Close #14588: added a PEP 3115 compliant dynamic type creation mechanism

Lib/types.py

@@ -40,3 +40,61 @@ GetSetDescriptorType = type(FunctionType.__code__)
 MemberDescriptorType = type(FunctionType.__globals__)
 
 del sys, _f, _g, _C,                              # Not for export
+
+
+# Provide a PEP 3115 compliant mechanism for class creation
+def new_class(name, bases=(), kwds=None, exec_body=None):
+    """Create a class object dynamically using the appropriate metaclass."""
+    meta, ns, kwds = prepare_class(name, bases, kwds)
+    if exec_body is not None:
+        exec_body(ns)
+    return meta(name, bases, ns, **kwds)
+
+def prepare_class(name, bases=(), kwds=None):
+    """Call the __prepare__ method of the appropriate metaclass.
+
+    Returns (metaclass, namespace, kwds) as a 3-tuple
+
+    *metaclass* is the appropriate metaclass
+    *namespace* is the prepared class namespace
+    *kwds* is an updated copy of the passed in kwds argument with any
+    'metaclass' entry removed. If no kwds argument is passed in, this will
+    be an empty dict.
+    """
+    if kwds is None:
+        kwds = {}
+    else:
+        kwds = dict(kwds) # Don't alter the provided mapping
+    if 'metaclass' in kwds:
+        meta = kwds.pop('metaclass')
+    else:
+        if bases:
+            meta = type(bases[0])
+        else:
+            meta = type
+    if isinstance(meta, type):
+        # when meta is a type, we first determine the most-derived metaclass
+        # instead of invoking the initial candidate directly
+        meta = _calculate_meta(meta, bases)
+    if hasattr(meta, '__prepare__'):
+        ns = meta.__prepare__(name, bases, **kwds)
+    else:
+        ns = {}
+    return meta, ns, kwds
+
+def _calculate_meta(meta, bases):
+    """Calculate the most derived metaclass."""
+    winner = meta
+    for base in bases:
+        base_meta = type(base)
+        if issubclass(winner, base_meta):
+            continue
+        if issubclass(base_meta, winner):
+            winner = base_meta
+            continue
+        # else:
+        raise TypeError("metaclass conflict: "
+                        "the metaclass of a derived class "
+                        "must be a (non-strict) subclass "
+                        "of the metaclasses of all its bases")
+    return winner