snag at jool.po
Mon Jun 23 16:40:47 CEST 2003
"Sean Ross" <sross at connectmail.carleton.ca> wrote in message
news:okuJa.1035$Fe3.161376 at news20.bellglobal.com...
> > > > The problem is division, I would like it that only exact division
> > > > aloud, I mean no remainders.
> > > > i would like if that was illegal because 5 doesnt go into 22 , 4
> > > > exactly, and have the eval function raise an execption which i could
> > > > handle.
> I'm not going to ask why you want this behaviour, I'm just going to offer
> quick, dirty, and possibly buggy solution.
> This code relies on the following assumptions:
>  You want to ensure that *only* exact division can occur
>  The only numbers your users can *ever* enter are integers, i.e.
> + 3.14' will raise an exception.
> So, for this to work, you'll need to somehow ensure . A more reliable
> solution would be to make your own simple interpreter. I would recommend
> checking out SPARK. There are examples available with the software that
> could extend to fit your problem domain. Anyway, here's some code. I won't
> guarantee its robustness, I'll just say it appears to work.
> import re
> class ExactDivisionError(Exception):
> class myint(int):
> def __div__(self, other):
> "does exact division only"
> quotient, remainder = divmod(self, other)
> if remainder:
> raise ExactDivisionError, "exact division only"
> return quotient
> def replaceint(match):
> "substitues 'myint(integer)' for each matched integer"
> return 'myint(%s)'% match.string[match.start():match.end()]
> # some testing...
> pattern = re.compile(r'\d+') # matches integers
> txt = '2 + 22/5'
> txt = pattern.sub(replaceint, txt)
> print txt # 'myint(2) + myint(22)/myint(5)'
> result = eval(txt) # this will raise an exception for 'txt'
> print result
> except ExactDivisionError:
> print "ExactDivisionError"
> Hope that's helpful,
That works great for simple 2 number ones
txt = pattern.sub(replaceint, "4/3")
Traceback (most recent call last):
File "<stdin>", line 1, in ?
File "<string>", line 0, in ?
File "<stdin>", line 6, in __div__
__main__.ExactDivisionError: exact division only
bit if the user types something more complex like
>>> txt = pattern.sub(replaceint, "((4 * 5) -2) / 5 ")
it doesnt raise an exception even though im dividing
18 / 5 which doesnt go evenly .
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