human
Solve the following python programming problem given the following code:
def _context_descriptor(attr = None) : 
    getter = operator.attrgetter (attr) 
    def _property_for_context(context) : 
        try : 
            transaction_context = context.transaction_ctx 
except exception.NoEngineContextEstablished : 
            raise exception.NoEngineContextEstablished (('No TransactionContext is established for this %s object within the current thread.
the %r attribute is unavailable.' % (context, attr))) 
else : 
            result = getter (transaction_context) 
            if (result is None) : 
                raise exception.ContextNotRequestedError (("The '%s' context attribute was requested but it has not been established for this context." % attr)) 
return result 
return property (<MASK>) 
    
    Given the code above, what is a proper replacement for <MASK>? Choose among: getter, attr
assistant
#Here is a proper replacement for <MASK>.
def _property_for_context(