gh-148690: Build Windows freethreaded binaries into separate directory and include python3t.dll on GIL-enabled (GH-149218)

Co-authored-by: Petr Viktorin <encukou@gmail.com>

Author: zooba

Include/pyabi.h

@@ -55,6 +55,8 @@
  *
  *  (Don't use Py_TARGET_ABI3T directly. It's currently only used to set these
  *   2 macros, and defined for users' convenience.)
+ *
+ * This logic is currently partially duplicated in PC/pyconfig.h.
  */
 #if defined(Py_LIMITED_API) && defined(Py_GIL_DISABLED) \
         && !defined(Py_TARGET_ABI3T)

Lib/test/test_cext/__init__.py

@@ -8,6 +8,8 @@
 import shlex
 import shutil
 import subprocess
+import sysconfig
+import sys
 import unittest
 from test import support
 
@@ -62,6 +64,9 @@ def run_cmd(operation, cmd):
                 env['CPYTHON_TEST_LIMITED'] = '1'
             if abi3t:
                 env['CPYTHON_TEST_ABI3T'] = '1'
+            if support.MS_WINDOWS and sysconfig.is_python_build():
+                env['CPYTHON_EXTRA_INCDIRS'] = os.path.split(sysconfig.get_config_h_filename())[0]
+                env['CPYTHON_EXTRA_LIBDIRS'] = os.path.split(sys.executable)[0]
             env['CPYTHON_TEST_EXT_NAME'] = extension_name
             env['TEST_INTERNAL_C_API'] = str(int(self.TEST_INTERNAL_C_API))
             if support.verbose:

Lib/test/test_cext/setup.py

@@ -1,7 +1,6 @@
 # gh-91321: Build a basic C test extension to check that the Python C API is
 # compatible with C and does not emit C compiler warnings.
 import os
-import platform
 import shlex
 import sys
 import sysconfig
@@ -66,6 +65,8 @@ def main():
     limited = bool(os.environ.get("CPYTHON_TEST_LIMITED", ""))
     abi3t = bool(os.environ.get("CPYTHON_TEST_ABI3T", ""))
     internal = bool(int(os.environ.get("TEST_INTERNAL_C_API", "0")))
+    incdirs = os.environ.get("CPYTHON_EXTRA_INCDIRS", "")
+    libdirs = os.environ.get("CPYTHON_EXTRA_LIBDIRS", "")
 
     sources = [SOURCE]
 
@@ -106,19 +107,16 @@ def main():
     if internal:
         cflags.append('-DTEST_INTERNAL_C_API=1')
 
-    # On Windows, add PCbuild\amd64\ to include and library directories
+    # Add additional include and library directories, typically for in-tree
+    # testing where not all directories are inferred
     include_dirs = []
     library_dirs = []
-    if support.MS_WINDOWS:
-        srcdir = sysconfig.get_config_var('srcdir')
-        machine = platform.uname().machine
-        pcbuild = os.path.join(srcdir, 'PCbuild', machine)
-        if os.path.exists(pcbuild):
-            # pyconfig.h is generated in PCbuild\amd64\
-            include_dirs.append(pcbuild)
-            # python313.lib is generated in PCbuild\amd64\
-            library_dirs.append(pcbuild)
-            print(f"Add PCbuild directory: {pcbuild}")
+    if incdirs:
+        print("Add incdirs:", incdirs)
+        include_dirs.extend(incdirs.split(os.pathsep))
+    if libdirs:
+        print("Add libdirs:", libdirs)
+        library_dirs.extend(libdirs.split(os.pathsep))
 
     # Display information to help debugging
     for env_name in ('CC', 'CFLAGS', 'CPPFLAGS'):

Lib/test/test_cppext/__init__.py

@@ -6,6 +6,7 @@
 import shutil
 import subprocess
 import sys
+import sysconfig
 import unittest
 from test import support
 
@@ -50,6 +51,9 @@ def run_cmd(operation, cmd):
                 env['CPYTHON_TEST_CPP_STD'] = std
             if limited:
                 env['CPYTHON_TEST_LIMITED'] = '1'
+            if support.MS_WINDOWS and sysconfig.is_python_build():
+                env['CPYTHON_EXTRA_INCDIRS'] = os.path.split(sysconfig.get_config_h_filename())[0]
+                env['CPYTHON_EXTRA_LIBDIRS'] = os.path.split(sys.executable)[0]
             env['CPYTHON_TEST_EXT_NAME'] = extension_name
             env['TEST_INTERNAL_C_API'] = str(int(self.TEST_INTERNAL_C_API))
             if extra_cflags:

Lib/test/test_cppext/setup.py

@@ -1,7 +1,6 @@
 # gh-91321: Build a basic C++ test extension to check that the Python C API is
 # compatible with C++ and does not emit C++ compiler warnings.
 import os
-import platform
 import shlex
 import sys
 import sysconfig
@@ -48,6 +47,8 @@ def main():
     module_name = os.environ["CPYTHON_TEST_EXT_NAME"]
     limited = bool(os.environ.get("CPYTHON_TEST_LIMITED", ""))
     internal = bool(int(os.environ.get("TEST_INTERNAL_C_API", "0")))
+    incdirs = os.environ.get("CPYTHON_EXTRA_INCDIRS", "")
+    libdirs = os.environ.get("CPYTHON_EXTRA_LIBDIRS", "")
 
     cppflags = list(CPPFLAGS)
     cppflags.append(f'-DMODULE_NAME={module_name}')
@@ -90,19 +91,16 @@ def main():
     if extra_cflags:
         cppflags.extend(shlex.split(extra_cflags))
 
