Another extension to reduce(). It can return a 4- or 5-tuple now.

The 4th item can be None or an iterator yielding list items, which are
used to append() or extend() the object.  The 5th item can be None or
an iterator yielding a dict's (key, value) pairs, which are stuffed
into the object using __setitem__.

Also (as a separate, though related, feature) add "batching" for list
and dict items.  If you pickled a dict or list with a million items in
the past, it would push a million items onto the stack.  It now pushes
only 1000 items at a time on the stack, using repeated APPENDS or
SETITEMS opcodes.  (For lists, I hope that using many short extend()
calls doesn't exhibit quadratic behavior.)

Lib/pickle.py

@@ -317,19 +317,14 @@ def save(self, obj):
         if type(rv) is not TupleType:
             raise PicklingError("%s must return string or tuple" % reduce)
 
-        # Assert that it returned a 2-tuple or 3-tuple, and unpack it
+        # Assert that it returned an appropriately sized tuple
         l = len(rv)
-        if l == 2:
-            func, args = rv
-            state = None
-        elif l == 3:
-            func, args, state = rv
-        else:
+        if not (2 <= l <= 5):
             raise PicklingError("Tuple returned by %s must have "
-                                "exactly two or three elements" % reduce)
+                                "two to five elements" % reduce)
 
         # Save the reduce() output and finally memoize the object
-        self.save_reduce(func, args, state, obj)
+        self.save_reduce(obj=obj, *rv)
 
     def persistent_id(self, obj):
         # This exists so a subclass can override it
@@ -343,7 +338,8 @@ def save_pers(self, pid):
         else:
             self.write(PERSID + str(pid) + '\n')
 
-    def save_reduce(self, func, args, state=None, obj=None):
+    def save_reduce(self, func, args, state=None,
+                    listitems=None, dictitems=None, obj=None):
         # This API is be called by some subclasses
 
         # Assert that args is a tuple or None
@@ -411,6 +407,17 @@ def save_reduce(self, func, args, state=None, obj=None):
         if obj is not None:
             self.memoize(obj)
 
+        # More new special cases (that work with older protocols as
+        # well): when __reduce__ returns a tuple with 4 or 5 items,
+        # the 4th and 5th item should be iterators that provide list
+        # items and dict items (as (key, value) tuples), or None.
+
+        if listitems is not None:
+            self._batch_appends(listitems)
+
+        if dictitems is not None:
+            self._batch_setitems(dictitems)
+
         if state is not None:
             save(state)
             write(BUILD)
@@ -434,28 +441,9 @@ def save_newobj(self, obj):
         self.memoize(obj)
 
         if isinstance(obj, list):
-            n = len(obj)
-            if n > 1:
-                write(MARK)
-                for x in obj:
-                    save(x)
-                write(APPENDS)
-            elif n == 1:
-                save(obj[0])
-                write(APPEND)
+            self._batch_appends(iter(obj))
         elif isinstance(obj, dict):
-            n = len(obj)
-            if n > 1:
-                write(MARK)
-                for k, v in obj.iteritems():
-                    save(k)
-                    save(v)
-                write(SETITEMS)
-            elif n == 1:
-                k, v = obj.items()[0]
-                save(k)
-                save(v)
-                write(SETITEM)
+            self._batch_setitems(obj.iteritems())
 
         getstate = getattr(obj, "__getstate__", None)
 
@@ -683,63 +671,100 @@ def save_empty_tuple(self, obj):
 
     def save_list(self, obj):
         write = self.write
-        save  = self.save
 
         if self.bin:
             write(EMPTY_LIST)
-            self.memoize(obj)
-            n = len(obj)
-            if n > 1:
-                write(MARK)
-                for element in obj:
-                    save(element)
-                write(APPENDS)
-            elif n:
-                assert n == 1
-                save(obj[0])
-                write(APPEND)
-            # else the list is empty, and we're already done
-
-        else:   # proto 0 -- can't use EMPTY_LIST or APPENDS
+        else:   # proto 0 -- can't use EMPTY_LIST
             write(MARK + LIST)
-            self.memoize(obj)
-            for element in obj:
-                save(element)
-                write(APPEND)
+
+        self.memoize(obj)
+        self._batch_appends(iter(obj))
 
     dispatch[ListType] = save_list
 
+    _BATCHSIZE = 1000
+
+    def _batch_appends(self, items):
+        # Helper to batch up APPENDS sequences
+        save = self.save
+        write = self.write
+
+        if not self.bin:
+            for x in items:
+                save(x)
+                write(APPEND)
+            return
+
+        r = xrange(self._BATCHSIZE)
+        while items is not None:
+            tmp = []
+            for i in r:
+                try:
+                    tmp.append(items.next())
+                except StopIteration:
+                    items = None
+                    break
+            n = len(tmp)
+            if n > 1:
+                write(MARK)
+                for x in tmp:
+                    save(x)
+                write(APPENDS)
+            elif n:
+                save(tmp[0])
+                write(APPEND)
+            # else tmp is empty, and we're done
+
     def save_dict(self, obj):
         write = self.write
-        save  = self.save
-        items = obj.iteritems()
 
         if self.bin:
             write(EMPTY_DICT)
-            self.memoize(obj)
-            if len(obj) > 1:
-                write(MARK)
-                for key, value in items:
-                    save(key)
-                    save(value)
-                write(SETITEMS)
-                return
-            # else (dict is empty or a singleton), fall through to the
-            # SETITEM code at the end
-        else:   # proto 0 -- can't use EMPTY_DICT or SETITEMS
+        else:   # proto 0 -- can't use EMPTY_DICT
             write(MARK + DICT)
-            self.memoize(obj)
 
-        # proto 0 or len(obj) < 2
-        for key, value in items:
-            save(key)
-            save(value)
-            write(SETITEM)
+        self.memoize(obj)
+        self._batch_setitems(obj.iteritems())
 
     dispatch[DictionaryType] = save_dict
     if not PyStringMap is None:
         dispatch[PyStringMap] = save_dict
 
+    def _batch_setitems(self, items):
+        # Helper to batch up SETITEMS sequences; proto >= 1 only
+        save = self.save
+        write = self.write
+
+        if not self.bin:
+            for k, v in items:
+                save(k)
+                save(v)
+                write(SETITEM)
+            return
+
+        r = xrange(self._BATCHSIZE)
+        while items is not None:
+            tmp = []
+            for i in r:
+                try:
+                    tmp.append(items.next())
+                except StopIteration:
+                    items = None
+                    break
+            n = len(tmp)
+            if n > 1:
+                write(MARK)
+                for k, v in tmp:
+                    save(k)
+                    save(v)
+                write(SETITEMS)
+            elif n:
+                k, v = tmp[0]
+                save(k)
+                save(v)
+                write(SETITEM)
+            # else tmp is empty, and we're done
+
     def save_inst(self, obj):
         cls = obj.__class__