remove internal use of metaclasses

All but one metaclass used internally can now
be replaced using __init_subclass__().  Within this
patch we remove:

* events._EventMeta
* sql.visitors.TraversibleType
* sql.visitors.InternalTraversibleType
* testing.fixtures.FindFixture
* testing.fixtures.FindFixtureDeclarative
* langhelpers.EnsureKWArgType
* sql.functions._GenericMeta
* sql.type_api.VisitableCheckKWArg (was a mixture of TraversibleType
  and EnsureKWArgType)

The remaining internal class is MetaOptions used by the
sql.Options object which is in turn currently mostly for
ORM internal use, as this type implements class level overrides
for the ``+`` operator.

For declarative, removing DeclarativeMeta in place of
an `__init_subclass__()` class would not be fully feasible as
it would break backwards compatibility with applications that
refer to this class explicitly, but also DeclarativeMeta intercepts
class-level attribute set and delete operations which is a widely
used pattern.   An option for declarative base to use
`__init_subclass__()` should be provided but this is out of
scope for this particular change.

Change-Id: I8aa898c7ab59d887739037d34b1cbab36521ab78
References: #6810

1 files changed, 12 insertions, 11 deletions

diff --git a/lib/sqlalchemy/sql/lambdas.py b/lib/sqlalchemy/sql/lambdas.py
index 2387e551e..d71c85d60 100644
--- a/lib/sqlalchemy/sql/lambdas.py
+++ b/lib/sqlalchemy/sql/lambdas.py
@@ -11,6 +11,7 @@ import operator
 import types
 import weakref
 
+from . import cache_key as _cache_key
 from . import coercions
 from . import elements
 from . import roles
@@ -185,7 +186,7 @@ class LambdaElement(elements.ClauseElement):
         else:
             parent_closure_cache_key = ()
 
-        if parent_closure_cache_key is not traversals.NO_CACHE:
+        if parent_closure_cache_key is not _cache_key.NO_CACHE:
             anon_map = traversals.anon_map()
             cache_key = tuple(
                 [
@@ -194,7 +195,7 @@ class LambdaElement(elements.ClauseElement):
                 ]
             )
 
-            if traversals.NO_CACHE not in anon_map:
+            if _cache_key.NO_CACHE not in anon_map:
                 cache_key = parent_closure_cache_key + cache_key
 
                 self.closure_cache_key = cache_key
@@ -204,17 +205,17 @@ class LambdaElement(elements.ClauseElement):
                 except KeyError:
                     rec = None
             else:
-                cache_key = traversals.NO_CACHE
+                cache_key = _cache_key.NO_CACHE
                 rec = None
 
         else:
-            cache_key = traversals.NO_CACHE
+            cache_key = _cache_key.NO_CACHE
             rec = None
 
         self.closure_cache_key = cache_key
 
         if rec is None:
-            if cache_key is not traversals.NO_CACHE:
+            if cache_key is not _cache_key.NO_CACHE:
                 rec = AnalyzedFunction(
                     tracker, self, apply_propagate_attrs, fn
                 )
@@ -233,7 +234,7 @@ class LambdaElement(elements.ClauseElement):
 
         self._rec = rec
 
-        if cache_key is not traversals.NO_CACHE:
+        if cache_key is not _cache_key.NO_CACHE:
             if self.parent_lambda is not None:
                 bindparams[:0] = self.parent_lambda._resolved_bindparams
 
@@ -326,8 +327,8 @@ class LambdaElement(elements.ClauseElement):
         return expr
 
     def _gen_cache_key(self, anon_map, bindparams):
-        if self.closure_cache_key is traversals.NO_CACHE:
-            anon_map[traversals.NO_CACHE] = True
+        if self.closure_cache_key is _cache_key.NO_CACHE:
+            anon_map[_cache_key.NO_CACHE] = True
             return None
 
         cache_key = (
@@ -808,7 +809,7 @@ class AnalyzedCode:
                     for tup_elem in opts.track_on[idx]
                 )
 
-        elif isinstance(elem, traversals.HasCacheKey):
+        elif isinstance(elem, _cache_key.HasCacheKey):
 
             def get(closure, opts, anon_map, bindparams):
                 return opts.track_on[idx]._gen_cache_key(anon_map, bindparams)
@@ -834,7 +835,7 @@ class AnalyzedCode:
 
         """
 
-        if isinstance(cell_contents, traversals.HasCacheKey):
+        if isinstance(cell_contents, _cache_key.HasCacheKey):
 
             def get(closure, opts, anon_map, bindparams):
 
@@ -1166,7 +1167,7 @@ class PyWrapper(ColumnOperators):
             and not isinstance(
                 # TODO: coverage where an ORM option or similar is here
                 value,
-                traversals.HasCacheKey,
+                _cache_key.HasCacheKey,
             )
         ):
             name = object.__getattribute__(self, "_name")