Note that genexps are function scopes too and therefore won't see class attributes.

Doc/reference/executionmodel.rst

@@ -52,7 +52,13 @@ variable is defined in a block, its scope includes that block.  If the
 definition occurs in a function block, the scope extends to any blocks contained
 within the defining one, unless a contained block introduces a different binding
 for the name.  The scope of names defined in a class block is limited to the
-class block; it does not extend to the code blocks of methods.
+class block; it does not extend to the code blocks of methods -- this includes
+generator expressions since they are implemented using a function scope.  This
+means that the following will fail::
+
+   class A:
+       a = 42
+       b = list(a + i for i in range(10))
 
 .. index:: single: environment