gh-158856: Avoid a linear scan per member when creating an Enum - #158857

Summary

Every time a member is added, _proto_member.__set_name__ runs value not in enum_class._hashable_values_. That's a list, so each new member scans every member before it, and building the class is O(n²).

The scan is only needed when the value was already in _value2member_map_, which happens for aliases and Flag pseudo-members. If setdefault just added the key, the value can't be in the list yet, because the list only ever gets values that were added to the map, and nothing removes from the map. So I compare the map's size before and after and only scan when it didn't grow:

-enum_class._value2member_map_.setdefault(value, enum_member)
-if value not in enum_class._hashable_values_:
+size = len(value2member_map)
+value2member_map.setdefault(value, enum_member)
+if (len(value2member_map) != size
+        or value not in enum_class._hashable_values_):
     enum_class._hashable_values_.append(value)

It's the same symptom as gh-89580, which was fixed back then; this scan was added later.

Evidence

Enum('Generated', {f'M{i:08x}': i for i in range(n)}), 3.16 dev build, macOS arm64, median of 5, two alternating runs:

New test test_hashable_values_creation_work counts __eq__ calls while building a 200-member enum:

• Before: AssertionError: 19900 not less than 800
• After: passes

test_hashable_values_with_aliases checks that aliases still don't add duplicates to _hashable_values_.

_hashable_values_ is identical before and after for all 70 enum classes created when importing ssl, signal, socket, re, http, inspect, uuid, ast and a few more. test_enum: 1,093 run, all pass. test_signal, test_http_cookies, test_re, test_ssl, test_inspect, test_pydoc and test_socket show the same 6 failures with and without the patch; they're local-environment failures in the CLI and online-docs tests.

Note: Enum('X', ['A', 'B', ...]) (a list of names, so auto values) is still quadratic, because _create_ copies last_values and _generate_next_value_ sorts it for every member. That's a public hook that people override, so I've left it alone here.

Merge Danger

Door: two-way

Three lines in one private code path. _hashable_values_ ends up with the same contents in the same order.

Blast Radius: stdlib

Every Enum class goes through this code. The only way behaviour could change is a value whose __eq__ and __hash__ disagree, which already breaks _value2member_map_ lookups.

AI disclosure: the profiling, the patch and the equivalence check were done with AI tools (Codex, Claude).

🤖 Generated with Claude Code