add python_2_bytes_compatible

six.py

@@ -955,6 +955,23 @@ def python_2_unicode_compatible(klass):
     return klass
 
 
+def python_2_bytes_compatible(klass):
+    """
+    A class decorator that defines __str__ and __bytes__ methods under Python 2.
+    Under Python 3 it does nothing.
+
+    To support Python 2 and 3 with a single code base, define a __bytes__ method
+    returning bytes and apply this decorator to the class.
+    """
+    if PY2:
+        if '__bytes__' not in klass.__dict__:
+            raise ValueError("@python_2_bytes_compatible cannot be applied "
+                             "to %s because it doesn't define __bytes__()." %
+                             klass.__name__)
+        klass.__str__ = klass.__bytes__
+    return klass
+
+
 # Complete the moves implementation.
 # This code is at the end of this module to speed up module loading.
 # Turn this module into a package.