diff -r 5b43d7984a63 Lib/test/test_io.py
--- a/Lib/test/test_io.py	Wed Dec 30 01:42:43 2015 +0200
+++ b/Lib/test/test_io.py	Tue Dec 29 21:24:05 2015 -0800
@@ -3015,6 +3015,42 @@
         F.tell = lambda x: 0
         t = self.TextIOWrapper(F(), encoding='utf-8')
 
+    def test_issue25863(self):
+        iso2022_charsets = ['utf-8',
+                            'iso-2022-jp',
+                            'iso-2022-jp-1',
+                            'iso-2022-jp-2',
+                            'iso-2022-jp-3',
+                            'iso-2022-jp-2004',
+                            'iso-2022-kr',
+                            # These charsets aren't implemented currently
+                            # but should also be tested if they are added
+                            #'iso-2022-cn',
+                            #'iso-2022-cn-ext'
+        ]
+        for charset in iso2022_charsets:
+            text = self.TextIOWrapper(io.BytesIO(), encoding=charset)
+            s = u"Panter 正孝"
+        
+            text.write(u"P")
+            
+            start = text.tell()
+            num_chars = text.write(u"anter 正")
+            pos = text.tell()
+
+            text.write(u"孝")
+            text.seek(pos)
+
+            # Report the encoding that failed
+            self.assertEqual(text.read(), u"孝")
+            
+            self.assertEqual(text.buffer.getvalue(), s.encode(encoding=charset))
+
+            # These lines test a bug where text.tell() raises a
+            # UnicodeDecodeError.
+            text.seek(start)
+            text.read(num_chars)
+            text.tell()
 
 class MemviewBytesIO(io.BytesIO):
     '''A BytesIO object whose read method returns memoryviews