Issue #16915: Clarify that mode parameter of socket.makefile() does not accept

the same values as open().

Doc/library/socket.rst

@@ -1006,7 +1006,8 @@ to sockets.
 
    Return a :term:`file object` associated with the socket.  The exact returned
    type depends on the arguments given to :meth:`makefile`.  These arguments are
-   interpreted the same way as by the built-in :func:`open` function.
+   interpreted the same way as by the built-in :func:`open` function, except
+   the only supported *mode* values are ``'r'`` (default), ``'w'`` and ``'b'``.
 
    The socket must be in blocking mode; it can have a timeout, but the file
    object's internal buffer may end up in an inconsistent state if a timeout

Lib/socket.py

@@ -209,10 +209,10 @@ class socket(_socket.socket):
                  encoding=None, errors=None, newline=None):
         """makefile(...) -> an I/O stream connected to the socket
 
-        The arguments are as for io.open() after the filename,
-        except the only mode characters supported are 'r', 'w' and 'b'.
-        The semantics are similar too.  (XXX refactor to share code?)
+        The arguments are as for io.open() after the filename, except the only
+        supported mode values are 'r' (default), 'w' and 'b'.
         """
+        # XXX refactor to share code?
         if not set(mode) <= {"r", "w", "b"}:
             raise ValueError("invalid mode %r (only r, w, b allowed)" % (mode,))
         writing = "w" in mode

Lib/test/test_socket.py

@@ -1374,6 +1374,20 @@ class GeneralModuleTests(unittest.TestCase):
             self.assertRaises(ValueError, fp.writable)
             self.assertRaises(ValueError, fp.seekable)
 
+    def test_makefile_mode(self):
+        for mode in 'r', 'rb', 'rw', 'w', 'wb':
+            with self.subTest(mode=mode):
+                with socket.socket() as sock:
+                    with sock.makefile(mode) as fp:
+                        self.assertEqual(fp.mode, mode)
+
+    def test_makefile_invalid_mode(self):
+        for mode in 'rt', 'x', '+', 'a':
+            with self.subTest(mode=mode):
+                with socket.socket() as sock:
+                    with self.assertRaisesRegex(ValueError, 'invalid mode'):
+                        sock.makefile(mode)
+
     def test_pickle(self):
         sock = socket.socket()
         with sock: