gh-106045: Keep searching from venv home when the resolved base executable has no stdlib

Lib/test/test_getpath.py

@@ -475,6 +475,109 @@ def test_venv_symlink_with_misleading_landmarks_posix(self):
         actual = getpath(ns, expected)
         self.assertEqual(expected, actual)
 
+    def test_venv_symlink_to_launcher_outside_runtime_posix(self):
+        "Test a venv whose base executable resolves away from its stdlib."
+        ns = MockPosixNamespace(
+            argv0="/venv/bin/python",
+            PREFIX="/usr",
+        )
+        ns.add_known_xfile("/out/launcher")
+        ns.add_known_xfile("/runtime/bin/python")
+        ns.add_known_link("/runtime/bin/python", "/out/launcher")
+        ns.add_known_xfile("/venv/bin/python")
+        ns.add_known_link("/venv/bin/python", "/runtime/bin/python")
+        ns.add_known_file("/runtime/lib/python9.8/os.py")
+        ns.add_known_dir("/runtime/lib/python9.8/lib-dynload")
+        ns.add_known_file("/venv/pyvenv.cfg", [
+            "home = /runtime/bin",
+        ])
+        expected = dict(
+            executable="/venv/bin/python",
+            prefix="/venv",
+            exec_prefix="/venv",
+            base_executable="/out/launcher",
+            base_prefix="/runtime",
+            base_exec_prefix="/runtime",
+            module_search_paths_set=1,
+            module_search_paths=[
+                "/runtime/lib/python98.zip",
+                "/runtime/lib/python9.8",
+                "/runtime/lib/python9.8/lib-dynload",
+            ],
+        )
+        actual = getpath(ns, expected)
+        self.assertEqual(expected, actual)
+
+    def test_venv_symlink_to_separate_exec_prefix_posix(self):
+        "Test a venv whose base executable lives under a separate exec_prefix."
+        ns = MockPosixNamespace(
+            argv0="/venv/bin/python",
+            PREFIX="/usr",
+            EXEC_PREFIX="/usr",
+        )
+        ns.add_known_xfile("/exec/bin/python")
+        ns.add_known_xfile("/runtime/bin/python")
+        ns.add_known_link("/runtime/bin/python", "/exec/bin/python")
+        ns.add_known_xfile("/venv/bin/python")
+        ns.add_known_link("/venv/bin/python", "/runtime/bin/python")
+        ns.add_known_file("/runtime/lib/python9.8/os.py")
+        ns.add_known_dir("/exec/lib/python9.8/lib-dynload")
+        ns.add_known_file("/venv/pyvenv.cfg", [
+            "home = /runtime/bin",
+        ])
+        expected = dict(
+            executable="/venv/bin/python",
+            prefix="/venv",
+            exec_prefix="/venv",
+            base_executable="/exec/bin/python",
+            base_prefix="/runtime",
+            base_exec_prefix="/exec",
+            module_search_paths_set=1,
+            module_search_paths=[
+                "/runtime/lib/python98.zip",
+                "/runtime/lib/python9.8",
+                "/exec/lib/python9.8/lib-dynload",
+            ],
+        )
+        actual = getpath(ns, expected)
+        self.assertEqual(expected, actual)
+
+    def test_venv_separate_exec_prefix_with_misleading_home_posix(self):
+        "Test that a stale lib-dynload near home does not override the base executable."
+        ns = MockPosixNamespace(
+            argv0="/venv/bin/python",
+            PREFIX="/stdlib",
+            EXEC_PREFIX="/build/exec",
+        )
+        ns.add_known_xfile("/exec/bin/python")
+        ns.add_known_xfile("/public/bin/python")
+        ns.add_known_link("/public/bin/python", "/exec/bin/python")
+        ns.add_known_xfile("/venv/bin/python")
+        ns.add_known_link("/venv/bin/python", "/public/bin/python")
+        ns.add_known_file("/stdlib/lib/python9.8/os.py")
+        ns.add_known_dir("/exec/lib/python9.8/lib-dynload")
+        # An empty directory near the symlink must not win.
+        ns.add_known_dir("/public/lib/python9.8/lib-dynload")
+        ns.add_known_file("/venv/pyvenv.cfg", [
+            "home = /public/bin",
+        ])
+        expected = dict(
+            executable="/venv/bin/python",
+            prefix="/venv",
+            exec_prefix="/venv",
+            base_executable="/exec/bin/python",
+            base_prefix="/stdlib",
+            base_exec_prefix="/exec",
+            module_search_paths_set=1,
+            module_search_paths=[
+                "/stdlib/lib/python98.zip",
+                "/stdlib/lib/python9.8",
+                "/exec/lib/python9.8/lib-dynload",
+            ],
+        )
+        actual = getpath(ns, expected)
+        self.assertEqual(expected, actual)
+
     def test_venv_non_installed_zip_path_posix(self):
         "Test a venv created from non-installed python has correct zip path."""
         ns = MockPosixNamespace(

Misc/NEWS.d/next/Core_and_Builtins/2026-10-22-12-30-11.gh-issue-106045.fkMW34.rst

@@ -0,0 +1,5 @@
+Fix :mod:`venv` path discovery when the base executable resolves to a
+location outside the installation pointed to by the ``home`` key in
+``pyvenv.cfg``. The standard library is now found from ``home``, and
+extension modules from the resolved executable's installation, instead of
+falling back to the compiled-in prefix.

Modules/getpath.py

@@ -232,6 +232,7 @@ def search_up(prefix, *landmarks, test=isfile):
 stdlib_dir_was_set_in_config = bool(stdlib_dir)
 
 real_executable_dir = None
+base_executable_dir = None
 platstdlib_dir = None
 stdlib_zip = None
 
@@ -412,7 +413,17 @@ def search_up(prefix, *landmarks, test=isfile):
                 if base_executable and isfile(base_executable):
                     # Search relative to the resolved base executable rather
                     # than the potentially external symlink in pyvenv.cfg.
-                    executable_dir = real_executable_dir = dirname(base_executable)
+                    # If nothing there locates a standard library (e.g. a
+                    # launcher binary stored outside the installation that
+                    # 'home' points into), keep searching from 'home'.
+                    # base_executable_dir is still used to find platform
+                    # dependent libraries (exec_prefix) in that case.
+                    base_executable_dir = dirname(base_executable)
+                    if (
+                        isfile(joinpath(base_executable_dir, BUILDDIR_TXT))
+                        or search_up(base_executable_dir, ZIP_LANDMARK, *STDLIB_LANDMARKS)
+                    ):
+                        executable_dir = real_executable_dir = base_executable_dir
             # home key found; stop iterating over lines
             break
 
@@ -622,6 +633,11 @@ def search_up(prefix, *landmarks, test=isfile):
             exec_prefix = prefix
         if not exec_prefix and prefix and isdir(joinpath(prefix, PLATSTDLIB_LANDMARK)):
             exec_prefix = prefix
+        if not exec_prefix and base_executable_dir and base_executable_dir != executable_dir:
+            # The venv's resolved base executable may live under a separate
+            # exec_prefix (e.g. ./configure --exec-prefix) even when the
+            # stdlib was found from 'home'.
+            exec_prefix = search_up(base_executable_dir, PLATSTDLIB_LANDMARK, test=isdir)
         if not exec_prefix and executable_dir:
             exec_prefix = search_up(executable_dir, PLATSTDLIB_LANDMARK, test=isdir)
         if not exec_prefix and EXEC_PREFIX: