code: def requiredAttribute(requiredAttributeName):
      
      class RequiredAttribute(attribute):
          def get(self):
              if requiredAttributeName not in self.__dict__:
                  raise AttributeError(
                      ('Required attribute %r has not been changed'
                       ' from its default value on %r' % (
                              requiredAttributeName, self)))
              return self.__dict__[requiredAttributeName]
          def set(self, value):
              self.__dict__[requiredAttributeName] = value
      return RequiredAttribute
docs: Utility for defining attributes on base classes/mixins which require their
      values to be supplied by their derived classes.  C{None} is a common, but
      almost never suitable default value for these kinds of attributes, as it
      may cause operations in the derived class to fail silently in peculiar
      ways.  If a C{requiredAttribute} is accessed before having its value
      changed, a C{AttributeError} will be raised with a helpful error message.
  
      @param requiredAttributeName: The name of the required attribute.
      @type requiredAttributeName: C{str}
  
      Example:
          >>> from epsilon.descriptor import requiredAttribute
          ...
          >>> class FooTestMixin:
          ...  expectedResult = requiredAttribute('expectedResult')
          ...
          >>> class BrokenFooTestCase(TestCase, FooTestMixin):
          ...  pass
          ...
          >>> brokenFoo = BrokenFooTestCase()
          >>> print brokenFoo.expectedResult
          Traceback (most recent call last):
              ...
          AttributeError: Required attribute 'expectedResult' has not been
                          changed from its default value on '<BrokenFooTestCase
                          instance>'.
          ...
          >>> class WorkingFooTestCase(TestCase, FooTestMixin):
          ...  expectedResult = 7
          ...
          >>> workingFoo = WorkingFooTestCase()
          >>> print workingFoo.expectedResult
          ... 7
          >>>