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'''
A class which presents the reverse of a sequence without duplicating it.
From: "Steven D. Majewski" <sdm7g@elvis.med.virginia.edu>
It works on mutable or inmutable sequences.
>>> Rev('Hello World!')
!dlroW olleH
The .forw is so you can use anonymous sequences in __init__, and still
keep a reference the forward sequence. )
If you give it a non-anonymous mutable sequence, the reverse sequence
will track the updated values. ( but not reassignment! - another
good reason to use anonymous values in creating the sequence to avoid
confusion. Maybe it should be change to copy input sequence to break
the connection completely ? )
>>> nnn = list(range(3))
>>> rnn = Rev(nnn)
>>> for n in rnn: n
...
2
1
0
>>> for n in range(4, 6): nnn.append(n) # update nnn
...
>>> for n in rnn: n # prints reversed updated values
...
5
4
2
1
0
>>> nnn = nnn[1:-1]
>>> nnn
[1, 2, 4]
>>> for n in rnn: n # prints reversed values of old nnn
...
5
4
2
1
0
#
>>> WH = Rev('Hello World!')
>>> print(WH.forw, WH.back)
Hello World! !dlroW olleH
>>> nnn = Rev(list(range(1, 10)))
>>> print(nnn.forw)
[1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> print(nnn.back)
[9, 8, 7, 6, 5, 4, 3, 2, 1]
>>> Rev(nnn)
<1, 2, 3, 4, 5, 6, 7, 8, 9>
'''
class Rev:
def __init__(self, seq):
self.forw = seq
self.back = self
def __len__(self):
return len(self.forw)
def __getitem__(self, j):
return self.forw[-(j + 1)]
def __repr__(self):
seq = self.forw
if isinstance(seq, list):
wrap = '[]'
sep = ', '
elif isinstance(seq, tuple):
wrap = '()'
sep = ', '
elif isinstance(seq, str):
wrap = ''
sep = ''
else:
wrap = '<>'
sep = ', '
outstrs = [str(item) for item in self.back]
return wrap[:1] + sep.join(outstrs) + wrap[-1:]
def _test():
import doctest, Rev
return doctest.testmod(Rev)
if __name__ == "__main__":
_test()
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