'''A class which presents the reverse of a sequence without duplicating it.From: "Steven D. Majewski" <sdm7g@elvis.med.virginia.edu> It works on mutable or inmutable sequences. >>> chars = list(Rev('Hello World!'))>>> print ''.join(chars)!dlroW olleH The .forw is so you can use anonymous sequences in __init__, and stillkeep a reference the forward sequence. )If you give it a non-anonymous mutable sequence, the reverse sequencewill track the updated values. ( but not reassignment! - anothergood reason to use anonymous values in creating the sequence to avoidconfusion. Maybe it should be change to copy input sequence to breakthe connection completely ? ) >>> nnn = range(3)>>> rnn = Rev(nnn)>>> for n in rnn: print n...210>>> for n in range(4, 6): nnn.append(n)   # update nnn...>>> for n in rnn: print n     # prints reversed updated values...54210>>> nnn = nnn[1:-1]>>> nnn[1, 2, 4]>>> for n in rnn: print n     # prints reversed values of old nnn...54210 #>>> WH = Rev('Hello World!')>>> print WH.forw, WH.backHello World! !dlroW olleH>>> nnn = Rev(range(1, 10))>>> print nnn.forw[1, 2, 3, 4, 5, 6, 7, 8, 9]>>> print nnn.back[9, 8, 7, 6, 5, 4, 3, 2, 1] >>> rrr = Rev(nnn)>>> rrr<1, 2, 3, 4, 5, 6, 7, 8, 9> ''' class Rev:    def __init__(self, seq):        self.forw = seq        self.back = self     def __len__(self):        return len(self.forw)     def __getitem__(self, j):        return self.forw[-(j + 1)]     def __repr__(self):        seq = self.forw        if isinstance(seq, list):            wrap = '[]'            sep = ', '        elif isinstance(seq, tuple):            wrap = '()'            sep = ', '        elif isinstance(seq, str):            wrap = ''            sep = ''        else:            wrap = '<>'            sep = ', '        outstrs = [str(item) for item in self.back]        return wrap[:1] + sep.join(outstrs) + wrap[-1:] def _test():    import doctest, Rev    return doctest.testmod(Rev) if __name__ == "__main__":    _test()