bpo-30977: rework code changes according to post-merge code review (GH-9106)

also mention the change and its consequences in What's New

Doc/whatsnew/3.8.rst

@@ -131,6 +131,10 @@ Optimizations
   objects (e.g. tuple, list, dict) size is reduced 4 or 8 bytes.
   (Contributed by Inada Naoki in :issue:`33597`)
 
+* :class:`uuid.UUID` now uses ``__slots__`` to reduce its memory footprint.
+  Note that this means that instances can no longer be weak-referenced and
+  that arbitrary attributes can no longer be added to them.
+
 
 Build and C API Changes
 =======================
@@ -275,6 +279,9 @@ Changes in the Python API
 * The function :func:`math.factorial` no longer accepts arguments that are not
   int-like. (Contributed by Pablo Galindo in :issue:`33083`.)
 
+* :class:`uuid.UUID` now uses ``__slots__``, therefore instances can no longer
+  be weak-referenced and attributes can no longer be added.
+
 
 CPython bytecode changes
 ------------------------

Lib/uuid.py

@@ -207,26 +207,19 @@ class UUID:
         object.__setattr__(self, 'is_safe', is_safe)
 
     def __getstate__(self):
-        d = {attr: getattr(self, attr) for attr in self.__slots__}
+        d = {'int': self.int}
+        if self.is_safe != SafeUUID.unknown:
             # is_safe is a SafeUUID instance.  Return just its value, so that
-        # it can be unpickled in older Python versions without SafeUUID.
-        d['is_safe'] = d['is_safe'].value
+            # it can be un-pickled in older Python versions without SafeUUID.
+            d['is_safe'] = self.is_safe.value
         return d
 
     def __setstate__(self, state):
-        # is_safe was added in 3.7
-        state.setdefault('is_safe', SafeUUID.unknown.value)
-
-        for attr in self.__slots__:
-            value = state[attr]
-
-            # for is_safe, restore the SafeUUID from the stored value
-            if attr == 'is_safe':
-                try:
-                    value = SafeUUID(value)
-                except ValueError:
-                    value = SafeUUID.unknown
-            object.__setattr__(self, attr, value)
+        object.__setattr__(self, 'int', state['int'])
+        # is_safe was added in 3.7; it is also omitted when it is "unknown"
+        object.__setattr__(self, 'is_safe',
+                           SafeUUID(state['is_safe'])
+                           if 'is_safe' in state else SafeUUID.unknown)
 
     def __eq__(self, other):
         if isinstance(other, UUID):