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, 35 insertions, 28 deletions

diff --git a/lib/sqlalchemy/sql/functions.py b/lib/sqlalchemy/sql/functions.py
index b7b9257b4..3b6da7175 100644
--- a/lib/sqlalchemy/sql/functions.py
+++ b/lib/sqlalchemy/sql/functions.py
@@ -37,8 +37,8 @@ from .elements import WithinGroup
 from .selectable import FromClause
 from .selectable import Select
 from .selectable import TableValuedAlias
+from .type_api import TypeEngine
 from .visitors import InternalTraversal
-from .visitors import TraversibleType
 from .. import util
 
 
@@ -48,7 +48,7 @@ _registry = util.defaultdict(dict)
 def register_function(identifier, fn, package="_default"):
     """Associate a callable with a particular func. name.
 
-    This is normally called by _GenericMeta, but is also
+    This is normally called by GenericFunction, but is also
     available by itself so that a non-Function construct
     can be associated with the :data:`.func` accessor (i.e.
     CAST, EXTRACT).
@@ -828,7 +828,11 @@ class Function(FunctionElement):
         ("type", InternalTraversal.dp_type),
     ]
 
-    type = sqltypes.NULLTYPE
+    name: str
+
+    identifier: str
+
+    type: TypeEngine = sqltypes.NULLTYPE
     """A :class:`_types.TypeEngine` object which refers to the SQL return
     type represented by this SQL function.
 
@@ -871,30 +875,7 @@ class Function(FunctionElement):
         )
 
 
-class _GenericMeta(TraversibleType):
-    def __init__(cls, clsname, bases, clsdict):
-        if annotation.Annotated not in cls.__mro__:
-            cls.name = name = clsdict.get("name", clsname)
-            cls.identifier = identifier = clsdict.get("identifier", name)
-            package = clsdict.pop("package", "_default")
-            # legacy
-            if "__return_type__" in clsdict:
-                cls.type = clsdict["__return_type__"]
-
-            # Check _register attribute status
-            cls._register = getattr(cls, "_register", True)
-
-            # Register the function if required
-            if cls._register:
-                register_function(identifier, cls, package)
-            else:
-                # Set _register to True to register child classes by default
-                cls._register = True
-
-        super(_GenericMeta, cls).__init__(clsname, bases, clsdict)
-
-
-class GenericFunction(Function, metaclass=_GenericMeta):
+class GenericFunction(Function):
     """Define a 'generic' function.
 
     A generic function is a pre-established :class:`.Function`
@@ -986,9 +967,34 @@ class GenericFunction(Function, metaclass=_GenericMeta):
     """
 
     coerce_arguments = True
-    _register = False
     inherit_cache = True
 
+    name = "GenericFunction"
+
+    def __init_subclass__(cls) -> None:
+        if annotation.Annotated not in cls.__mro__:
+            cls._register_generic_function(cls.__name__, cls.__dict__)
+        super().__init_subclass__()
+
+    @classmethod
+    def _register_generic_function(cls, clsname, clsdict):
+        cls.name = name = clsdict.get("name", clsname)
+        cls.identifier = identifier = clsdict.get("identifier", name)
+        package = clsdict.get("package", "_default")
+        # legacy
+        if "__return_type__" in clsdict:
+            cls.type = clsdict["__return_type__"]
+
+        # Check _register attribute status
+        cls._register = getattr(cls, "_register", True)
+
+        # Register the function if required
+        if cls._register:
+            register_function(identifier, cls, package)
+        else:
+            # Set _register to True to register child classes by default
+            cls._register = True
+
     def __init__(self, *args, **kwargs):
         parsed_args = kwargs.pop("_parsed_args", None)
         if parsed_args is None:
@@ -1006,6 +1012,7 @@ class GenericFunction(Function, metaclass=_GenericMeta):
         self.clause_expr = ClauseList(
             operator=operators.comma_op, group_contents=True, *parsed_args
         ).self_group()
+
         self.type = sqltypes.to_instance(
             kwargs.pop("type_", None) or getattr(self, "type", None)
         )