Enable support for free-threading (#1295)

This PR:
1. marks the `libcst.native` module as free-threading-compatible
2. replaces the use of ProcessPoolExecutor with ThreadPoolExecutor if free-threaded CPython is detected at runtime

Author: zsol

.github/workflows/ci.yml

@@ -13,8 +13,13 @@ jobs:
       fail-fast: false
       matrix:
         os: [macos-latest, ubuntu-latest, windows-latest]
-        python-version: ["3.9", "3.10", "3.11", "3.12", "3.13"]
+        python-version: ["3.9", "3.10", "3.11", "3.12", "3.13", "3.13t"]
     steps:
+      - uses: actions/setup-python@v5
+        with:
+          python-version: "3.10"
+      - name: Install hatch
+        run: pip install -U hatch
       - uses: actions/checkout@v4
         with:
           fetch-depth: 0
@@ -24,9 +29,6 @@ jobs:
           cache: pip
           cache-dependency-path: "pyproject.toml"
           python-version: ${{ matrix.python-version }}
-      - name: Install hatch
-        run: |
-          pip install -U hatch
       - uses: actions-rs/toolchain@v1
         with:
           toolchain: stable
@@ -44,50 +46,6 @@ jobs:
           hatch run coverage combine .coverage.pure
           hatch run coverage report
 
-  # TODO:
-  # merge into regular CI once hatch has support for creating environments on
-  # the free-threaded build: https://github.com/pypa/hatch/issues/1931
-  free-threaded-tests:
-    name: "test (${{ matrix.os }}, 3.13t)"
-    runs-on: ${{ matrix.os }}
-    strategy:
-      fail-fast: false
-      matrix:
-        os: [macos-latest, ubuntu-latest, windows-latest]
-    steps:
-      - uses: actions/checkout@v4
-        with:
-          fetch-depth: 0
-          persist-credentials: false
-      - uses: actions/setup-python@v5
-        with:
-          cache: pip
-          cache-dependency-path: "pyproject.toml"
-          python-version: '3.13t'
-      - name: Build LibCST
-        run: |
-          # Install build-system.requires dependencies
-          pip install setuptools setuptools-scm setuptools-rust wheel
-          # Jupyter is annoying to install on free-threaded Python
-          pip install -e .[dev-without-jupyter]
-      - name: Native Parser Tests
-        # TODO: remove when native modules declare free-threaded support
-        env:
-          PYTHON_GIL: '0'
-        run: |
-          python -m coverage run -m libcst.tests
-      - name: Pure Parser Tests
-        env:
-          COVERAGE_FILE: .coverage.pure
-          LIBCST_PARSER_TYPE: pure
-        run: |
-          python -m coverage run -m libcst.tests
-      - name: Coverage
-        run: |
-          python -m coverage combine .coverage.pure
-          python -m coverage report
-
-
   # Run linters
   lint:
     runs-on: ubuntu-latest
@@ -139,7 +97,7 @@ jobs:
       - name: Install hatch
         run: pip install -U hatch
       - uses: ts-graphviz/setup-graphviz@v2
-      - run: hatch run docs
+      - run: hatch run docs:docs
       - name: Archive Docs
         uses: actions/upload-artifact@v4
         with:

libcst/codemod/_cli.py

@@ -8,18 +8,19 @@
 """
 
 import difflib
+import functools
 import os.path
 import re
 import subprocess
 import sys
 import time
 import traceback
-from concurrent.futures import as_completed, Executor, ProcessPoolExecutor
+from concurrent.futures import as_completed, Executor
 from copy import deepcopy
 from dataclasses import dataclass
 from multiprocessing import cpu_count
 from pathlib import Path
-from typing import AnyStr, cast, Dict, List, Optional, Sequence, Type, Union
+from typing import AnyStr, Callable, cast, Dict, List, Optional, Sequence, Type, Union
 from warnings import warn
 
 from libcst import parse_module, PartialParserConfig
@@ -624,14 +625,20 @@ def parallel_exec_transform_with_prettyprint(  # noqa: C901
         python_version=python_version,
     )
 
-    pool_impl: type[Executor]
+    pool_impl: Callable[[], Executor]
     if total == 1 or jobs == 1:
         # Simple case, we should not pay for process overhead.
         # Let's just use a dummy synchronous executor.
         jobs = 1
         pool_impl = DummyExecutor
+    elif getattr(sys, "_is_gil_enabled", lambda: False)():  # pyre-ignore[16]
+        from concurrent.futures import ThreadPoolExecutor
+
+        pool_impl = functools.partial(ThreadPoolExecutor, max_workers=jobs)
     else:
-        pool_impl = ProcessPoolExecutor
+        from concurrent.futures import ProcessPoolExecutor
+
+        pool_impl = functools.partial(ProcessPoolExecutor, max_workers=jobs)
         # Warm the parser, pre-fork.
         parse_module(
             "",
@@ -650,7 +657,7 @@ def parallel_exec_transform_with_prettyprint(  # noqa: C901
         deepcopy(transform.context.scratch) if isinstance(transform, Codemod) else {}
     )
 
