bpo-39674: Revert "bpo-37330: open() no longer accept 'U' in file mode (GH-16959)" (GH-18767)

This reverts commit e471e72977c83664f13d041c78549140c86c92de.

The mode will be removed from Python 3.10.

4 files changed, 15 insertions, 19 deletions

diff --git a/Doc/library/codecs.rst b/Doc/library/codecs.rst
index ec6a053..f071057 100644
--- a/Doc/library/codecs.rst
+++ b/Doc/library/codecs.rst
@@ -197,9 +197,6 @@ wider range of codecs when working with binary files:
    *buffering* has the same meaning as for the built-in :func:`open` function.
    It defaults to -1 which means that the default buffer size will be used.
 
-   .. versionchanged:: 3.9
-      The ``'U'`` mode has been removed.
-
 
 .. function:: EncodedFile(file, data_encoding, file_encoding=None, errors='strict')
 
diff --git a/Doc/library/fileinput.rst b/Doc/library/fileinput.rst
index 236f5d5..cc4039a 100644
--- a/Doc/library/fileinput.rst
+++ b/Doc/library/fileinput.rst
@@ -148,8 +148,8 @@ available for subclassing as well:
    The sequence must be accessed in strictly sequential order; random access
    and :meth:`~io.TextIOBase.readline` cannot be mixed.
 
-   With *mode* you can specify which file mode will be passed to :func:`open`.
-   It must be ``'r'`` or ``'rb'``.
+   With *mode* you can specify which file mode will be passed to :func:`open`. It
+   must be one of ``'r'``, ``'rU'``, ``'U'`` and ``'rb'``.
 
    The *openhook*, when given, must be a function that takes two arguments,
    *filename* and *mode*, and returns an accordingly opened file-like object. You
@@ -166,14 +166,15 @@ available for subclassing as well:
    .. versionchanged:: 3.2
       Can be used as a context manager.
 
+   .. deprecated:: 3.4
+      The ``'rU'`` and ``'U'`` modes.
+
    .. deprecated:: 3.8
       Support for :meth:`__getitem__` method is deprecated.
 
    .. versionchanged:: 3.8
       The keyword parameter *mode* and *openhook* are now keyword-only.
 
-   .. versionchanged:: 3.9
-      The ``'rU'`` and ``'U'`` modes have been removed.
 
 
 **Optional in-place filtering:** if the keyword argument ``inplace=True`` is
diff --git a/Doc/library/functions.rst b/Doc/library/functions.rst
index cc48597..ca09e6f 100644
--- a/Doc/library/functions.rst
+++ b/Doc/library/functions.rst
@@ -1090,6 +1090,12 @@ are always available.  They are listed here in alphabetical order.
    first decoded using a platform-dependent encoding or using the specified
    *encoding* if given.
 
+   There is an additional mode character permitted, ``'U'``, which no longer
+   has any effect, and is considered deprecated. It previously enabled
+   :term:`universal newlines` in text mode, which became the default behaviour
+   in Python 3.0. Refer to the documentation of the
+   :ref:`newline <open-newline-parameter>` parameter for further details.
+
    .. note::
 
       Python doesn't depend on the underlying operating system's notion of text
@@ -1246,6 +1252,10 @@ are always available.  They are listed here in alphabetical order.
 
          * The file is now non-inheritable.
 
+   .. deprecated-removed:: 3.4 3.10
+
+      The ``'U'`` mode.
+
    .. versionchanged::
       3.5
 
@@ -1261,10 +1271,6 @@ are always available.  They are listed here in alphabetical order.
          * On Windows, opening a console buffer may return a subclass of
            :class:`io.RawIOBase` other than :class:`io.FileIO`.
 
-   .. versionchanged:: 3.9
-      The ``'U'`` mode has been removed.
-
-
 .. function:: ord(c)
 
    Given a string representing one Unicode character, return an integer
diff --git a/Doc/whatsnew/3.9.rst b/Doc/whatsnew/3.9.rst
index d072b8d..a59de48 100644
--- a/Doc/whatsnew/3.9.rst
+++ b/Doc/whatsnew/3.9.rst
@@ -625,14 +625,6 @@ that may require changes to your code.
 Changes in the Python API
 -------------------------
 
-* :func:`open`, :func:`io.open`, :func:`codecs.open` and
-  :class:`fileinput.FileInput` no longer accept ``'U'`` ("universal newline")
-  in the file mode. This flag was deprecated since Python 3.3. In Python 3, the
-  "universal newline" is used by default when a file is open in text mode.  The
-  :ref:`newline parameter <open-newline-parameter>` of :func:`open` controls
-  how universal newlines works.
-  (Contributed by Victor Stinner in :issue:`37330`.)
-
 * :func:`__import__` and :func:`importlib.util.resolve_name` now raise
   :exc:`ImportError` where it previously raised :exc:`ValueError`. Callers
   catching the specific exception type and supporting both Python 3.9 and