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/Demo/classes/Rev.py

http://unladen-swallow.googlecode.com/
Python | 95 lines | 93 code | 0 blank | 2 comment | 0 complexity | e8b6a62f19162bab884894634538e095 MD5 | raw file
 1'''
 2A class which presents the reverse of a sequence without duplicating it.
 3From: "Steven D. Majewski" <sdm7g@elvis.med.virginia.edu>
 4
 5It works on mutable or inmutable sequences.
 6
 7>>> chars = list(Rev('Hello World!'))
 8>>> print ''.join(chars)
 9!dlroW olleH
10
11The .forw is so you can use anonymous sequences in __init__, and still
12keep a reference the forward sequence. )
13If you give it a non-anonymous mutable sequence, the reverse sequence
14will track the updated values. ( but not reassignment! - another
15good reason to use anonymous values in creating the sequence to avoid
16confusion. Maybe it should be change to copy input sequence to break
17the connection completely ? )
18
19>>> nnn = range(3)
20>>> rnn = Rev(nnn)
21>>> for n in rnn: print n
22...
232
241
250
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...
305
314
322
331
340
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...
405
414
422
431
440
45
46#
47>>> WH = Rev('Hello World!')
48>>> print WH.forw, WH.back
49Hello 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
62class 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
90def _test():
91    import doctest, Rev
92    return doctest.testmod(Rev)
93
94if __name__ == "__main__":
95    _test()