Demo/classes/Rev.py

   1 '''
   2 A class which presents the reverse of a sequence without duplicating it.
   3 From: "Steven D. Majewski" <sdm7g@elvis.med.virginia.edu>
   4
   5 It works on mutable or inmutable sequences.
   6
   7 >>> chars = list(Rev('Hello World!'))
   8 >>> print ''.join(chars)
   9 !dlroW olleH
  10
  11 The .forw is so you can use anonymous sequences in __init__, and still
  12 keep a reference the forward sequence. )
  13 If you give it a non-anonymous mutable sequence, the reverse sequence
  14 will track the updated values. ( but not reassignment! - another
  15 good reason to use anonymous values in creating the sequence to avoid
  16 confusion. Maybe it should be change to copy input sequence to break
  17 the connection completely ? )
  18
  19 >>> nnn = range(3)
  20 >>> rnn = Rev(nnn)
  21 >>> for n in rnn: print n
  22 ...
  23 2
  24 1
  25 0
  26 >>> for n in range(4, 6): nnn.append(n)   # update nnn
  27 ...
  28 >>> for n in rnn: print n     # prints reversed updated values
  29 ...
  30 5
  31 4
  32 2
  33 1
  34 0
  35 >>> nnn = nnn[1:-1]
  36 >>> nnn
  37 [1, 2, 4]
  38 >>> for n in rnn: print n     # prints reversed values of old nnn
  39 ...
  40 5
  41 4
  42 2
  43 1
  44 0
  45
  46 #
  47 >>> WH = Rev('Hello World!')
  48 >>> print WH.forw, WH.back
  49 Hello World! !dlroW olleH
  50 >>> nnn = Rev(range(1, 10))
  51 >>> print nnn.forw
  52 [1, 2, 3, 4, 5, 6, 7, 8, 9]
  53 >>> print nnn.back
  54 [9, 8, 7, 6, 5, 4, 3, 2, 1]
  55
  56 >>> rrr = Rev(nnn)
  57 >>> rrr
  58 <1, 2, 3, 4, 5, 6, 7, 8, 9>
  59
  60 '''
  61
  62 class Rev:
  63     def __init__(self, seq):
  64         self.forw = seq
  65         self.back = self
  66
  67     def __len__(self):
  68         return len(self.forw)
  69
  70     def __getitem__(self, j):
  71         return self.forw[-(j + 1)]
  72
  73     def __repr__(self):
  74         seq = self.forw
  75         if isinstance(seq, list):
  76             wrap = '[]'
  77             sep = ', '
  78         elif isinstance(seq, tuple):
  79             wrap = '()'
  80             sep = ', '
  81         elif isinstance(seq, str):
  82             wrap = ''
  83             sep = ''
  84         else:
  85             wrap = '<>'
  86             sep = ', '
  87         outstrs = [str(item) for item in self.back]
  88         return wrap[:1] + sep.join(outstrs) + wrap[-1:]
  89
  90 def _test():
  91     import doctest, Rev
  92     return doctest.testmod(Rev)
  93
  94 if __name__ == "__main__":
  95     _test()