Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > comp.lang.python > #36120

Re: Yet another attempt at a safe eval() call

Show key headers only | View raw

On Sat, Jan 5, 2013 at 3:38 AM, Grant Edwards <invalid@invalid.invalid> wrote:
> I've added equals, backslash, commas, square/curly brackets, colons and semicolons to the
> prohibited character list. I also reduced the maximum length to 60
> characters.  It's unfortunate that parentheses are overloaded for both
> expression grouping and for function calling...

I have to say that an expression evaluator that can't handle parens
for grouping is badly flawed. Can you demand that open parenthesis be
preceded by an operator (or beginning of line)? For instance:

(1+2)*3+4 # Valid
1+2*(3+4) # Valid
1+2(3+4) # Invalid, this will attempt to call 2

You could explain it as a protection against mistaken use of algebraic
notation (in which the last two expressions have the same meaning and
evaluate to 15). Or, alternatively, you could simply insert the
asterisk yourself, though that could potentially be VERY confusing.

Without parentheses, your users will be forced to store intermediate
results in variables, which gets tiresome fast.

discriminant = b*b-4*a*c
denominator = 2*a
# Okay, this expression demands a square rooting, but let's pretend that's done.
sol1 = -b+discriminant
sol2 = -b-discrminant
sol1 = sol1/denominator
sol2 /= denominator # if they know about augmented assignment

You can probably recognize the formula I'm working with there, but
it's far less obvious and involves six separate statements rather than
two. And this is a fairly simple formula. It'll get a lot worse in
production.

ChrisA

Back to comp.lang.python | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread

Thread

Yet another attempt at a safe eval() call Grant Edwards <invalid@invalid.invalid> - 2013-01-03 23:25 +0000
  Re: Yet another attempt at a safe eval() call Tim Chase <python.list@tim.thechases.com> - 2013-01-03 19:11 -0600
    Re: Yet another attempt at a safe eval() call Grant Edwards <invalid@invalid.invalid> - 2013-01-04 02:34 +0000
  Re: Yet another attempt at a safe eval() call Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-01-04 07:47 +0000
    Re: Yet another attempt at a safe eval() call Grant Edwards <invalid@invalid.invalid> - 2013-01-04 15:53 +0000
      Re: Yet another attempt at a safe eval() call Michael Torrie <torriem@gmail.com> - 2013-01-04 09:05 -0700
        Re: Yet another attempt at a safe eval() call Grant Edwards <invalid@invalid.invalid> - 2013-01-04 16:16 +0000
      Re: Yet another attempt at a safe eval() call Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2013-01-05 15:56 +0000
        Re: Yet another attempt at a safe eval() call Grant Edwards <invalid@invalid.invalid> - 2013-01-06 15:12 +0000
          Re: Yet another attempt at a safe eval() call Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2013-01-07 00:08 +0000
      Re: Yet another attempt at a safe eval() call Chris Angelico <rosuav@gmail.com> - 2013-01-06 03:01 +1100
      Re: Yet another attempt at a safe eval() call Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2013-01-05 16:17 +0000
        Re: Yet another attempt at a safe eval() call matt.newville@gmail.com - 2013-01-05 08:40 -0800
        Re: Yet another attempt at a safe eval() call matt.newville@gmail.com - 2013-01-05 08:40 -0800
    Re: Yet another attempt at a safe eval() call Grant Edwards <invalid@invalid.invalid> - 2013-01-04 16:38 +0000
      Re: Yet another attempt at a safe eval() call Chris Angelico <rosuav@gmail.com> - 2013-01-05 03:51 +1100
        Re: Yet another attempt at a safe eval() call Grant Edwards <invalid@invalid.invalid> - 2013-01-04 17:14 +0000
          Re: Yet another attempt at a safe eval() call Chris Angelico <rosuav@gmail.com> - 2013-01-05 04:21 +1100
            Re: Yet another attempt at a safe eval() call Grant Edwards <invalid@invalid.invalid> - 2013-01-04 18:09 +0000
              Re: Yet another attempt at a safe eval() call Chris Angelico <rosuav@gmail.com> - 2013-01-05 05:23 +1100
                Re: Yet another attempt at a safe eval() call Grant Edwards <invalid@invalid.invalid> - 2013-01-04 18:43 +0000
                Re: Yet another attempt at a safe eval() call Chris Angelico <rosuav@gmail.com> - 2013-01-05 06:02 +1100
  Re: Yet another attempt at a safe eval() call Chris Rebert <clp2@rebertia.com> - 2013-01-03 23:50 -0800
  Re: Yet another attempt at a safe eval() call Terry Reedy <tjreedy@udel.edu> - 2013-01-04 07:24 -0500
    Re: Yet another attempt at a safe eval() call Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-01-04 13:33 +0000
      Re: Yet another attempt at a safe eval() call Grant Edwards <invalid@invalid.invalid> - 2013-01-04 15:59 +0000
      Re: Yet another attempt at a safe eval() call Alister <alister.ware@ntlworld.com> - 2013-01-04 18:13 +0000

csiph-web