Demo/classes/Rev.py

   1 # A class which presents the reverse of a sequence without duplicating it.
   2 # From: "Steven D. Majewski" <sdm7g@elvis.med.virginia.edu>
   3
   4 # It works on mutable or inmutable sequences.
   5 #
   6 # >>> for c in Rev( 'Hello World!' ) : sys.stdout.write( c )
   7 # ... else: sys.stdout.write( '\n' )
   8 # ...
   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( 0, 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 # nnn = Rev( range( 1, 10 ) )
  50 # print nnn.forw
  51 # print nnn
  52 #
  53 # produces output:
  54 #
  55 # Hello World! !dlroW olleH
  56 # [1, 2, 3, 4, 5, 6, 7, 8, 9]
  57 # [9, 8, 7, 6, 5, 4, 3, 2, 1]
  58 #
  59 # >>>rrr = Rev( nnn )
  60 # >>>rrr
  61 # <1, 2, 3, 4, 5, 6, 7, 8, 9>
  62
  63 from string import joinfields
  64 class Rev:
  65         def __init__( self, seq ):
  66                 self.forw = seq
  67                 self.back = self
  68         def __len__( self ):
  69                 return len( self.forw )
  70         def __getitem__( self, j ):
  71                 return self.forw[ -( j + 1 ) ]
  72         def __repr__( self ):
  73                 seq = self.forw
  74                 if type(seq) == type( [] ) :
  75                         wrap = '[]'
  76                         sep = ', '
  77                 elif type(seq) == type( () ) :
  78                         wrap = '()'
  79                         sep = ', '
  80                 elif type(seq) == type( '' ) :
  81                         wrap = ''
  82                         sep = ''
  83                 else:
  84                         wrap = '<>'
  85                         sep = ', '
  86                 outstrs = []
  87                 for item in self.back :
  88                         outstrs.append( str( item ) )
  89                 return wrap[:1] + joinfields( outstrs, sep ) + wrap[-1:]