-    # On Windows, add PCbuild\amd64\ to include and library directories
+    # Add additional include and library directories, typically for in-tree
+    # testing where not all directories are inferred
     include_dirs = []
     library_dirs = []
-    if support.MS_WINDOWS:
-        srcdir = sysconfig.get_config_var('srcdir')
-        machine = platform.uname().machine
-        pcbuild = os.path.join(srcdir, 'PCbuild', machine)
-        if os.path.exists(pcbuild):
-            # pyconfig.h is generated in PCbuild\amd64\
-            include_dirs.append(pcbuild)
-            # python313.lib is generated in PCbuild\amd64\
-            library_dirs.append(pcbuild)
-            print(f"Add PCbuild directory: {pcbuild}")
+    if incdirs:
+        print("Add incdirs:", incdirs)
+        include_dirs.extend(incdirs.split(os.pathsep))
+    if libdirs:
+        print("Add libdirs:", libdirs)
+        library_dirs.extend(libdirs.split(os.pathsep))
 
     # Display information to help debugging
     for env_name in ('CC', 'CXX', 'CFLAGS', 'CPPFLAGS', 'CXXFLAGS'):

Lib/venv/__init__.py

@@ -358,6 +358,9 @@ def setup_python(self, context):
                 exe_t = f'3.{sys.version_info[1]}t'
                 python_exe = os.path.join(dirname, f'python{exe_t}{exe_d}.exe')
                 pythonw_exe = os.path.join(dirname, f'pythonw{exe_t}{exe_d}.exe')
+                if not os.path.isfile(python_exe):
+                    python_exe = os.path.join(dirname, f'python{exe_d}.exe')
+                    pythonw_exe = os.path.join(dirname, f'pythonw{exe_d}.exe')
                 link_sources = {
                     'python.exe': python_exe,
                     f'python{exe_d}.exe': python_exe,

Misc/NEWS.d/next/Build/2026-05-01-12-01-54.gh-issue-148690.oTtYk-.rst

@@ -0,0 +1,4 @@
+Windows free-threaded builds now output to a different default path with
+default filenames, for example, ``PCbuild/amd64t/python.exe`` rather than
+``PCbuild/amd64/python3.15t.exe``. The ``PC/layout`` script has been updated
+to ensure compatibility of generated layouts.

Misc/NEWS.d/next/Windows/2026-05-01-12-03-39.gh-issue-148690.TMV8dU.rst

@@ -0,0 +1,3 @@
+Non-freethreaded builds on Windows now support extensions linked to
+``python3t.dll``, and will include a copy of that library in normal installs
+that references the non-freethreaded runtime.

PC/layout/main.py

@@ -127,7 +127,10 @@ def get_tcltk_lib(ns):
 def get_layout(ns):
     def in_build(f, dest="", new_name=None, no_lib=False):
         n, _, x = f.rpartition(".")
-        n = new_name or n
+        if new_name and new_name.endswith(f".{x}"):
+            n = new_name.rpartition(".")[0]
+        else:
+            n = new_name or n
         src = ns.build / f
         if ns.debug and src not in REQUIRED_DLLS:
             if not "_d." in src.name:
@@ -161,11 +164,12 @@ def in_build(f, dest="", new_name=None, no_lib=False):
         source = "python_uwp.exe"
         sourcew = "pythonw_uwp.exe"
     elif ns.include_freethreaded:
-        source = "python{}t.exe".format(VER_DOT)
-        sourcew = "pythonw{}t.exe".format(VER_DOT)
         if not ns.include_alias:
             alias = []
             aliasw = []
+        if (VER_MAJOR, VER_MINOR, VER_MICRO, VER_FIELD4) < (3, 15, 0, 0xB0):
+            source = "python{}t.exe".format(VER_DOT)
+            sourcew = "pythonw{}t.exe".format(VER_DOT)
         alias.extend([
             "python{}t".format(VER_DOT),
             "python{}t".format(VER_MAJOR) if ns.include_alias3 else None,
@@ -196,6 +200,8 @@ def in_build(f, dest="", new_name=None, no_lib=False):
             yield from in_build(FREETHREADED_PYTHON_STABLE_DLL_NAME)
         else:
             yield from in_build(PYTHON_STABLE_DLL_NAME)
+            if (VER_MAJOR, VER_MINOR, VER_MICRO, VER_FIELD4) >= (3, 15, 0, 0xB0):
+                yield from in_build(FREETHREADED_PYTHON_STABLE_DLL_NAME)
 
     found_any = False
     for dest, src in rglob(ns.build, "vcruntime*.dll"):

PC/pyconfig.h

@@ -331,14 +331,16 @@ Py_NO_ENABLE_SHARED to find out.  Also support MS_NO_COREDLL for b/w compat */
 #                       if defined(Py_GIL_DISABLED)
 #                       if defined(Py_DEBUG)
 #                               pragma comment(lib,"python315t_d.lib")
-#                       elif defined(Py_LIMITED_API)
+#                       elif defined(Py_LIMITED_API) || defined(Py_TARGET_ABI3T)
 #                               pragma comment(lib,"python3t.lib")
 #                       else
 #                               pragma comment(lib,"python315t.lib")
 #                       endif /* Py_DEBUG */
 #                       else /* Py_GIL_DISABLED */
 #                       if defined(Py_DEBUG)
 #                               pragma comment(lib,"python315_d.lib")
+#                       elif defined(Py_TARGET_ABI3T)
+#                               pragma comment(lib,"python3t.lib")
 #                       elif defined(Py_LIMITED_API)
 #                               pragma comment(lib,"python3.lib")
 #                       else