Re: Transforming a str to an operator
On Fri, 28 Aug 2009, Ben Finney wrote:
> Duke Normandin writes:
>
> > Hey
> >
> > I'm a Python noob
> >
> > So far so good!
> >
> > I've written the following:
> >
> > num1 = raw_input('Enter the first number: ')
> > num2 = raw_input('Enter the second number: ')
> > op = raw_input('Select one of the following [+-*/]: ')
> > print 'The answer is: ', int(num1), eval(op), int(num2)
> >
> >
> > How do I convert the contents of "op" from a string to an actual
> > arithmetic operator? eval() does not seem to be the answer. TIA!
>
> In general, ‘eval’ on unsanitised input is not the answer.
Agreed! If I were to expose "eval" to the 'net, I would have some
input error checking and "type" checks to insure that only integers
and valid operators were being input.
>
> I would use the following approach:
>
> import operator
>
> op_funcs = {
> '+': operator.add,
> '-': operator.sub,
> '*': operator.mul,
> '/': operator.div,
> }
>
> num_1 = int(raw_input('Enter the first number: '))
> num_2 = int(raw_input('Enter the second number: '))
> op_prompt = (
> "Select an operator "
> + "[" + "".join(s for s in op_funcs.keys()) + "]"
> + ": ")
> op_symbol = raw_input(op_prompt)
> op_func = op_funcs[op_symbol]
> print 'The answer is: ', op_func(num_1, num_2)
>
> This has several advantages:
>
> * The input isn't evaluated directly as code.
>
> * The operator symbols are specified in one place, the ‘op_funcs’
> mapping; if you want to change the set of possible operators, you just
> change it there.
>
> * If the input results in an operator that's not defined, it won't
> attempt to perform it; instead, a simple KeyError will result when
> trying to find the corresponding operator function.
Cool! Something useful to study...
Thanks for the input!
--
duke--
http://mail.python.org/mailman/listinfo/python-list
Re: Transforming a str to an operator
On Thu, 27 Aug 2009, r wrote: > On Aug 27, 10:52 pm, Duke Normandin wrote: > > How do I convert the contents of "op" from a string to an actual > > arithmetic operator? eval() does not seem to be the answer. TIA! > > > Try this.. > > >>> op = '+' > >>> one = '1' > >>> two = '2' > >>> one+op+two > '1+2' > >>> eval(one+op+two) > 3 > > > you could also use string formatting. I see! Concatenate the strings within the "eval()" function. Of course, it's prudent not to expose "eval" to the outside world. But for learning purposes Thanks for the input! -- duke-- http://mail.python.org/mailman/listinfo/python-list
Re: Transforming a str to an operator
On Thu, 27 Aug 2009, Stephen Hansen wrote:
> >
> > num1 = raw_input('Enter the first number: ')
> > num2 = raw_input('Enter the second number: ')
> > op = raw_input('Select one of the following [+-*/]: ')
> > print 'The answer is: ', int(num1), eval(op), int(num2)
> >
> >
> > How do I convert the contents of "op" from a string to an actual
> > arithmetic operator? eval() does not seem to be the answer. TIA!
> >
>
> You could eval(num1+op+num2), but it'd be safer to do:
>
> import operator
> operators = {"+": operator.add, "-": operator.sub, "*": operator.mul, "/":
> operator.div}
> fn = operators[op]
> print "The answer is:", fn(int(num1), int(num2))
>
> Its best to avoid eval when possible :)
>
> --S
>
In *any* language "eval" is dangerous, so your second example would
also be my choice. Thanks for the clue.
BTW, I hunted hi-n-lo for something that would address my question at
http://docs.python.org. I obviously didn't have much luck. Something
about those docs that is confusing
--
duke
--
http://mail.python.org/mailman/listinfo/python-list
Transforming a str to an operator
Hey
I'm a Python noob
So far so good!
I've written the following:
num1 = raw_input('Enter the first number: ')
num2 = raw_input('Enter the second number: ')
op = raw_input('Select one of the following [+-*/]: ')
print 'The answer is: ', int(num1), eval(op), int(num2)
How do I convert the contents of "op" from a string to an actual
arithmetic operator? eval() does not seem to be the answer. TIA!
--
duke
--
http://mail.python.org/mailman/listinfo/python-list
