Yet another attempt at a safe eval() call

Grant Edwards 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
>>>> calling...
>>>
>>> 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
>> similar.
>>
>> 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

  http://stackoverflow.com/questions/2371436/evaluating-a-mathematical-expression-in-a-string

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:

import ast,operator

operators = \
    {
    ast.Add:    operator.iadd,
    ast.Sub:    operator.isub,
    ast.Mult:   operator.imul,
    ast.Div:    operator.idiv,
    ast.BitXor: operator.ixor,
    ast.BitAnd: operator.iand,
    ast.BitOr:  operator.ior,
    ast.LShift: operator.lshift,
    ast.RShift: operator.rshift,
    ast.Invert: operator.invert,
    ast.USub:   operator.neg,
    ast.UAdd:   operator.pos,
    }

def _eval_expr(node):
    global symbolTable
    if isinstance(node, ast.Name):
        if node.id not in symbolTable:
            raise ParseError("name '%s' undefined" % node.id)
        return symbolTable[node.id]
    elif isinstance(node, ast.Num):
        return node.n
    elif isinstance(node, ast.operator) or isinstance(node, ast.unaryop):
        return operators[type(node)]
    elif isinstance(node, ast.BinOp):
        return _eval_expr(node.op)(_eval_expr(node.left), _eval_expr(node.right))
    elif isinstance(node, ast.UnaryOp):
        return _eval_expr(node.op)(_eval_expr(node.operand))
    else:
        raise ParseError("error parsing expression at node %s" %  node)

def eval_expr(expr):
    return _eval_expr(ast.parse(expr).body[0].value)
    

-- 
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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