"Steven D'Aprano" <steve@pearwood.info> wrote in message news:200805281007.59902.steve@pearwood.info... Just throwing a suggestion out there... def atomic(obj, _atomic=(basestring,)): try: return bool(obj.__atomic__) except AttributeError: if isinstance(obj, _atomic): return True else: try: iter(obj) except TypeError: return True return False assert atomic("abc") assert not atomic(['a', 'b', 'c']) If built-in objects grew an __atomic__ attribute, you could simplify the atomic() function greatly: def atomic(obj): return bool(obj.__atomic__) However atomic() is defined, now flatten() is easy: def flatten(obj): if atomic(obj): yield obj else: for item in obj: for i in flatten(item): yield i If you needed more control, you could customise it using standard techniques e.g. shadow the atomic() function with your own version, sub-class the types you wish to treat differently, make __atomic__ a computed property instead of a simple attribute, etc. ================== This is a lot of work to avoid being explicit about either atomic or non-atomic classes on an site, package, module, or call basis ;-)