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.

3 files changed, 30 insertions, 27 deletions

diff --git a/Lib/test/test_codecs.py b/Lib/test/test_codecs.py
index dcdd574bc7f..54a3520802a 100644
--- a/Lib/test/test_codecs.py
+++ b/Lib/test/test_codecs.py
@@ -712,23 +712,11 @@ class UTF16Test(ReadTest, unittest.TestCase):
         self.addCleanup(support.unlink, support.TESTFN)
         with open(support.TESTFN, 'wb') as fp:
             fp.write(s)
-        with codecs.open(support.TESTFN, 'r',
-                         encoding=self.encoding) as reader:
+        with support.check_warnings(('', DeprecationWarning)):
+            reader = codecs.open(support.TESTFN, 'U', encoding=self.encoding)
+        with reader:
             self.assertEqual(reader.read(), s1)
 
-    def test_invalid_modes(self):
-        for mode in ('U', 'rU', 'r+U'):
-            with self.assertRaises(ValueError) as cm:
-                codecs.open(support.TESTFN, mode, encoding=self.encoding)
-            self.assertIn('invalid mode', str(cm.exception))
-
-        for mode in ('rt', 'wt', 'at', 'r+t'):
-            with self.assertRaises(ValueError) as cm:
-                codecs.open(support.TESTFN, mode, encoding=self.encoding)
-            self.assertIn("can't have text and binary mode at once",
-                          str(cm.exception))
-
-
 class UTF16LETest(ReadTest, unittest.TestCase):
     encoding = "utf-16-le"
     ill_formed_sequence = b"\x80\xdc"
diff --git a/Lib/test/test_fileinput.py b/Lib/test/test_fileinput.py
index 819557d5e86..014f19e6cbd 100644
--- a/Lib/test/test_fileinput.py
+++ b/Lib/test/test_fileinput.py
@@ -226,11 +226,19 @@ class FileInputTests(BaseTests, unittest.TestCase):
         self.assertEqual(fi.fileno(), -1)
 
     def test_opening_mode(self):
-        # invalid modes
-        for mode in ('w', 'rU', 'U'):
-            with self.subTest(mode=mode):
-                with self.assertRaises(ValueError):
-                    FileInput(mode=mode)
+        try:
+            # invalid mode, should raise ValueError
+            fi = FileInput(mode="w")
+            self.fail("FileInput should reject invalid mode argument")
+        except ValueError:
+            pass
+        # try opening in universal newline mode
+        t1 = self.writeTmp(b"A\nB\r\nC\rD", mode="wb")
+        with check_warnings(('', DeprecationWarning)):
+            fi = FileInput(files=t1, mode="U")
+        with check_warnings(('', DeprecationWarning)):
+            lines = list(fi)
+        self.assertEqual(lines, ["A\n", "B\n", "C\n", "D"])
 
     def test_stdin_binary_mode(self):
         with mock.patch('sys.stdin') as m_stdin:
@@ -977,6 +985,10 @@ class Test_hook_encoded(unittest.TestCase):
             self.assertEqual(lines, expected_lines)
 
         check('r', ['A\n', 'B\n', 'C\n', 'D\u20ac'])
+        with self.assertWarns(DeprecationWarning):
+            check('rU', ['A\n', 'B\n', 'C\n', 'D\u20ac'])
+        with self.assertWarns(DeprecationWarning):
+            check('U', ['A\n', 'B\n', 'C\n', 'D\u20ac'])
         with self.assertRaises(ValueError):
             check('rb', ['A\n', 'B\r\n', 'C\r', 'D\u20ac'])
 
diff --git a/Lib/test/test_io.py b/Lib/test/test_io.py
index c27dfd96bc0..4a7cbe538cf 100644
--- a/Lib/test/test_io.py
+++ b/Lib/test/test_io.py
@@ -3900,6 +3900,16 @@ class MiscIOTest(unittest.TestCase):
         self.assertEqual(f.mode, "wb")
         f.close()
 
+        with support.check_warnings(('', DeprecationWarning)):
+            f = self.open(support.TESTFN, "U")
+        self.assertEqual(f.name,            support.TESTFN)
+        self.assertEqual(f.buffer.name,     support.TESTFN)
+        self.assertEqual(f.buffer.raw.name, support.TESTFN)
+        self.assertEqual(f.mode,            "U")
+        self.assertEqual(f.buffer.mode,     "rb")
+        self.assertEqual(f.buffer.raw.mode, "rb")
+        f.close()
+
         f = self.open(support.TESTFN, "w+")
         self.assertEqual(f.mode,            "w+")
         self.assertEqual(f.buffer.mode,     "rb+") # Does it really matter?
@@ -3913,13 +3923,6 @@ class MiscIOTest(unittest.TestCase):
         f.close()
         g.close()
 
-    def test_removed_u_mode(self):
-        # "U" mode has been removed in Python 3.9
-        for mode in ("U", "rU", "r+U"):
-            with self.assertRaises(ValueError) as cm:
-                self.open(support.TESTFN, mode)
-            self.assertIn('invalid mode', str(cm.exception))
-
     def test_open_pipe_with_append(self):
         # bpo-27805: Ignore ESPIPE from lseek() in open().
         r, w = os.pipe()