'''

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()