WeavePy

WeavePy is an experimental high-performance Python interpreter written in Rust, designed to be a 100% compatible, drop-in replacement for CPython. The goal is simple but ambitious: run existing Python code, packages, tools, and workflows unchanged while dramatically improving execution speed, startup time, memory usage, and runtime scalability. WeavePy treats CPython compatibility as the baseline, not a stretch goal — using CPython's own test suite as a guiding standard while exploring a modern Rust-based runtime architecture built for aggressive optimization, native interoperability, and long-term performance work.

Status: drop-in replacement for the documented CPython 3.13 surface, with a measured conformance baseline and a live C-extension entry point. The bundled regression suite at tests/regrtest/ covers the seven semantic groups exercised by RFC 0027 (object model, iterators/ generators/coroutines, numerics/strings/format, containers, exceptions/ context, serialization/compression/codecs, and IO/OS/argparse/inspect/ typing) and is green on main. RFC 0028 adds the PEP 3118 buffer protocol, PEP 590 vectorcall, the full PyType_FromSpec[WithBases] slot surface, and a _ndarray.c C-extension fixture exercising the stack end-to-end. RFC 0029 closes the loop: the datetime C-API, the full PyCapsule surface, keyword-aware PyArg_ParseTupleAndKeywords, property-aware descriptor dispatch in tp_getset, a numpy-shaped _numpylike.c fixture exercising dtype/ufuncs/buffer-protocol/ reshape/mask_select/PyDateTime, a PEP 425 wheel-tag matcher in _minipip (so binary wheels resolve), and an end-to-end regression test that installs a binary wheel under a private prefix and imports the bundled extension through the regular ExtensionFileLoader path — proving the numpy install-and-run story works mechanically. RFC 0030 ships the pure-Python drop-in surface: a real PyPI-compatible pip (PEP 440/503/508/425, dependency resolver, PEP 517 sdist builds, full CLI), a numpy facade with pure-Python fallback so import numpy works without compiling _numpylike, a bundled pytest + pluggy + iniconfig + exceptiongroup stack, and sys.settrace / sys.setprofile / sys.monitoring (PEP 669) + tracemalloc observability so debuggers, coverage tools, and profilers boot. RFC 0031 closes the observability loop: the VM dispatcher actually fires the registered hooks (call / line / return / yield / exception for settrace + setprofile; the PEP 669 event table for sys.monitoring; record_alloc from container- construction opcodes for tracemalloc; PEP 578 audit dispatch at open / compile / exec / eval / import / marshal sites). The same commit lands PEP 684 sub-interpreters (_xxsubinterpreters

• a high-level interpreters frontend with cross-interpreter channels), wires pdb / bdb on top of the now-firing settrace, and grows _pytest to handle @pytest.mark.parametrize Cartesian matrices, indirect fixtures, request.addfinalizer LIFO ordering, and per-scope (function / class / module / session) fixture caching. RFC 0036 wires a real CPython 3.13 Lib/test/ checkout into the regrtest harness (--cpython-dir, crash-isolated --mode subprocess, --jobs) and rewrites the touched rows of tests/regrtest/expectations.toml from guesses to a measured baseline (unexpected 0 on a fresh sweep). RFC 0049 (wave 5) retires the curated allowlist as a scope mechanism: discovery now schedules every vendored test_*.py file and test_*/ package (504 labels, up from 227), and expectations.toml is a measured whole-suite baseline — 226 of the 427 vendored-CPython labels pass under the sweep budget (plus all 77 bundled fixtures), and every red row carries a measured first-failure reason. The same wave landed the SETUP_ANNOTATIONS opcode (block-entry __annotations__, lazy type/module getsets), CPython-strict bool()/__bool__/__len__ semantics, str argument-clinic arity across ~30 native methods, full-mapping-protocol str.format_map, saturating int shift semantics, code-object value equality (code_richcompare), Py_ReprEnter-style recursive-repr guards on dict views and the io stack (fixing two native stack overflows), a CPython-shaped codeop, verbatim configparser, and the six built-in codecs error-handler callables. Expect small breaking changes around the edges as the long tail catches up.

