A tentative fix for SF bug #503837 (Roeland Rengelink):

gvanrossum · gvanrossum · commit 340cbe74b9e2 · 2002-01-15T21:06:07.000Z
type.__module__ problems (again?)

This simply initializes the __module__ local in a class statement from
the __name__ global.  I'm not 100% sure that this is the correct fix,
although it usually does the right thing.  The problem is that if the
class statement executes in a custom namespace, the __name__ global
may be taken from __builtins__, in which case it would have the value
__builtin__, or it may not exist at all (if the custom namespace also
has a custom __builtins__), in which case the class statement will
fail.

Nevertheless, unless someone finds a better solution, this is a 2.2.1
bugfix too.
diff --git a/Python/compile.c b/Python/compile.c
@@ -3926,6 +3926,9 @@ compile_classdef(struct compiling *c, node *n)
 	/* classdef: 'class' NAME ['(' testlist ')'] ':' suite */
 	c->c_name = STR(CHILD(n, 1));
 	c->c_private = c->c_name;
+	/* Initialize local __module__ from global __name__ */
+	com_addop_name(c, LOAD_GLOBAL, "__name__");
+	com_addop_name(c, STORE_NAME, "__module__");
 	ch = CHILD(n, NCH(n)-1); /* The suite */
 	doc = get_docstring(c, ch);
 	if (doc != NULL) {
