Set parity of a string
snacktime
snacktime at gmail.com
Wed Jan 26 14:50:46 EST 2005
I'm trying to figure out why the following code transforms ascii STX
(control-b) into "\x82". The perl module I use returns a value of
"^B", and that is also what the application I am communicating with
expects to see. Some ascii characters such as FS and GS come through
fine, but STX and ETX get transformed incorrectly (at least for what I
need they do).
What am I missing here?
Chris
> But here's one for you anyway. It raises an exception if any
> input character is non-ASCII, otherwise when you call set_parity()
> with a string and a parity parameter of 'even', 'mark', 'none',
> 'odd', or 'space', it returns a string of the same length with
> the high (parity) bits set appropriately.
>
> '''parity module for python'''
> # Copyright 2005 Engenuity Corporation
> # Released under the "MIT license"
> # at http://www.opensource.org/licenses/mit-license.php
>
> import string
> import operator
>
> def _makeTable(parity):
> bitMap = {'even': [0,128], 'odd': [128,0], 'mark': [128,128]}
> table = []
> for c in range(128):
> even_odd = (sum(bool(c & 1<<b) for b in range(8))) & 1
> cp = chr(c | bitMap.get(parity, [0,0])[even_odd])
> table.append(cp)
> table.extend(chr(c) for c in range(128, 256))
> table = ''.join(table)
> return table, table[128:]
>
> _map = {}
> for parity in 'even odd mark space none'.split():
> _map[parity] = _makeTable(parity)
>
> def set_parity(data, parity='none'):
> '''return string with parity bits, accepting only ASCII characters
> (0..127) as input'''
> out = string.translate(data, *_map[parity.lower()])
> if len(out) != len(data):
> raise ValueError('non-ASCII characters in input')
> else:
> return out
>
> if __name__ == '__main__':
> print 'Running self-tests for parity.py'
>
> # check for invalid input handling
> for input in [ '\xff', '\x80', 'test\xA0ing' ]:
> try:
> set_parity(input)
> except ValueError:
> pass
> else:
> assert False, 'no exception for non-ASCII input'
>
> # check for various valid inputs
> for i, (parity, input, expected) in enumerate([
> ('space', 'test', 'test'),
> ('SPACE', '\x00\x7f', '\x00\x7f'),
> ('mark', 'test', '\xf4\xe5\xf3\xf4'),
> ('Odd', '\x00test\x7f', '\x80\xf4\xe5s\xf4\x7f'),
> ('even', '\x00\x01\x02\x03test\x7d\x7e\x7f',
> '\x00\x81\x82\x03te\xf3t\x7d\x7e\xff'),
> ]):
> actual = set_parity(input, parity)
> assert expected == actual, 'case %d: %r != %r' % (i, expected, actual)
>
> print 'All tests passed.'
> --
> http://mail.python.org/mailman/listinfo/python-list
>
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