RFC 0033 makes WeavePy introspectable like CPython. It ships a CPython-faithful code-object surface (co_code, co_linetable (PEP 626), co_exceptiontable, co_positions() (PEP 657), co_stacksize, co_qualname, co_lines(), replace(), …) backed by a new cpython_code codec that re-encodes WeavePy's instruction stream into CPython 3.13's 16-bit _Py_CODEUNIT form (EXTENDED_ARG + inline CACHE entries). On top of that it lands the four introspection modules every serious tool reaches for — import ast, import dis, import opcode, import symtable — as frozen Python over thin Rust cores (_ast, _symtable), a marshal that serialises code objects byte-compatibly with CPython 3.13 (TYPE_CODE + FLAG_REF shared refs

• exact 15-bit bigint digits), and real .pyc read/write under __pycache__ using CPython's b"\xf3\r\r\n" magic + PEP 552 header (kept collision-safe by a distinct weavepy-3.13 cache tag). Six bundled regrtests cross-check the whole surface against CPython 3.13.

RFC 0054 lands the async wave: a native _asyncio C-accelerator (the real Future/Task state machines, current_task/all_tasks, eager task factories, cancellation bookkeeping) that CPython's frozen asyncio adopts via its normal import hook, plus an OpenSSL-shaped _ssl over rustls — full getpeercert() X.509→dict parsing, SNI servername callbacks via a two-phase rustls::server::Acceptor handshake, server-side ALPN, session stats, options/verify_flags bitmasks (VERIFY_X509_STRICT, CRL checks), dual RSA/ECC certificate slots, encrypted PKCS#8 keys with password callbacks, per-message handshake callbacks, and TLS 1.3 post-handshake-auth emulation. The vendored test_asyncio package now grades as 31 per-submodule rows — all 31 pass over real loopback sockets, real subprocess transports, and rustls TLS — and the network tail graduates to measured pass rows: test_ssl (191 tests), test_urllib2, test_poplib, joining the already-green httplib/imaplib/ftplib/smtplib/socketserver family.

RFC 0055 is the daily-driver wave: the acceptance bar moves from "passes CPython's tests" to "runs real installed packages". A new ecosystem conformance lane (tests/ecosystem/) creates a scratch venv per manifest row with the WeavePy binary under test, installs real PyPI packages through the in-tree pip (online or fully offline via a wheel cache), runs a behaviour-asserting probe, and grades against a checked-in baseline — all nine launch rows pass: six, attrs, click, jinja2, requests, python-dateutil, typing_extensions, packaging, and real pytest (8.4). Getting there landed the mypyc C-API tail (charset_normalizer's compiled .so loads and runs), dependency-closure resolution in pip install --no-index --find-links, the 3.11+ importlib.resources package layout (.abc, .readers, NamespaceLoader), int-subclass SystemExit payloads (sys.exit(pytest.ExitCode.OK)), and site-packages precedence over the bundled third-party facades — pip install packaging (or pytest, numpy, …) now actually changes what import packaging returns. The same wave finished the CLI/REPL residuals (test_cmd_line, test_repl, test_cmd_line_script all pass).

RFC 0056 is the modern-stack wave: a faithful _sqlite3 over rusqlite + verbatim Lib/sqlite3, real vendored expat behind pyexpat, the ctypes/mock/warnings residual burns, an abi3/PyO3 surface audit (_abi3check), and an ecosystem matrix grown from 9 to 27 rows — all pass, offline from the wheel cache. Capstone: Django migrates against the new sqlite3 backend, runs ORM CRUD + atomic() rollback, and serves a request through django.test.Client. Net regrtest flips include test_sqlite3, test_dbm_sqlite3, the XML family (test_pyexpat/test_sax/test_minidom/test_pulldom/ test_plistlib/test_xml_etree*), test_htmlparser, test_unittest/test_doctest/test_warnings, and test_compileall.

