Add new infrastructure to support greater use of __slots__

* Changed AliasedInsp to use __slots__
* Migrated all of strategy_options to use __slots__ for objects.
  Adds new infrastructure to traversals to support shallow
  copy, to dict and from dict based on internal traversal
  attributes.  Load / _LoadElement then leverage this to
  provide clone / generative / getstate without the need
  for __dict__ or explicit attribute lists.

Doing this change revealed that there are lots of things that
trigger off of whether or not a class has a __visit_name__ attribute.
so to suit that we've gone back to having Visitable, which is
a better name than Traversible at this point  (I think
Traversible is mis-spelled too).

Change-Id: I13d04e494339fac9dbda0b8e78153418abebaf72
References: #7527

1 files changed, 15 insertions, 0 deletions

diff --git a/lib/sqlalchemy/sql/cache_key.py b/lib/sqlalchemy/sql/cache_key.py
index 8dd44dbf0..42bd60353 100644
--- a/lib/sqlalchemy/sql/cache_key.py
+++ b/lib/sqlalchemy/sql/cache_key.py
@@ -47,6 +47,11 @@ class CacheTraverseTarget(enum.Enum):
 class HasCacheKey:
     """Mixin for objects which can produce a cache key.
 
+    This class is usually in a hierarchy that starts with the
+    :class:`.HasTraverseInternals` base, but this is optional.  Currently,
+    the class should be able to work on its own without including
+    :class:`.HasTraverseInternals`.
+
     .. seealso::
 
         :class:`.CacheKey`
@@ -55,6 +60,8 @@ class HasCacheKey:
 
     """
 
+    __slots__ = ()
+
     _cache_key_traversal = NO_CACHE
 
     _is_has_cache_key = True
@@ -106,11 +113,17 @@ class HasCacheKey:
             _cache_key_traversal = getattr(cls, "_cache_key_traversal", None)
             if _cache_key_traversal is None:
                 try:
+                    # this would be HasTraverseInternals
                     _cache_key_traversal = cls._traverse_internals
                 except AttributeError:
                     cls._generated_cache_key_traversal = NO_CACHE
                     return NO_CACHE
 
+            assert _cache_key_traversal is not NO_CACHE, (
+                f"class {cls} has _cache_key_traversal=NO_CACHE, "
+                "which conflicts with inherit_cache=True"
+            )
+
             # TODO: wouldn't we instead get this from our superclass?
             # also, our superclass may not have this yet, but in any case,
             # we'd generate for the superclass that has it.   this is a little
@@ -323,6 +336,8 @@ class HasCacheKey:
 
 
 class MemoizedHasCacheKey(HasCacheKey, HasMemoized):
+    __slots__ = ()
+
     @HasMemoized.memoized_instancemethod
     def _generate_cache_key(self):
         return HasCacheKey._generate_cache_key(self)