-    with pool_impl(max_workers=jobs) as executor:  # type: ignore
+    with pool_impl() as executor:  # type: ignore
         try:
             futures = [
                 executor.submit(

libcst/codemod/_dummy_pool.py

@@ -22,9 +22,6 @@ class DummyExecutor(Executor):
     Synchronous dummy `concurrent.futures.Executor` analogue.
     """
 
-    def __init__(self, max_workers: Optional[int] = None) -> None:
-        pass
-
     def submit(
         self,
         fn: Callable[Params, Return],

native/libcst/src/py.rs

@@ -6,7 +6,7 @@
 use crate::nodes::traits::py::TryIntoPy;
 use pyo3::prelude::*;
 
-#[pymodule]
+#[pymodule(gil_used = false)]
 #[pyo3(name = "native")]
 pub fn libcst_native(_py: Python, m: &Bound<PyModule>) -> PyResult<()> {
     #[pyfn(m)]

pyproject.toml

@@ -18,33 +18,9 @@ classifiers = [
     "Typing :: Typed",
 ]
 requires-python = ">=3.9"
-dependencies = ["pyyaml>=5.2"]
-
-[project.optional-dependencies]
-dev = [
-    "libcst[dev-without-jupyter]",
-    "jupyter>=1.0.0",
-    "nbsphinx>=0.4.2",
-]
-dev-without-jupyter = [
-    "black==25.1.0",
-    "coverage[toml]>=4.5.4",
-    "build>=0.10.0",
-    "fixit==2.1.0",
-    "flake8==7.2.0",
-    "Sphinx>=5.1.1",
-    "hypothesis>=4.36.0",
-    "hypothesmith>=0.0.4",
-    "maturin>=1.7.0,<1.8",
-    "prompt-toolkit>=2.0.9",
-    "pyre-check==0.9.18; platform_system != 'Windows'",
-    "setuptools_scm>=6.0.1",
-    "sphinx-rtd-theme>=0.4.3",
-    "ufmt==2.8.0",
-    "usort==1.0.8.post1",
-    "setuptools-rust>=1.5.2",
-    "slotscheck>=0.7.1",
-    "jinja2==3.1.6",
+dependencies = [
+    "pyyaml>=5.2; python_version < '3.13'",
+    "pyyaml-ft; python_version >= '3.13'",
 ]
 
 [project.urls]
@@ -63,10 +39,26 @@ show_missing = true
 skip_covered = true
 
 [tool.hatch.envs.default]
-features = ["dev"]
+installer = "uv"
+dependencies = [
+    "black==25.1.0",
+    "coverage[toml]>=4.5.4",
+    "build>=0.10.0",
+    "fixit==2.1.0",
+    "flake8==7.2.0",
+    "hypothesis>=4.36.0",
+    "hypothesmith>=0.0.4",
+    "maturin>=1.7.0,<1.8",
+    "prompt-toolkit>=2.0.9",
+    "pyre-check==0.9.18; platform_system != 'Windows'",
+    "setuptools_scm>=6.0.1",
+    "ufmt==2.8.0",
+    "usort==1.0.8.post1",
+    "setuptools-rust>=1.5.2",
+    "slotscheck>=0.7.1",
+]
 
 [tool.hatch.envs.default.scripts]
-docs = "sphinx-build -ab html docs/source docs/build"
 fixtures = ["python scripts/regenerate-fixtures.py", "git diff --exit-code"]
 format = "ufmt format libcst scripts"
 lint = [
@@ -78,6 +70,17 @@ lint = [
 test = ["python --version", "python -m coverage run -m libcst.tests"]
 typecheck = ["pyre --version", "pyre check"]
 
+[tool.hatch.envs.docs]
+extra-dependencies = [
+    "Sphinx>=5.1.1",
+    "sphinx-rtd-theme>=0.4.3",
+    "jupyter>=1.0.0",
+    "nbsphinx>=0.4.2",
+    "jinja2==3.1.6",
+]
+[tool.hatch.envs.docs.scripts]
+docs = "sphinx-build -ab html docs/source docs/build"
+
 [tool.slotscheck]
 exclude-modules = '^libcst\.(testing|tests)'