RFC 0057 is the long-tail wave: the measured whole-suite baseline moves from 418 to 496 of 543 Lib/test files passing (+78 net flips, zero timeout rows, unexpected 0), with the ecosystem lane still 27/27 offline. The wave lands the comprehension-scope root-cause fix (the test_listcomps/test_dictcomps/test_setcomps/ test_named_expressions quartet flips), exception args as a real slot, the slot-descriptor error taxonomy, compile() from AST with the PyCF_* flags (test_ast residual: 169F/80E → 1F), frozen module specs + AppleFrameworkLoader (test_import/test_types now run end-to-end), a _decimal that passes the decTest corpus (test_decimal is a measured pass row), pickle protocol 5 with out-of-band PickleBuffer round-trips (test_pickle/ test_picklebuffer/test_pickletools all pass), CPython-faithful pattern-match codegen + jump threading for trace-event exactness, and the retirement of every timeout row (test_deque/test_mmap/ test_weakref pass under measured budgets). The re-baseline itself caught two engine bugs: a greedy TLS shutdown drain that could eat post-close_notify plaintext under load (intermittent test_ssl STARTTLS deadlock), and a datetime_CAPI stand-in shadowing the real capsule (segfaulting any extension doing PyDateTime_IMPORT, e.g. orjson) — both fixed and re-measured.

RFC 0060 is the conformance endgame wave: the measured whole-suite baseline moves to 515 of 548 files passing (fail 27, error 0, skip 6, timeout 0, unexpected 0; +14 net flips), and the ecosystem lane grows to 29/29 with two capstones — pandas (binary wheel, real numpy underneath) and FastAPI (pydantic v2 routes through TestClient). The wave lands the _testcapi/_testinternalcapi fixture surface (vectorcall fixture types un-gate test_call's matrix, dict watchers, rare-event counters, frame probes, normalize_path, and an instruction-sequence assemble stage — test_compiler_assemble runs CPython-3.13 pseudo-op streams through to executable code objects), a Python-constructible types.CodeType, the full blake2/sha3/shake constructor surface, sys.orig_argv over the WTF-8 argv bridge, PEP 578 audit-hook blocking semantics, and retires the test.libregrtest shim for the verbatim package. The capstones caught real engine bugs — a PyType_FromMetaclass NULL-tp_alloc segfault, Cython's PyType_CheckExact rejecting the _ImmutableTypeMeta metaclass (retired for a truthful Py_TPFLAGS_IMMUTABLETYPE), zoneinfo restructured as a real package, str-subclass __slots__, and a CALL_FUNCTION_EX kwargs clone that defeated prompt reaping (an SSLContext leak under asyncio timeouts). Flips include test_call, test_hashlib, test_re, test_ast, test_builtin, test_frame, test_zoneinfo, and the fixture-gated introspection rows; the honestly-enumerated remainder (the test_compile/test_peepholer codegen-stage cluster, test_capi's fixture fractal, and the unboxed-value identity legs of test_marshal) carries measured reasons in expectations.toml.

Repository layout

This is a Cargo workspace organized along the classical interpreter pipeline. Each crate owns one phase of execution and depends only on the phases before it, so implementation work in any layer can proceed mostly in isolation.

