Rev.py revision 946c53ed7ff53f38792ac35e5da21de3e0a48ef2 
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
63from string import joinfields
64class 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:]
90