Document absoluteness of sys.executable

Closes #13402.

Doc/library/sys.rst

@@ -202,8 +202,10 @@ always available.
 
 .. data:: executable
 
-   A string giving the name of the executable binary for the Python interpreter, on
-   systems where this makes sense.
+   A string giving the absolute path of the executable binary for the Python
+   interpreter, on systems where this makes sense. If Python is unable to retrieve
+   the real path to its executable, :data:`sys.executable` will be an empty string
+   or ``None``.
 
 
 .. function:: exit([arg])

Lib/test/test_sys.py

@@ -532,6 +532,9 @@ class SysModuleTest(unittest.TestCase):
         self.assertEqual(out, b'?')
 
     def test_executable(self):
+        # sys.executable should be absolute
+        self.assertEqual(os.path.abspath(sys.executable), sys.executable)
+
         # Issue #7774: Ensure that sys.executable is an empty string if argv[0]
         # has been set to an non existent program name and Python is unable to
         # retrieve the real program name

Misc/NEWS

@@ -454,6 +454,8 @@ Extension Modules
 Documentation
 -------------
 
+- Issue #13402: Document absoluteness of sys.executable.
+
 - Issue #13883: PYTHONCASEOK also used on OS X and OS/2.
 
 - Issue #12949: Document the kwonlyargcount argument for the PyCode_New

Python/sysmodule.c

@@ -1263,7 +1263,7 @@ version_info -- version information as a named tuple\n\
 hexversion -- version information encoded as a single integer\n\
 copyright -- copyright notice pertaining to this interpreter\n\
 platform -- platform identifier\n\
-executable -- pathname of this Python interpreter\n\
+executable -- absolute path of the executable binary of the Python interpreter\n\
 prefix -- prefix used to find the Python library\n\
 exec_prefix -- prefix used to find the machine-specific Python library\n\
 float_repr_style -- string indicating the style of repr() output for floats\n\