weavepy/
├── Cargo.toml                  # workspace root (shared metadata, deps, lints)
├── rust-toolchain.toml         # pinned to stable + rustfmt + clippy
├── rustfmt.toml                # formatting rules
├── .cargo/config.toml          # workspace cargo aliases
├── crates/
│   ├── weavepy-lexer/          # Python source -> tokens
│   ├── weavepy-parser/         # tokens -> AST (re-exports the AST module)
│   ├── weavepy-compiler/       # AST   -> bytecode (CodeObject + opcodes)
│   ├── weavepy-vm/             # bytecode interpreter + object model
│   ├── weavepy/                # umbrella library: public Rust embedding API
│   ├── weavepy-cli/            # the `weavepy` binary, argv-compatible with `python`
│   └── weavepy-conformance/    # CPython-as-oracle harness (dev-only, not on crates.io)
├── conformance/
│   └── corpus/                 # in-tree Python fixtures graded against CPython
├── docs/
│   ├── ARCHITECTURE.md         # design overview + open questions
│   ├── CONFORMANCE.md          # how WeavePy is graded against CPython
│   └── rfcs/                   # design documents
└── .github/workflows/ci.yml    # fmt + clippy + tests on Linux/macOS/Windows + conformance

Building

WeavePy targets stable Rust. The toolchain is pinned via rust-toolchain.toml, so a fresh rustup install will pick up the right channel automatically.

# Build everything.
cargo build --workspace

# Run the test suite.
cargo test --workspace

# Lint and format checks (matches CI).
cargo fmt --all -- --check
cargo clippy --workspace --all-targets --all-features -- -D warnings

# Convenience aliases (defined in .cargo/config.toml).
cargo xtest
cargo xclippy

Running

The CLI binary is named weavepy and aims to be argv-compatible with python.

# Inline source (mirrors `python -c`).
cargo run -p weavepy-cli -- -c "print('hello, weavepy')"

# Run a script file.
cargo run -p weavepy-cli -- path/to/script.py

# Print the version (mirrors `python -V`).
cargo run -p weavepy-cli -- --version

CPython conformance

Compatibility is graded automatically. The weavepy-conformance crate runs the host's python3 as an oracle (tokenize, ast.parse + ast.dump, compile + dis.dis) and reports per-phase agreement on a corpus of Python fixtures. CI runs the harness on every PR and uploads the report as an artifact.

cargo run -p weavepy-conformance -- run            # all phases
cargo run -p weavepy-conformance -- diff tokens    # one phase

# End-to-end: run CPython's own Lib/test/ files under WeavePy and grade
# against the measured baseline (RFC 0036).
cargo run -p weavepy-conformance -- regrtest \
    --cpython-dir vendor/cpython/Lib/test --mode subprocess --jobs 8

# Ecosystem lane: venv + pip install + probe per real PyPI package,
# graded against tests/ecosystem/expectations.toml (RFC 0055).
cargo run -p weavepy-conformance -- ecosystem                # online
python3 tools/ecosystem_fetch.py --dest target/ecosystem-wheels
cargo run -p weavepy-conformance -- ecosystem \
    --wheels target/ecosystem-wheels                         # offline

See docs/CONFORMANCE.md for the model, the corpus layout, and the now-live CPython regrtest-style runner (RFC 0034 built it; RFC 0036 wired a real CPython 3.13 checkout into the CLI).

Project goals

1. Compatibility first. CPython's behavior — including dark corners, PEP 8 grammar minutiae, and the reference C-API — is the spec. The CPython test suite is the acceptance harness. Performance work that breaks compatibility is rejected.
2. Performance second, but seriously. Once a feature is correct, the architecture should make it fast: tiered execution, inline caches, specialization, and a JIT are all on the long-term roadmap.
3. Modern, safe foundation. Written in safe Rust where possible, with unsafe confined to small, well-audited boundaries (object header layout, FFI to native extensions, etc.).
4. Embeddable. The weavepy crate is a library first; the weavepy CLI is just one consumer.

Contributing

See CONTRIBUTING.md for development setup, coding standards, and how to propose larger changes via the RFC process in docs/rfcs/.

License

WeavePy is dual-licensed under either of:

• Apache License, Version 2.0 (LICENSE-APACHE)
• MIT License (LICENSE-MIT)

at your option. This matches the rest of the Rust ecosystem, so contributions to and from common Rust crates remain straightforward.

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in WeavePy by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.