[Tutor] Input to python executable code and design question

[Liam Clarke]

Eep.


On Mon, 10 Jan 2005 13:04:33 +1300, Liam Clarke <cyresse at gmail.com> wrote:
> I think you're looking for eval() - but that's a big security hole,
> and wouldn't handle f(x) notation overly well, unless you parse like
> John said.
> 
> 
> On Mon, 10 Jan 2005 12:52:24 +1300 (NZDT), jfouhy at paradise.net.nz
> <jfouhy at paradise.net.nz> wrote:
> > Quoting Ismael Garrido <ismaelgf at adinet.com.uy>:
> >
> > > I am trying to make a program that will plot functions. For that, I need
> > > to be able to get an input (the function to be plotted) and execute it.
> > > So, my question is, how do I use the input? I have found no way to
> > > convert the string to some kind of executable code.
> >
> > So you want the user to be able to type something like "f(x) = sin(2*x)" and
> > then your program will plot it --- is that correct?
> >
> > Maybe you could parse it yourself?  I have found SimpleParse quite easy to use
> > --- http://simpleparse.sourceforge.net/ .  You will need to write your own
> > grammar to describe functions --- something like this, I guess:
> >
> > eqn := fname, '(', varlist, ')=', expr
> > varlist := var, (',', var)*
> >
> > expr := atom, (binop, atom)?
> > atom := var / (fun, '(', expr, ')') / num
> > fun := 'sin' / 'cos' / 'tan' / ...
> >
> > var := char
> > fname := char+
> > num := digit+
> > binop := '+' / '*' / '/' / '-'
> >
> > char := [a-zA-Z]
> > digit := [0-9]
> >
> > although you will need to work on that to get the precedence right and avoid
> > undesired recursion and the like.  SimpleParse will give you a tree (made of
> > nested tuples) representing your function.  You can then have a go at converting
> > the tree to a function.
> >
> > I guess the standard way to do this would be something like:
> >
> > def convert(node):
> >     functionName = node[0]
> >     children = node[1]
> >     if functionName == '*':
> >         return convert(children[0]) * convert(children[1])
> >     elif functionName == '+':
> >         ...
> >
> > But you may be able to come up with something more clever.
> >
> > Hope this helps.
> >
> > --
> > John.
> > _______________________________________________
> > Tutor maillist  -  Tutor at python.org
> > http://mail.python.org/mailman/listinfo/tutor
> >
> 
> 
> --
> 'There is only one basic human right, and that is to do as you damn well please.
> And with it comes the only basic human duty, to take the consequences.
> 


-- 
'There is only one basic human right, and that is to do as you damn well please.
And with it comes the only basic human duty, to take the consequences.