A method to unset category initialisation

Author: simon-king-jena

src/sage/structure/parent.pyx

@@ -503,6 +503,67 @@ cdef class Parent(category_object.CategoryObject):
             print 'hash of {0} changed in Parent._refine_category_ during refinement' \
                 .format(str(self.__class__))
 
+    def _unset_category(self):
+        """
+        Remove the information on ``self``'s category.
+
+        NOTE:
+
+        This may change ``self``'s class!
+
+        EXAMPLES:
+
+        Let us create a parent in the category of rings::
+
+            sage: class MyParent(Parent):
+            ....:     def __init__(self):
+            ....:         Parent.__init__(self, category=Rings())
+            ....:
+            sage: P = MyParent()
+            sage: P.category()
+            Category of rings
+
+        Of course, its category is initialised::
+
+            sage: P._is_category_initialized()
+            True
+
+        We may now refine the category to the category of fields.
+        Note that this changes the class::
+
+            sage: C = type(P)
+            sage: C == MyParent
+            False
+            sage: P._refine_category_(Fields())
+            sage: P.category()
+            Category of fields
+            sage: C == type(P)
+            False
+
+        Now we may have noticed that the category refinement was a
+        mistake. We do not need to worry, because we can undo category
+        initialisation totally::
+
+            sage: P._unset_category()
+            sage: P._is_category_initialized()
+            False
+            sage: type(P) == MyParent
+            True
+
+        Hence, we can now initialise the parent again in the original
+        category, i.e., the category of rings. We find that not only
+        the category, but also theclass of the parent is brought back
+        to what it was after the original category initialisation::
+
+            sage: P._init_category_(Rings())
+            sage: type(P) == C
+            True
+
+        """
+        self._category = None
+        if not is_extension_type(self.__class__):
+            while issubclass(self.__class__, Sets_parent_class):
+                self.__class__ = self.__class__.__base__
 
     # This probably should go into Sets().Parent
     @lazy_attribute