frame: support importing under sub-interpreters - #340
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Python 3.12 won't load single-phase C extensions inside a sub-interpreter. Convert _frame to multi-phase init so it imports there, and declare it safe to run with a per-interpreter GIL. _frame keeps no shared global state — each compression/decompression context is allocated per call — so this is safe. No API change, and older Python versions are unaffected.
This was referenced Jul 27, 2026
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What
Python 3.12+ won't import single-phase C extensions inside a sub-interpreter.
This converts
_frameto multi-phase init so it imports there, and declares itsafe to run with a per-interpreter GIL.
Part of #345 - multi-phase init for the
_framemodule (see also #341_block, #342_stream, #343_version).Why it's safe
_framekeeps no shared global state: each compression/decompression context isallocated per call and freed with the object it's wrapped in, and there are no
custom module globals or static types. The bundled liblz4 is reentrant. So the
module is safe even in fully isolated interpreters.
Compatibility
No API change. The new declaration is guarded for Python 3.12+, so builds on
older Python are unaffected.
Scope
This covers the
framesubmodule only.block,stream, and_versionarestill single-phase, so
import lz4as a whole doesn't work under asub-interpreter yet — those are follow-ups. This PR makes the
_frameextensionitself compatible, which is the foundation for the rest.
Testing
tests/frame/passes (12,587 tests)._frameimports and round-trips inside an isolated, own-GILsub-interpreter (checked with both the
_xxsubinterpretersAPI and a C harnessthat enables the multi-interpreter check).
Refs: PEP 489, Isolating Extension Modules