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() 96