Yet another attempt at a safe eval() call
invalid at invalid.invalid
Fri Jan 4 19:09:34 CET 2013
On 2013-01-04, Chris Angelico <rosuav at gmail.com> wrote:
> On Sat, Jan 5, 2013 at 4:14 AM, Grant Edwards <invalid at invalid.invalid> wrote:
>> On 2013-01-04, Chris Angelico <rosuav at gmail.com> wrote:
>>> On Sat, Jan 5, 2013 at 3:38 AM, Grant Edwards <invalid at 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
>>> I have to say that an expression evaluator that can't handle parens
>>> for grouping is badly flawed.
>> Indeed. That's why I didn't disallow parens.
>> What I was implying was that since you have to allow parens for
>> grouping, there's no simple way to disallow function calls.
> Yeah, and a safe evaluator that allows function calls is highly vulnerable.
>>> Can you demand that open parenthesis be preceded by an operator (or
>>> beginning of line)?
>> Yes, but once you've parsed the expression to the point where you can
>> enforce rules like that, you're probably most of the way to doing the
>> "right" thing and evaluating the expression using ast or pyparsing or
>> Some might argue that repeated tweaking of and adding limitiations to
>> a "safe eval" is just heading down that same road in a different car.
>> They'd probably be right: in the end, it will probably have been less
>> work to just do it with ast. But it's still interesting to try. :)
> Yep, have fun with it. As mentioned earlier, though, security isn't
> all that critical; so in this case, chances are you can just leave
> parens permitted and let function calls potentially happen.
An ast-based evaluator wasn't as complicated as I first thought: the
examples I'd been looking at implemented far more features than I
needed. This morning I found a simpler example at
The error messages are still pretty cryptic, so improving
that will add a few more lines. One nice thing about the ast code is
that it's simple to add code to allow C-like character constants such
that ('A' === 0x41). Here's the first pass at ast-based code:
operators = \
if isinstance(node, ast.Name):
if node.id not in symbolTable:
raise ParseError("name '%s' undefined" % node.id)
elif isinstance(node, ast.Num):
elif isinstance(node, ast.operator) or isinstance(node, ast.unaryop):
elif isinstance(node, ast.BinOp):
return _eval_expr(node.op)(_eval_expr(node.left), _eval_expr(node.right))
elif isinstance(node, ast.UnaryOp):
raise ParseError("error parsing expression at node %s" % node)
Grant Edwards grant.b.edwards Yow! A can of ASPARAGUS,
at 73 pigeons, some LIVE ammo,
gmail.com and a FROZEN DAQUIRI!!
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