refactor: use CPython macros to construct PYBIND11_VERSION_HEX (#5683)

* refactor: use CPython macros to construct `PYBIND11_VERSION_HEX`

* docs: update release guide

* tests: add test to keep version values in sync

Signed-off-by: Henry Schreiner <henryschreineriii@gmail.com>

* style: pre-commit fixes

* test: update version test

* test: update version test

* test: update version test

* chore: update code comments

* Update docs/release.rst

---------

Signed-off-by: Henry Schreiner <henryschreineriii@gmail.com>
Co-authored-by: Henry Schreiner <henryschreineriii@gmail.com>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

docs/release.rst

@@ -10,11 +10,13 @@ For example:
 
     #define PYBIND11_VERSION_MAJOR X
     #define PYBIND11_VERSION_MINOR Y
+    #define PYBIND11_VERSION_MICRO Z
+    #define PYBIND11_VERSION_RELEASE_LEVEL PY_RELEASE_LEVEL_ALPHA
+    #define PYBIND11_VERSION_RELEASE_SERIAL 0
     #define PYBIND11_VERSION_PATCH Za0
 
 For beta, ``PYBIND11_VERSION_PATCH`` should be ``Zb1``. RC's can be ``Zrc1``.
-For a final release, this must be a simple integer. There is also
-``PYBIND11_VERSION_HEX`` just below that needs to be updated.
+For a final release, this must be a simple integer.
 
 
 To release a new version of pybind11:
@@ -26,9 +28,7 @@ If you don't have nox, you should either use ``pipx run nox`` instead, or use
 - Update the version number
 
   - Update ``PYBIND11_VERSION_MAJOR`` etc. in
-    ``include/pybind11/detail/common.h``. PATCH should be a simple integer.
-
-  - Update ``PYBIND11_VERSION_HEX`` just below as well.
+    ``include/pybind11/detail/common.h``. MICRO should be a simple integer.
 
   - Run ``nox -s tests_packaging`` to ensure this was done correctly.
 

include/pybind11/detail/common.h

@@ -14,13 +14,35 @@
 #    error "PYTHON < 3.8 IS UNSUPPORTED. pybind11 v2.13 was the last to support Python 3.7."
 #endif
 
+// Similar to Python's convention: https://docs.python.org/3/c-api/apiabiversion.html
+// See also: https://github.com/python/cpython/blob/HEAD/Include/patchlevel.h
+/* -- start version constants -- */
 #define PYBIND11_VERSION_MAJOR 3
 #define PYBIND11_VERSION_MINOR 0
+#define PYBIND11_VERSION_MICRO 0
+// ALPHA = 0xA, BETA = 0xB, GAMMA = 0xC (release candidate), FINAL = 0xF (stable release)
+// - The release level is set to "alpha" for development versions.
+//   Use 0xA0 (LEVEL=0xA, SERIAL=0) for development versions.
+// - For stable releases, set the serial to 0.
+#define PYBIND11_VERSION_RELEASE_LEVEL PY_RELEASE_LEVEL_GAMMA
+#define PYBIND11_VERSION_RELEASE_SERIAL 1
+// String version of (micro, release level, release serial), e.g.: 0a0, 0b1, 0rc1, 0
 #define PYBIND11_VERSION_PATCH 0rc1
+/* -- end version constants -- */
 
-// Similar to Python's convention: https://docs.python.org/3/c-api/apiabiversion.html
-// Use 0xA0 for dev
-#define PYBIND11_VERSION_HEX 0x030000C1
+#if !defined(Py_PACK_FULL_VERSION)
+// Stable API since Python 3.14.0a4
+#    define Py_PACK_FULL_VERSION(X, Y, Z, LEVEL, SERIAL)                                          \
+        ((((X) & 0xff) << 24) | (((Y) & 0xff) << 16) | (((Z) & 0xff) << 8)                        \
+         | (((LEVEL) & 0xf) << 4) | (((SERIAL) & 0xf) << 0))
+#endif
+// Version as a single 4-byte hex number, e.g. 0x030C04B5 == 3.12.4b5.
+#define PYBIND11_VERSION_HEX                                                                      \
+    Py_PACK_FULL_VERSION(PYBIND11_VERSION_MAJOR,                                                  \
+                         PYBIND11_VERSION_MINOR,                                                  \
+                         PYBIND11_VERSION_MICRO,                                                  \
+                         PYBIND11_VERSION_RELEASE_LEVEL,                                          \
+                         PYBIND11_VERSION_RELEASE_SERIAL)
 
 #include "pybind11_namespace_macros.h"
 

tests/extra_python_package/test_files.py

@@ -347,3 +347,36 @@ def tests_build_global_wheel(monkeypatch, tmpdir):
 
     pkgconfig_expected = PKGCONFIG.format(VERSION=simple_version)
     assert pkgconfig_expected == pkgconfig
+
+
+def test_version_matches():
+    header = MAIN_DIR / "include/pybind11/detail/common.h"
+    text = header.read_text()
+
+    # Extract the relevant macro values
+    regex_prefix = r"#\s*define\s+PYBIND11_VERSION_"
+    micro = re.search(rf"{regex_prefix}MICRO\s+(\d+)\b", text).group(1)
+    release_level = re.search(rf"{regex_prefix}RELEASE_LEVEL\s+(\w+)\b", text).group(1)
+    release_serial = re.search(
+        rf"{regex_prefix}RELEASE_SERIAL\s+(\d+)\b",
+        text,
+    ).group(1)
+    patch = re.search(rf"{regex_prefix}PATCH\s+([\w.-]+)\b", text).group(1)
+
+    # Map release level macro to string
+    level_map = {
+        "PY_RELEASE_LEVEL_ALPHA": "a",
+        "PY_RELEASE_LEVEL_BETA": "b",
+        "PY_RELEASE_LEVEL_GAMMA": "rc",
+        "PY_RELEASE_LEVEL_FINAL": "",
+    }
+    level_str = level_map[release_level]
+
+    if release_level == "PY_RELEASE_LEVEL_FINAL":
+        assert level_str == ""
+        assert release_serial == "0"
+        expected_patch = micro
+    else:
+        expected_patch = f"{micro}{level_str}{release_serial}"
+
+    assert patch == expected_patch