fix: install into free-threaded python3.14t/site-packages prefixes (#911)

* fix: support free-threaded python3.14t/site-packages prefixes

* fix: append sys.abiflags to e.g. python3.14 in env.sh

Author: wdconinc

cmake/podioBuild.cmake

@@ -214,13 +214,27 @@ endif()
 # Setup the python install dir. See the discussion in
 # https://github.com/AIDASoft/podio/pull/599 for more details on why this is
 # done the way it is
+#
+# For Python 3.13+ with free-threading (PEP 703), the site-packages directory
+# includes a 't' suffix (e.g., python3.14t/site-packages). We use Python's
+# own SITEARCH to get the correct path including any ABI suffixes.
+#
 set(podio_python_lib_dir lib)
 if("${Python3_SITEARCH}" MATCHES "/lib64/")
   set(podio_python_lib_dir lib64)
 endif()
 
+# Extract the python-specific part of the path (e.g., python3.14t/site-packages)
+# from Python3_SITEARCH
+string(REGEX MATCH "python[0-9]+\\.[0-9]+[a-z]*/site-packages" _python_site_subdir "${Python3_SITEARCH}")
+if(NOT _python_site_subdir)
+  # Fallback to manual construction if regex fails
+  set(_python_site_subdir "python${Python3_VERSION_MAJOR}.${Python3_VERSION_MINOR}/site-packages")
+  message(WARNING "Could not extract site-packages path from Python3_SITEARCH (${Python3_SITEARCH}), using fallback: ${_python_site_subdir}")
+endif()
+
 set(podio_PYTHON_INSTALLDIR
-  "${CMAKE_INSTALL_PREFIX}/${podio_python_lib_dir}/python${Python3_VERSION_MAJOR}.${Python3_VERSION_MINOR}/site-packages"
+  "${CMAKE_INSTALL_PREFIX}/${podio_python_lib_dir}/${_python_site_subdir}"
   CACHE STRING
   "The install prefix for the python bindings and the generator and templates"
 )

env.sh

@@ -25,7 +25,7 @@ elif [[ "$unamestr" = 'Darwin' ]]; then
     fi
 fi
 
-python_stem=python3.$(python -c 'import sys; print(sys.version_info[1])')/site-packages
+python_stem=python3.$(python -c 'import sys; print(str(sys.version_info[1]) + sys.abiflags )')/site-packages
 if [ -d $PODIO/lib64/${python_stem} ]; then
   if ! echo $PYTHONPATH | grep -o $PODIO/lib64/${python_stem} > /dev/null 2>&1; then
     export PYTHONPATH=$PODIO/lib64/${python_stem}:$PYTHONPATH