combin.py

def permute(choices):                                                                                                                                                                                               
    if not choices:                                                                                                                                                                                                 
        yield ()                                                                                                                                                                                                    
    else:                                                                                                                                                                                                           
        for item in choices[0]:                                                                                                                                                                                     
            for suffix in permute(choices[1:]):                                                                                                                                                                     
                yield (item,) + suffix                                                                                                                                                                              
                                                                                                                                                                                                                    
# A unique object that is not a member of any possible powerset                                                                                                                                                     
NON_MEMBER = object()                                                                                                                                                                                               
                                                                                                                                                                                                                    
# Another sensible seq_class is set, but duplicate sets might be generated                                                                                                                                          
def powerset(members, seq_class = tuple):                                                                                                                                                                           
    choices = tuple((NON_MEMBER, i) for i in members)                                                                                                                                                               
    for permutation in permute(choices):                                                                                                                                                                            
        yield seq_class(filter(lambda i: i is not NON_MEMBER, permutation))