py/objslice: Ensure slice is not hashable.

As per https://bugs.python.org/issue408326, the slice object should not be
hashable.  Since MicroPython has an implicit fallback when the unary_op
slot is empty, we need to fill this slot.

Signed-off-by: David Lechner <david@pybricks.com>

py/objslice.c

@@ -46,6 +46,12 @@ STATIC void slice_print(const mp_print_t *print, mp_obj_t o_in, mp_print_kind_t
     mp_print_str(print, ")");
 }
 
+STATIC mp_obj_t slice_unary_op(mp_unary_op_t op, mp_obj_t o_in) {
+    // Needed to explicitly opt out of default __hash__.
+    // REVISIT: CPython implements comparison operators for slice.
+    return MP_OBJ_NULL;
+}
+
 #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);
@@ -104,6 +110,7 @@ MP_DEFINE_CONST_OBJ_TYPE(
     mp_type_slice,
     MP_QSTR_slice,
     MP_TYPE_FLAG_NONE,
+    unary_op, slice_unary_op,
     SLICE_TYPE_ATTR_OR_LOCALS_DICT
     print, slice_print
     );

tests/basics/slice_op.py

@@ -0,0 +1,15 @@
+
+try:
+    t = [][:]
+except:
+    print("SKIP")
+    raise SystemExit
+
+
+# REVISIT: slice comparison operators are not implemented in MicroPython
+
+# test that slice is not hashable, i.e. it can't be used to copy a dict
+try:
+    {}[:] = {}
+except TypeError:
+    print('TypeError')