[1.5.x] Fixed #20212 - __reduce__ should only be defined for Py3+.

1 files changed, 19 insertions, 5 deletions

diff --git a/django/utils/functional.py b/django/utils/functional.py
index 1dcbd55664..839dd4ef85 100644
--- a/django/utils/functional.py
+++ b/django/utils/functional.py
@@ -5,6 +5,12 @@ import sys
 
 from django.utils import six
 
+try:
+    import copyreg
+except ImportError:
+    import copy_reg as copyreg
+
+
 # You can't trivially replace this `functools.partial` because this binds to
 # classes and returns bound instances, whereas functools.partial (on CPython)
 # is a type and its instances don't bind.
@@ -294,15 +300,23 @@ class SimpleLazyObject(LazyObject):
             self._setup()
         return self._wrapped.__dict__
 
-    # Python 3.3 will call __reduce__ when pickling; these methods are needed
-    # to serialize and deserialize correctly. They are not called in earlier
-    # versions of Python.
+    # Python 3.3 will call __reduce__ when pickling; this method is needed
+    # to serialize and deserialize correctly.
     @classmethod
     def __newobj__(cls, *args):
         return cls.__new__(cls, *args)
 
-    def __reduce__(self):
-        return (self.__newobj__, (self.__class__,), self.__getstate__())
+    def __reduce_ex__(self, proto):
+        if proto >= 2:
+            # On Py3, since the default protocol is 3, pickle uses the
+            # ``__newobj__`` method (& more efficient opcodes) for writing.
+            return (self.__newobj__, (self.__class__,), self.__getstate__())
+        else:
+            # On Py2, the default protocol is 0 (for back-compat) & the above
+            # code fails miserably (see regression test). Instead, we return
+            # exactly what's returned if there's no ``__reduce__`` method at
+            # all.
+            return (copyreg._reconstructor, (self.__class__, object, None), self.__getstate__())
 
     # Need to pretend to be the wrapped class, for the sake of objects that care
     # about this (especially in equality tests)