Message 276083 - Python tracker (original) (raw)

Xiang: Preserving insertion order in shared-key dict is possible, but it's hard. If we allow it, we should add tons of test for shared-key dict.

I'll describe why it's harder than you think.

Current implementation allow insertion to split table only when:

/* When insertion order is different from shared key, we can't share
 * the key anymore.  Convert this instance to combine table.
 */
if (_PyDict_HasSplitTable(mp) &&
    ((ix >= 0 && *value_addr == NULL && mp->ma_used != ix) ||
     (ix == DKIX_EMPTY && mp->ma_used != mp->ma_keys->dk_nentries))) {
    if (insertion_resize(mp) < 0) {

ix >= 0 && *value_addr == NULL means it's pending slot. mp->ma_used == ix ensure insertion order. And if we allow deletion, this check will be broken.

For example:

a, b = C() a.a, a.b, a.c = 1, 2, 3 # shared-key order b.a, b.b, b.c = 1, 2, 3 # same order, no problem

del b.a # mp->ma_used = 2 del b.b # mp->ma_used = 1

b.b = 42 # It's OK because mp->ma_used == ix == 1. Keep sharing.

assert(list(b.dict.keys()) == ["c", "b"]) # AssertionError! Actual order is [(PENDING "a",) "b", "c"]