- A new pickle protocol (protocol 3) is added with explicit support

for bytes.  This is the default protocol.  It intentionally cannot
  be unpickled by Python 2.x.

- When a pickle	written	by Python 2.x contains an (8-bit) str
  instance, this is now decoded to a (Unicode) str instance.  The
  encoding used to do this defaults to ASCII, but can be overridden
  via two new keyword arguments to the Unpickler class.  Previously
  this would create bytes instances, which is usually wrong: str
  instances are often used to pickle attribute names etc., and text is
  more common than binary data anyway.

Lib/pickle.py

@@ -42,19 +42,22 @@ __all__ = ["PickleError", "PicklingError", "UnpicklingError", "Pickler",
 bytes_types = (bytes, bytearray, memoryview)
 
 # These are purely informational; no code uses these.
-format_version = "2.0"                  # File format version we write
+format_version = "3.0"                  # File format version we write
 compatible_formats = ["1.0",            # Original protocol 0
                       "1.1",            # Protocol 0 with INST added
                       "1.2",            # Original protocol 1
                       "1.3",            # Protocol 1 with BINFLOAT added
                       "2.0",            # Protocol 2
+                      "3.0",            # Protocol 3
                       ]                 # Old format versions we can read
 
 # This is the highest protocol number we know how to read.
-HIGHEST_PROTOCOL = 2
+HIGHEST_PROTOCOL = 3
 
 # The protocol we write by default.  May be less than HIGHEST_PROTOCOL.
-DEFAULT_PROTOCOL = 2
+# We intentionally write a protocol that Python 2.x cannot read;
+# there are too many issues with that.
+DEFAULT_PROTOCOL = 3
 
 # Why use struct.pack() for pickling but marshal.loads() for
 # unpickling?  struct.pack() is 40% faster than marshal.dumps(), but
@@ -161,6 +164,10 @@ LONG4          = b'\x8b'  # push really big long
 
 _tuplesize2code = [EMPTY_TUPLE, TUPLE1, TUPLE2, TUPLE3]
 
+# Protocol 3 (Python 3.x)
+
+BINBYTES       = b'B'   # push bytes; counted binary string argument
+SHORT_BINBYTES = b'C'   #  "     "   ;    "      "       "      " < 256 bytes
 
 __all__.extend([x for x in dir() if re.match("[A-Z][A-Z0-9_]+$",x)])
 
@@ -494,20 +501,19 @@ class Pickler:
             self.write(FLOAT + repr(obj).encode("ascii") + b'\n')
     dispatch[float] = save_float
 
-    def save_string(self, obj, pack=struct.pack):
-        if self.bin:
-            n = len(obj)
-            if n < 256:
-                self.write(SHORT_BINSTRING + bytes([n]) + bytes(obj))
-            else:
-                self.write(BINSTRING + pack("<i", n) + bytes(obj))
+    def save_bytes(self, obj, pack=struct.pack):
+        if self.proto < 3:
+            self.save_reduce(bytes, (list(obj),))
+            return
+        n = len(obj)
+        if n < 256:
+            self.write(SHORT_BINBYTES + bytes([n]) + bytes(obj))
         else:
-            # Strip leading 'b' due to repr() of bytes() returning b'...'
-            self.write(STRING + repr(obj).lstrip("b").encode("ascii") + b'\n')
+            self.write(BINBYTES + pack("<i", n) + bytes(obj))
         self.memoize(obj)
-    dispatch[bytes] = save_string
+    dispatch[bytes] = save_bytes
 
-    def save_unicode(self, obj, pack=struct.pack):
+    def save_str(self, obj, pack=struct.pack):
         if self.bin:
             encoded = obj.encode('utf-8')
             n = len(encoded)
@@ -518,7 +524,7 @@ class Pickler:
             self.write(UNICODE + bytes(obj.encode('raw-unicode-escape')) +
                        b'\n')
         self.memoize(obj)
-    dispatch[str] = save_unicode
+    dispatch[str] = save_str
 
     def save_tuple(self, obj):
         write = self.write
@@ -775,7 +781,7 @@ def whichmodule(func, funcname):
 
 class Unpickler:
 
-    def __init__(self, file):
+    def __init__(self, file, *, encoding="ASCII", errors="strict"):
         """This takes a binary file for reading a pickle data stream.
 
         The protocol version of the pickle is detected automatically, so no
@@ -787,10 +793,16 @@ class Unpickler:
         Thus file-like object can be a binary file object opened for
         reading, a BytesIO object, or any other custom object that
         meets this interface.
+
+        Optional keyword arguments are encoding and errors, which are
+        used to decode 8-bit string instances pickled by Python 2.x.
+        These default to 'ASCII' and 'strict', respectively.
         """
         self.readline = file.readline
         self.read = file.read
         self.memo = {}
+        self.encoding = encoding
+        self.errors = errors
 
     def load(self):
         """Read a pickled object representation from the open file.
@@ -831,7 +843,7 @@ class Unpickler:
 
     def load_proto(self):
         proto = ord(self.read(1))
-        if not 0 <= proto <= 2:
+        if not 0 <= proto <= HIGHEST_PROTOCOL:
             raise ValueError("unsupported pickle protocol: %d" % proto)
     dispatch[PROTO[0]] = load_proto
 
@@ -924,9 +936,16 @@ class Unpickler:
 
     def load_binstring(self):
         len = mloads(b'i' + self.read(4))
-        self.append(self.read(len))
+        data = self.read(len)
+        value = str(data, self.encoding, self.errors)
+        self.append(value)
     dispatch[BINSTRING[0]] = load_binstring
 
+    def load_binbytes(self):
+        len = mloads(b'i' + self.read(4))
+        self.append(self.read(len))
+    dispatch[BINBYTES[0]] = load_binbytes
+
     def load_unicode(self):
         self.append(str(self.readline()[:-1], 'raw-unicode-escape'))
     dispatch[UNICODE[0]] = load_unicode
@@ -938,9 +957,16 @@ class Unpickler:
 
     def load_short_binstring(self):
         len = ord(self.read(1))
-        self.append(bytes(self.read(len)))
+        data = bytes(self.read(len))
+        value = str(data, self.encoding, self.errors)
+        self.append(value)
     dispatch[SHORT_BINSTRING[0]] = load_short_binstring
 
+    def load_short_binbytes(self):
+        len = ord(self.read(1))
+        self.append(bytes(self.read(len)))
+    dispatch[SHORT_BINBYTES[0]] = load_short_binbytes
+
     def load_tuple(self):
         k = self.marker()
         self.stack[k:] = [tuple(self.stack[k+1:])]