--- a/Doc/library/functions.rst
+++ b/Doc/library/functions.rst
@@ -776,7 +776,7 @@
    :meth:`__index__` method that returns an integer.
 
 
-.. function:: open(file, mode='r', buffering=-1, encoding=None, errors=None, newline=None, closefd=True)
+.. function:: open(file, mode='r', buffering=-1, encoding=None, errors=None, newline=None, closefd=True, opener=None)
 
    Open *file* and return a corresponding stream.  If the file cannot be opened,
    an :exc:`OSError` is raised.
@@ -883,6 +883,12 @@
    closed.  If a filename is given *closefd* has no effect and must be ``True``
    (the default).
 
+   A custom opener can be used by passing a callable as *opener*. The underlying
+   file descriptor for the file object is then obtained by calling *opener* with
+   (*file*, *flags*). *opener* must return an open file descriptor (passing
+   :mod:`os.open` as *opener* results in functionality similar to passing
+   ``None``).
+
    The type of file object returned by the :func:`open` function depends on the
    mode.  When :func:`open` is used to open a file in a text mode (``'w'``,
    ``'r'``, ``'wt'``, ``'rt'``, etc.), it returns a subclass of