feat: add Python 3.15 support

CPython 3.15.0 is final, but requires-python capped the package at <3.15:
pip on 3.15 falls back to diff-diff 2.3.1, and uv builds the sdist. Raise
the cap, add the classifier, build cp315 wheels on every platform and add
3.15 to the Rust-backend CI matrix; the pure-Python fallback and the doc
snippets move to 3.15 as they moved to 3.14 in #284. PyO3 0.29 already
supports 3.15.

Author: ilgrad

.github/workflows/build-wheels.yml

@@ -50,7 +50,7 @@ jobs:
       - name: Build wheels
         run: |
           expected=0
-          for pyver in 39 310 311 312 313 314; do
+          for pyver in 39 310 311 312 313 314 315; do
             pybin="/opt/python/cp${pyver}-cp${pyver}/bin/python"
             if [ ! -f "$pybin" ]; then
               echo "ERROR: Expected Python interpreter not found: $pybin"
@@ -80,7 +80,7 @@ jobs:
     runs-on: macos-14
     strategy:
       matrix:
-        python-version: ['3.9', '3.10', '3.11', '3.12', '3.13', '3.14']
+        python-version: ['3.9', '3.10', '3.11', '3.12', '3.13', '3.14', '3.15']
     steps:
       - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7
 
@@ -113,7 +113,7 @@ jobs:
     runs-on: windows-latest
     strategy:
       matrix:
-        python-version: ['3.9', '3.10', '3.11', '3.12', '3.13', '3.14']
+        python-version: ['3.9', '3.10', '3.11', '3.12', '3.13', '3.14', '3.15']
     steps:
       - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7
 

.github/workflows/docs-tests.yml

@@ -92,10 +92,10 @@ jobs:
       - name: Set up Python
         uses: actions/setup-python@5fda3b95a4ea91299a34e894583c3862153e4b97 # v7.0.0
         with:
-          # 3.14 to mirror Pure Python Fallback (the only existing job
+          # 3.15 to mirror Pure Python Fallback (the only existing job
           # that actually ran these tests). notebooks.yml uses 3.11 for
           # nbmake compat, not relevant here.
-          python-version: '3.14'
+          python-version: '3.15'
 
       - name: Install dependencies
         # Keep in sync with pyproject.toml [project.dependencies] and [project.optional-dependencies.dev]
@@ -248,7 +248,7 @@ jobs:
 
       - name: Run changelog-fragment check on the floor Python
         # The compiler promises 3.9 compatibility but its behavioral tests
-        # run in the 3.14 doc-snippets job; this stdlib-only invocation is
+        # run in the 3.15 doc-snippets job; this stdlib-only invocation is
         # the floor's continuous syntax + behavior smoke for the script.
         run: python .claude/scripts/changelog_compile.py check
 

.github/workflows/release-build-check.yml

@@ -2,7 +2,7 @@ name: Release build check
 
 # Pre-merge guard for the PyPI release build path. publish.yml only runs on
 # `release: published`, so its manylinux container build (checkout@v7 inside
-# glibc-2.28, openblas, py3.9-3.14) is never exercised by PR CI. This calls the
+# glibc-2.28, openblas, py3.9-3.15) is never exercised by PR CI. This calls the
 # SAME reusable workflow build-only (no PyPI upload) so a PR that would break our
 # ability to mint a release fails here instead of at release time.
 #

.github/workflows/rust-test.yml

@@ -131,7 +131,7 @@ jobs:
       fail-fast: false
       matrix:
         os: [ubuntu-latest, macos-latest, windows-latest, ubuntu-24.04-arm]
-        python-version: ['3.11', '3.13', '3.14']
+        python-version: ['3.11', '3.13', '3.14', '3.15']
         include:
           # Python 3.9 floor guard (single Linux leg): mypy >= 2.2 can no
           # longer type-check with python_version = "3.9", so this runtime
@@ -251,7 +251,7 @@ jobs:
       - name: Set up Python
         uses: actions/setup-python@5fda3b95a4ea91299a34e894583c3862153e4b97 # v7.0.0
         with:
-          python-version: '3.14'
+          python-version: '3.15'
 
       - name: Install dependencies
         # Keep in sync with pyproject.toml [project.dependencies] and [project.optional-dependencies.dev]

README.md

@@ -153,7 +153,7 @@ No other Python or R DiD package offers design-based variance estimation for mod
 
 ## Requirements
 
-- Python 3.9 - 3.14
+- Python 3.9 - 3.15
 - numpy >= 1.20
 - pandas >= 1.3
 - scipy >= 1.10

changelog.d/20261011-python-3-15.md

@@ -0,0 +1,6 @@
+### Added
+- **Python 3.15 support**: `requires-python` now allows 3.15 (`<3.16`) and
+  the 3.15 classifier is declared; release wheels are built for CPython 3.15
+  on Linux (x86_64, aarch64), macOS arm64 and Windows, and CI runs the
+  Rust-backend matrix and the pure-Python fallback on 3.15. PyO3 0.29
+  already supports 3.15, so no Rust dependency changes.

pyproject.toml

@@ -8,7 +8,7 @@ version = "3.12.0"
 description = "Difference-in-Differences causal inference with sklearn-like API. Callaway-Sant'Anna, Synthetic DiD, Honest DiD, event studies, parallel trends."
 readme = "README.md"
 license = "MIT"
-requires-python = ">=3.9,<3.15"
+requires-python = ">=3.9,<3.16"
 authors = [
     {name = "diff-diff contributors"}
 ]
@@ -40,6 +40,7 @@ classifiers = [
     "Programming Language :: Python :: 3.12",
     "Programming Language :: Python :: 3.13",
     "Programming Language :: Python :: 3.14",
+    "Programming Language :: Python :: 3.15",
     "Topic :: Scientific/Engineering :: Mathematics",
     "Topic :: Scientific/Engineering :: Information Analysis",
     "Topic :: Scientific/Engineering",

rust/Cargo.toml

@@ -25,7 +25,7 @@ openblas = ["ndarray/blas"]
 alloc-profile = []
 
 [dependencies]
-# PyO3 0.29 supports Python 3.9-3.14
+# PyO3 0.29 supports Python 3.9-3.15
 pyo3 = "0.29"
 numpy = "0.29"
 ndarray = { version = "0.17", features = ["rayon"] }