gh-154160: Make sys.executable absolute when resolved from a relative PATH entry

When the interpreter is started by a bare program name and found through a
relative directory in PATH, getpath left sys.executable as that relative path,
contrary to its documented behaviour of being an absolute path. This is a
regression from the getpath.c rewrite in 3.11.

Absolutize the executable resolved from PATH, matching the sibling branch that
already calls abspath() when the program name itself contains a separator.

Author: fedonman

Lib/test/test_getpath.py

@@ -290,6 +290,32 @@ def test_normal_posix(self):
         actual = getpath(ns, expected)
         self.assertEqual(expected, actual)
 
+    def test_relative_path_env_posix(self):
+        "Resolving the executable from a relative PATH entry yields an absolute path (gh-154160)"
+        ns = MockPosixNamespace(
+            PREFIX="/usr",
+            argv0="python",
+            ENV_PATH="usr/bin",
+        )
+        ns.add_known_xfile("usr/bin/python")
+        ns.add_known_xfile("/Absolute/usr/bin/python")
+        ns.add_known_file("/usr/lib/python9.8/os.py")
+        ns.add_known_dir("/usr/lib/python9.8/lib-dynload")
+        expected = dict(
+            executable="/Absolute/usr/bin/python",
+            base_executable="/Absolute/usr/bin/python",
+            prefix="/usr",
+            exec_prefix="/usr",
+            module_search_paths_set=1,
+            module_search_paths=[
+                "/usr/lib/python98.zip",
+                "/usr/lib/python9.8",
+                "/usr/lib/python9.8/lib-dynload",
+            ],
+        )
+        actual = getpath(ns, expected)
+        self.assertEqual(expected, actual)
+
     def test_buildpath_posix(self):
         """Test an in-build-tree layout on POSIX.
 

Misc/NEWS.d/next/Core_and_Builtins/2026-07-19-18-30-00.gh-issue-154160.XnXeqS.rst

@@ -0,0 +1,3 @@
+Fixed :data:`sys.executable` being a relative path when the interpreter was
+found through a relative entry in :envvar:`PATH`.  It is now made absolute, as
+documented.

Modules/getpath.py

@@ -285,7 +285,9 @@ def search_up(prefix, *landmarks, test=isfile):
     for p in ENV_PATH.split(DELIM):
         p = joinpath(p, program_name)
         if isxfile(p):
-            executable = p
+            # A relative PATH entry would otherwise leave executable
+            # relative, contrary to the documented behaviour (gh-154160).
+            executable = abspath(p)
             break
 
 if not executable: