Added tests for issue #20501.

Lib/test/test_fileinput.py

@@ -260,6 +260,24 @@ class FileInputTests(unittest.TestCase):
             fi.readline()
         self.assertTrue(custom_open_hook.invoked, "openhook not invoked")
 
+    def test_readline(self):
+        with open(TESTFN, 'wb') as f:
+            f.write(b'A\nB\r\nC\r')
+            # Fill TextIOWrapper buffer.
+            f.write(b'123456789\n' * 1000)
+            # Issue #20501: readline() shouldn't read whole file.
+            f.write(b'\x80')
+        self.addCleanup(safe_unlink, TESTFN)
+
+        with FileInput(files=TESTFN,
+                       openhook=hook_encoded('ascii'), bufsize=8) as fi:
+            self.assertEqual(fi.readline(), 'A\n')
+            self.assertEqual(fi.readline(), 'B\n')
+            self.assertEqual(fi.readline(), 'C\n')
+            with self.assertRaises(UnicodeDecodeError):
+                # Read to the end of file.
+                list(fi)
+
     def test_context_manager(self):
         try:
             t1 = writeTmp(1, ["A\nB\nC"])
@@ -837,6 +855,26 @@ class Test_hook_encoded(unittest.TestCase):
         self.assertIs(kwargs.pop('encoding'), encoding)
         self.assertFalse(kwargs)
 
+    def test_modes(self):
+        # Unlikely UTF-7 is locale encoding
+        with open(TESTFN, 'wb') as f:
+            f.write(b'A\nB\r\nC\rD+IKw-')
+        self.addCleanup(safe_unlink, TESTFN)
+
+        def check(mode, expected_lines):
+            with FileInput(files=TESTFN, mode=mode,
+                           openhook=hook_encoded('utf-7')) as fi:
+                lines = list(fi)
+            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'])
+
 
 if __name__ == "__main__":
     unittest.main()