Unpacking a sub-iterable with * in a generator expression (PEP 798) now delegates to the sub-iterable using yield from semantics, rather than re-yielding each value with a manual loop.

Sync generator expressions

(*sub for sub in subs) now forwards send() and throw() to the sub-iterator currently being unpacked, and discards the sub-iterator's return value:

>>> def sub():
...     while True:
...         print("sub got", (yield "value"))
>>> g = (*sub() for _ in range(1))
>>> next(g)
'value'
>>> g.send(42)
sub got 42
'value'

Asynchronous generator expressions

* unpacking is synchronous (per PEP 448), so the sub-iterable is always a sync iterable. But it is delegated to from inside an async generator, where each yielded value must be wrapped as an async-generator value — otherwise the async-generator machinery treats it as an awaited value. The throw path is the subtle case: _gen_throw forwards throws to the sub in the runtime and returns the result bypassing any bytecode-level wrap (and that same path is shared with await/async for passthrough, which must stay unwrapped).

To handle this, a small internal iterator _PyAsyncGenUnpack wraps the sync iterable so that every value it produces — via __next__, am_send, and throw — is wrapped, while the return value and exceptions pass through unwrapped. It is constructed via a new INTRINSIC_ASYNC_GEN_UNPACK intrinsic emitted only in the async-generator case. As a result asend(), athrow() and aclose() are forwarded to the sub-iterator's send(), throw() and close().

No async yield from / PEP 828 syntax is introduced; this is purely about delegating PEP 798 unpacking.

Notes

• The PYC_MAGIC_NUMBER and the managed static type count are bumped (new helper type + changed codegen).
• This makes 3.16 generator-expression unpacking semantically differ from 3.15 (which shipped the re-yield behaviour). It may be worth considering for 3.15 as well.

Tests

New unittest.TestCase classes in Lib/test/test_unpack_ex.py cover sync send/throw/close forwarding and async asend/athrow/aclose forwarding. The async tests drive the generators by hand (no IsolatedAsyncioTestCase/asyncio), matching the convention in test_asyncgen.py.

🤖 Generated with Claude Code

• Issue: Implement PEP 798 – Unpacking in Comprehensions #143055