diff --git a/py/sequence.c b/py/sequence.c index 0acdd25be0..bc2cfc077c 100644 --- a/py/sequence.c +++ b/py/sequence.c @@ -53,15 +53,35 @@ bool mp_seq_get_fast_slice_indexes(mp_uint_t len, mp_obj_t slice, mp_bound_slice mp_int_t start, stop; mp_obj_slice_get(slice, &ostart, &ostop, &ostep); + if (ostep != mp_const_none && ostep != MP_OBJ_NEW_SMALL_INT(1)) { + indexes->step = mp_obj_get_int(ostep); + if (indexes->step == 0) { + mp_raise_ValueError("slice step cannot be zero"); + } + } else { + indexes->step = 1; + } + if (ostart == mp_const_none) { - start = 0; + if (indexes->step > 0) { + start = 0; + } else { + start = len - 1; + } } else { start = mp_obj_get_int(ostart); } if (ostop == mp_const_none) { - stop = len; + if (indexes->step > 0) { + stop = len; + } else { + stop = 0; + } } else { stop = mp_obj_get_int(ostop); + if (stop >= 0 && indexes->step < 0) { + stop += 1; + } } // Unlike subscription, out-of-bounds slice indexes are never error @@ -70,29 +90,31 @@ bool mp_seq_get_fast_slice_indexes(mp_uint_t len, mp_obj_t slice, mp_bound_slice if (start < 0) { start = 0; } - } else if ((mp_uint_t)start > len) { + } else if (indexes->step > 0 && (mp_uint_t)start > len) { start = len; + } else if (indexes->step < 0 && (mp_uint_t)start > len - 1) { + start = len - 1; } if (stop < 0) { stop = len + stop; + if (indexes->step < 0) { + stop += 1; + } } else if ((mp_uint_t)stop > len) { stop = len; } // CPython returns empty sequence in such case, or point for assignment is at start - if (start > stop) { + if (indexes->step > 0 && start > stop) { stop = start; + } else if (indexes->step < 0 && start < stop) { + stop = start + 1; } indexes->start = start; indexes->stop = stop; - if (ostep != mp_const_none && ostep != MP_OBJ_NEW_SMALL_INT(1)) { - indexes->step = mp_obj_get_int(ostep); - return false; - } - indexes->step = 1; - return true; + return indexes->step == 1; } #endif @@ -106,10 +128,9 @@ mp_obj_t mp_seq_extract_slice(mp_uint_t len, const mp_obj_t *seq, mp_bound_slice 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; + while (start >= stop) { + mp_obj_list_append(res, seq[start]); + start += step; } } else { while (start < stop) { diff --git a/tests/basics/list_slice_3arg.py b/tests/basics/list_slice_3arg.py index b98ca3e4f2..8578d5855e 100644 --- a/tests/basics/list_slice_3arg.py +++ b/tests/basics/list_slice_3arg.py @@ -7,3 +7,22 @@ x = list(range(9)) print(x[::-1]) print(x[::2]) print(x[::-2]) + +x = list(range(5)) +print(x[:0:-1]) +print(x[:1:-1]) +print(x[:2:-1]) +print(x[0::-1]) +print(x[1::-1]) +print(x[2::-1]) + +x = list(range(5)) +print(x[0:0:-1]) +print(x[4:4:-1]) +print(x[5:5:-1]) + +x = list(range(10)) +print(x[-1:-1:-1]) +print(x[-1:-2:-1]) +print(x[-1:-11:-1]) +print(x[-10:-11:-1])