Edit: Rev.py
''' 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 still keep a reference the forward sequence. ) If you give it a non-anonymous mutable sequence, the reverse sequence will track the updated values. ( but not reassignment! - another good reason to use anonymous values in creating the sequence to avoid confusion. Maybe it should be change to copy input sequence to break the connection completely ? ) >>> nnn = range(3) >>> rnn = Rev(nnn) >>> for n in rnn: print n ... 2 1 0 >>> for n in range(4, 6): nnn.append(n) # update nnn ... >>> for n in rnn: print n # prints reversed updated values ... 5 4 2 1 0 >>> nnn = nnn[1:-1] >>> nnn [1, 2, 4] >>> for n in rnn: print n # prints reversed values of old nnn ... 5 4 2 1 0 # >>> WH = Rev('Hello World!') >>> print WH.forw, WH.back Hello 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()
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