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.

2 files changed, 29 insertions, 3 deletions

diff --git a/Modules/_io/_iomodule.c b/Modules/_io/_iomodule.c
index d609fa4..534d7de 100644
--- a/Modules/_io/_iomodule.c
+++ b/Modules/_io/_iomodule.c
@@ -138,6 +138,7 @@ Character Meaning
 'b'       binary mode
 't'       text mode (default)
 '+'       open a disk file for updating (reading and writing)
+'U'       universal newline mode (deprecated)
 ========= ===============================================================
 
 The default mode is 'rt' (open for reading text). For binary random
@@ -153,6 +154,10 @@ bytes objects without any decoding. In text mode (the default, or when
 returned as strings, the bytes having been first decoded using a
 platform-dependent encoding or using the specified encoding if given.
 
+'U' mode is deprecated and will raise an exception in future versions
+of Python.  It has no effect in Python 3.  Use newline to control
+universal newlines mode.
+
 buffering is an optional integer used to set the buffering policy.
 Pass 0 to switch buffering off (only allowed in binary mode), 1 to select
 line buffering (only usable in text mode), and an integer > 1 to indicate
@@ -228,12 +233,12 @@ static PyObject *
 _io_open_impl(PyObject *module, PyObject *file, const char *mode,
               int buffering, const char *encoding, const char *errors,
               const char *newline, int closefd, PyObject *opener)
-/*[clinic end generated code: output=aefafc4ce2b46dc0 input=1543f4511d2356a5]*/
+/*[clinic end generated code: output=aefafc4ce2b46dc0 input=7295902222e6b311]*/
 {
     unsigned i;
 
     int creating = 0, reading = 0, writing = 0, appending = 0, updating = 0;
-    int text = 0, binary = 0;
+    int text = 0, binary = 0, universal = 0;
 
     char rawmode[6], *m;
     int line_buffering, is_number;
@@ -291,6 +296,10 @@ _io_open_impl(PyObject *module, PyObject *file, const char *mode,
         case 'b':
             binary = 1;
             break;
+        case 'U':
+            universal = 1;
+            reading = 1;
+            break;
         default:
             goto invalid_mode;
         }
@@ -313,6 +322,18 @@ _io_open_impl(PyObject *module, PyObject *file, const char *mode,
     *m = '\0';
 
     /* Parameters validation */
+    if (universal) {
+        if (creating || writing || appending || updating) {
+            PyErr_SetString(PyExc_ValueError,
+                            "mode U cannot be combined with 'x', 'w', 'a', or '+'");
+            goto error;
+        }
+        if (PyErr_WarnEx(PyExc_DeprecationWarning,
+                         "'U' mode is deprecated", 1) < 0)
+            goto error;
+        reading = 1;
+    }
+
     if (text && binary) {
         PyErr_SetString(PyExc_ValueError,
                         "can't have text and binary mode at once");
diff --git a/Modules/_io/clinic/_iomodule.c.h b/Modules/_io/clinic/_iomodule.c.h
index c1f518f..1a9651d 100644
--- a/Modules/_io/clinic/_iomodule.c.h
+++ b/Modules/_io/clinic/_iomodule.c.h
@@ -36,6 +36,7 @@ PyDoc_STRVAR(_io_open__doc__,
 "\'b\'       binary mode\n"
 "\'t\'       text mode (default)\n"
 "\'+\'       open a disk file for updating (reading and writing)\n"
+"\'U\'       universal newline mode (deprecated)\n"
 "========= ===============================================================\n"
 "\n"
 "The default mode is \'rt\' (open for reading text). For binary random\n"
@@ -51,6 +52,10 @@ PyDoc_STRVAR(_io_open__doc__,
 "returned as strings, the bytes having been first decoded using a\n"
 "platform-dependent encoding or using the specified encoding if given.\n"
 "\n"
+"\'U\' mode is deprecated and will raise an exception in future versions\n"
+"of Python.  It has no effect in Python 3.  Use newline to control\n"
+"universal newlines mode.\n"
+"\n"
 "buffering is an optional integer used to set the buffering policy.\n"
 "Pass 0 to switch buffering off (only allowed in binary mode), 1 to select\n"
 "line buffering (only usable in text mode), and an integer > 1 to indicate\n"
@@ -318,4 +323,4 @@ _io_open_code(PyObject *module, PyObject *const *args, Py_ssize_t nargs, PyObjec
 exit:
     return return_value;
 }
-/*[clinic end generated code: output=680e4b488c7da8a1 input=a9049054013a1b77]*/
+/*[clinic end generated code: output=3df6bc6d91697545 input=a9049054013a1b77]*/