It seems pretty common to me to have an argument, in a dynamically typed language that is either an Object or a key to lookup that object. For instance when I'm working with a database I might have a method getMyRelatedStuff(person)

All I really need to lookup the related stuff is the id of the person so my method could look like this in python:

def getMyRelatedStuff(person_or_id):
    id = person_or_id.id if isinstance(person,User) else person_or_id
    #do some lookup

Or going the other direction:

def someFileStuff(file_or_name):
    file = file_or_name if hasattr(file,'write') else open(file_or_name)