE_INDICES,
disabled by default, enabled on unix, stm32 and esp32 ports.
This commit also adds new tests for slice indices and for slicing unicode
strings.
diff --git a/py/objslice.c b/py/objslice.c
index cfc819e..d17dbf6 100644
--- a/py/objslice.c
+++ b/py/objslice.c
@@ -27,7 +27,7 @@
#include <stdlib.h>
#include <assert.h>
-#include "py/obj.h"
+#include "py/runtime.h"
/******************************************************************************/
/* slice object */
@@ -53,6 +53,22 @@
mp_print_str(print, ")");
}
+#if MICROPY_PY_BUILTINS_SLICE_INDICES
+STATIC mp_obj_t slice_indices(mp_obj_t self_in, mp_obj_t length_obj) {
+ mp_int_t length = mp_obj_int_get_checked(length_obj);
+ mp_bound_slice_t bound_indices;
+ mp_obj_slice_indices(self_in, length, &bound_indices);
+
+ mp_obj_t results[3] = {
+ MP_OBJ_NEW_SMALL_INT(bound_indices.start),
+ MP_OBJ_NEW_SMALL_INT(bound_indices.stop),
+ MP_OBJ_NEW_SMALL_INT(bound_indices.step),
+ };
+ return mp_obj_new_tuple(3, results);
+}
+STATIC MP_DEFINE_CONST_FUN_OBJ_2(slice_indices_obj, slice_indices);
+#endif
+
#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) {
@@ -60,22 +76,37 @@
return;
}
mp_obj_slice_t *self = MP_OBJ_TO_PTR(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;
+ #if MICROPY_PY_BUILTINS_SLICE_INDICES
+ } else if (attr == MP_QSTR_indices) {
+ dest[0] = MP_OBJ_FROM_PTR(&slice_indices_obj);
+ dest[1] = self_in;
+ #endif
}
}
#endif
+#if MICROPY_PY_BUILTINS_SLICE_INDICES && !MICROPY_PY_BUILTINS_SLICE_ATTRS
+STATIC const mp_rom_map_elem_t slice_locals_dict_table[] = {
+ { MP_ROM_QSTR(MP_QSTR_indices), MP_ROM_PTR(&slice_indices_obj) },
+};
+STATIC MP_DEFINE_CONST_DICT(slice_locals_dict, slice_locals_dict_table);
+#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,
+#elif MICROPY_PY_BUILTINS_SLICE_INDICES
+ .locals_dict = (mp_obj_dict_t*)&slice_locals_dict,
#endif
};
@@ -96,4 +127,69 @@
*step = self->step;
}
+// Return the real index and step values for a slice when applied to a sequence of
+// the given length, resolving missing components, negative values and values off
+// the end of the sequence.
+void mp_obj_slice_indices(mp_obj_t self_in, mp_int_t length, mp_bound_slice_t *result) {
+ mp_obj_slice_t *self = MP_OBJ_TO_PTR(self_in);
+ mp_int_t start, stop, step;
+
+ if (self->step == mp_const_none) {
+ step = 1;
+ } else {
+ step = mp_obj_get_int(self->step);
+ if (step == 0) {
+ mp_raise_ValueError("slice step cannot be zero");
+ }
+ }
+
+ if (step > 0) {
+ // Positive step
+ if (self->start == mp_const_none) {
+ start = 0;
+ } else {
+ start = mp_obj_get_int(self->start);
+ if (start < 0) {
+ start += length;
+ }
+ start = MIN(length, MAX(start, 0));
+ }
+
+ if (self->stop == mp_const_none) {
+ stop = length;
+ } else {
+ stop = mp_obj_get_int(self->stop);
+ if (stop < 0) {
+ stop += length;
+ }
+ stop = MIN(length, MAX(stop, 0));
+ }
+ } else {
+ // Negative step
+ if (self->start == mp_const_none) {
+ start = length - 1;
+ } else {
+ start = mp_obj_get_int(self->start);
+ if (start < 0) {
+ start += length;
+ }
+ start = MIN(length - 1, MAX(start, -1));
+ }
+
+ if (self->stop == mp_const_none) {
+ stop = -1;
+ } else {
+ stop = mp_obj_get_int(self->stop);
+ if (stop < 0) {
+ stop += length;
+ }
+ stop = MIN(length - 1, MAX(stop, -1));
+ }
+ }
+
+ result->start = start;
+ result->stop = stop;
+ result->step = step;
+}
+
#endif