gh-149671: Restore compatibility with setuptools nspkg.pth files in site module

Inject "sitedir" variable in the frame which executes pth code.

Author: vstinner

Lib/site.py

@@ -254,6 +254,7 @@ class StartupState:
         '_path_entries',
         '_importexecs',
         '_entrypoints',
+        '_sitedir',
     )
 
     def __init__(self, known_paths=None):
@@ -286,6 +287,7 @@ def __init__(self, known_paths=None):
         # when an entry fails.
         self._importexecs = {}
         self._entrypoints = {}
+        self._sitedir = None
 
     def addsitedir(self, sitedir):
         """Add a site directory and accumulate its .pth and .start startup data.
@@ -397,6 +399,8 @@ def _read_pth_file(self, sitedir, name):
         accepted paths are added to it so that subsequent .pth files in
         the same batch don't add them more than once.
         """
+        self._sitedir = sitedir
+
         lines, filename = _read_pthstart_file(sitedir, name, ".pth")
         if lines is None:
             return
@@ -500,6 +504,11 @@ def _extend_syspath(self):
                 )
 
     def _exec_imports(self):
+        # Inject 'sitedir' local variable in the current frame for
+        # compatibility with Python 3.14. Especially, "-nspkg.pth" files
+        # generated by setuptools use: sys._getframe(1).f_locals['sitedir'].
+        sitedir = self._sitedir
+
         # For each `import` line we've seen in a .pth file, exec() it in
         # order, unless the .pth has a matching .start file in this same
         # batch.  In that case, PEP 829 says the import lines are

Lib/test/test_site.py

@@ -233,6 +233,22 @@ def test_addsitedir_hidden_file_attribute(self):
             self.assertNotIn(site.makepath(pth_file.good_dir_path)[0], sys.path)
             self.assertIn(pth_file.base_dir, sys.path)
 
+    def test_sitedir_variable(self):
+        # gh-149671: Provide 'sitedir' local variable for compatibility with
+        # Python 3.14
+        custom_path = '/my/custom/path'
+        code = '; '.join((
+            "import sys",
+            # Code used by "-nspkg.pth" files generated by setuptools
+            "sitedir = sys._getframe(1).f_locals['sitedir']",
+            "print(sitedir)",
+        ))
+        pth_dir, pth_fn = self.make_pth(code)
+        with support.captured_stdout() as stdout:
+            known_paths = site.addpackage(pth_dir, pth_fn, set())
+        sitedir = stdout.getvalue().rstrip()
+        self.assertEqual(sitedir, pth_dir)
+
     # This tests _getuserbase, hence the double underline
     # to distinguish from a test for getuserbase
     def test__getuserbase(self):

Misc/NEWS.d/next/Library/2026-06-11-11-52-23.gh-issue-149671.6Rpr5r.rst

@@ -0,0 +1,3 @@
+Restore compatibility with setuptools ``nspkg.pth`` files in the :mod:`site`
+module. Inject ``sitedir`` variable in the frame which executes pth code.
+Patch by Victor Stinner.