frozenset({...}) freezes a set that is still referenced #158490

Bug description:

Since gh-150027, frozenset() of a set comprehension freezes the temporary set in place (make_frozenset() in Python/intrinsics.c), even when something else still holds it. A trace function that keeps the return value of a comprehension in a class annotation is enough:

import sys

kept = []
def tracer(frame, event, arg):
    if event == 'return' and isinstance(arg, set):
        kept.append(arg)
    return tracer

class C:
    x: frozenset({i for i in range(3)})

sys.settrace(tracer)
ann = C.__annotations__
sys.settrace(None)
print(type(kept[0]).__name__, kept[0] is ann['x'])

3.15 prints set False. A release build of main prints frozenset True: the set the tracer kept has become the returned frozenset. A debug build aborts:

python: Python/intrinsics.c:215: make_frozenset: Assertion `_PyObject_IsUniquelyReferenced(set)' failed.

Stepping into such an annotation with pdb's s command aborts a debug build the same way.

Expected: the kept object stays a set and frozenset() returns a new object, as in 3.15.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Output from running 'python -VV' on the command line:

Python 3.16.0a0 (heads/fix-frozenset-shared-set:77c06751571, Sep 30 2026, 13:07:14) [GCC 15.2.0]

Linked PRs

• gh-158490: Don't freeze a shared set in the frozenset() fast path #158492