diff -r 4f6499fc2f09 Lib/encodings/cp65001.py
--- a/Lib/encodings/cp65001.py	Sun Feb 09 13:11:53 2014 +0100
+++ b/Lib/encodings/cp65001.py	Sun Feb 09 14:14:02 2014 +0100
@@ -11,20 +11,23 @@ if not hasattr(codecs, 'code_page_encode
 ### Codec APIs
 
 encode = functools.partial(codecs.code_page_encode, 65001)
-decode = functools.partial(codecs.code_page_decode, 65001)
+_decode = functools.partial(codecs.code_page_decode, 65001)
+
+def decode(input, errors='strict'):
+    return codecs.code_page_decode(65001, input, errors, True)
 
 class IncrementalEncoder(codecs.IncrementalEncoder):
     def encode(self, input, final=False):
         return encode(input, self.errors)[0]
 
 class IncrementalDecoder(codecs.BufferedIncrementalDecoder):
-    _buffer_decode = decode
+    _buffer_decode = _decode
 
 class StreamWriter(codecs.StreamWriter):
     encode = encode
 
 class StreamReader(codecs.StreamReader):
-    decode = decode
+    decode = _decode
 
 ### encodings module API
 
diff -r 4f6499fc2f09 Lib/test/test_codecs.py
--- a/Lib/test/test_codecs.py	Sun Feb 09 13:11:53 2014 +0100
+++ b/Lib/test/test_codecs.py	Sun Feb 09 14:14:02 2014 +0100
@@ -890,10 +890,6 @@ class CP65001Test(ReadTest, unittest.Tes
                          "\U00010fff\uD800")
         self.assertTrue(codecs.lookup_error("surrogatepass"))
 
-    def test_readline(self):
-        self.skipTest("issue #20571: code page 65001 codec does not "
-                      "support partial decoder yet")
-
 
 class UTF7Test(ReadTest, unittest.TestCase):
     encoding = "utf-7"
@@ -2750,15 +2746,15 @@ class CodePageTest(unittest.TestCase):
         self.assertRaisesRegex(UnicodeEncodeError, 'cp932',
             codecs.code_page_encode, 932, '\xff')
         self.assertRaisesRegex(UnicodeDecodeError, 'cp932',
-            codecs.code_page_decode, 932, b'\x81\x00')
+            codecs.code_page_decode, 932, b'\x81\x00', 'strict', True)
         self.assertRaisesRegex(UnicodeDecodeError, 'CP_UTF8',
-            codecs.code_page_decode, self.CP_UTF8, b'\xff')
+            codecs.code_page_decode, self.CP_UTF8, b'\xff', 'strict', True)
 
     def check_decode(self, cp, tests):
         for raw, errors, expected in tests:
             if expected is not None:
                 try:
-                    decoded = codecs.code_page_decode(cp, raw, errors)
+                    decoded = codecs.code_page_decode(cp, raw, errors, True)
                 except UnicodeDecodeError as err:
                     self.fail('Unable to decode %a from "cp%s" with '
                               'errors=%r: %s' % (raw, cp, errors, err))
@@ -2770,7 +2766,7 @@ class CodePageTest(unittest.TestCase):
                 self.assertLessEqual(decoded[1], len(raw))
             else:
                 self.assertRaises(UnicodeDecodeError,
-                    codecs.code_page_decode, cp, raw, errors)
+                    codecs.code_page_decode, cp, raw, errors, True)
 
     def check_encode(self, cp, tests):
         for text, errors, expected in tests:
diff -r 4f6499fc2f09 Objects/unicodeobject.c
--- a/Objects/unicodeobject.c	Sun Feb 09 13:11:53 2014 +0100
+++ b/Objects/unicodeobject.c	Sun Feb 09 14:14:02 2014 +0100
@@ -6817,28 +6817,6 @@ code_page_name(UINT code_page, PyObject 
     return PyBytes_AS_STRING(*obj);
 }
 
-static int
-is_dbcs_lead_byte(UINT code_page, const char *s, int offset)
-{
-    const char *curr = s + offset;
-    const char *prev;
-
-    if (!IsDBCSLeadByteEx(code_page, *curr))
-        return 0;
-
-    prev = CharPrevExA(code_page, s, curr, 0);
-    if (prev == curr)
-        return 1;
-    /* FIXME: This code is limited to "true" double-byte encodings,
-       as it assumes an incomplete character consists of a single
-       byte. */
-    if (curr - prev == 2)
-        return 1;
-    if (!IsDBCSLeadByteEx(code_page, *prev))
-        return 1;
-    return 0;
-}
-
 static DWORD
 decode_code_page_flags(UINT code_page)
 {
@@ -6913,7 +6891,7 @@ static int
 decode_code_page_errors(UINT code_page,
                         PyObject **v,
                         const char *in, const int size,
-                        const char *errors)
+                        const char *errors, int final)
 {
     const char *startin = in;
     const char *endin = in + size;
@@ -6940,7 +6918,7 @@ decode_code_page_errors(UINT code_page,
     if (encoding == NULL)
         return -1;
 
-    if (errors == NULL || strcmp(errors, "strict") == 0) {
+    if ((errors == NULL || strcmp(errors, "strict") == 0) && final) {
         /* The last error was ERROR_NO_UNICODE_TRANSLATION, then we raise a
            UnicodeDecodeError. */
         make_decode_exception(&exc, encoding, in, size, 0, 0, reason);
@@ -7003,6 +6981,10 @@ decode_code_page_errors(UINT code_page,
         if (outsize <= 0) {
             Py_ssize_t startinpos, endinpos, outpos;
 
+            /* last character in partial decode? */
+            if (in + insize >= endin && !final)
+                break;
+
             startinpos = in - startin;
             endinpos = startinpos + 1;
             outpos = out - PyUnicode_AS_UNICODE(*v);
@@ -7031,7 +7013,7 @@ decode_code_page_errors(UINT code_page,
     assert(outsize <= PyUnicode_WSTR_LENGTH(*v));
     if (unicode_resize(v, outsize) < 0)
         goto error;
-    ret = size;
+    ret = in - startin;
 
 error:
     Py_XDECREF(encoding_obj);
@@ -7072,24 +7054,19 @@ decode_code_page_stateful(int code_page,
             done = 1;
         }
 
-        /* Skip trailing lead-byte unless 'final' is set */
-        if (!final && is_dbcs_lead_byte(code_page, s, chunk_size - 1))
-            --chunk_size;
-
         if (chunk_size == 0 && done) {
             if (v != NULL)
                 break;
             _Py_RETURN_UNICODE_EMPTY();
         }
 
-
         converted = decode_code_page_strict(code_page, &v,
                                             s, chunk_size);
         if (converted == -2)
             converted = decode_code_page_errors(code_page, &v,
                                                 s, chunk_size,
-                                                errors);
-        assert(converted != 0);
+                                                errors, final);
+        assert(converted != 0 || done);
 
         if (converted < 0) {
             Py_XDECREF(v);