Constructing an if statement from the client data in python

[Vishal Rana]

Thanks Chris

On Tue, Apr 13, 2010 at 1:08 PM, Chris Rebert <clp2 at rebertia.com> wrote:

> > On Tue, Apr 13, 2010 at 12:29 PM, Chris Rebert <clp2 at rebertia.com>
> wrote:
> >> On Tue, Apr 13, 2010 at 8:56 AM, Vishal Rana <ranavishal at gmail.com>
> wrote:
> >> > Hi,
> >> >
> >> > I need to construct an if statement from the data coming from the
> client
> >> > as
> >> > below:
> >> >
> >> > conditions: condition1, condition2, condition3, condition4 logical
> >> > operators: lo1, lo2, lo3 (Possible values: "and" "or")
> >> >
> >> > Eg.
> >> >
> >> > if condition1 lo1 condition2 lo3 condition4:
> >> >
> >> >     # Do something
> >> >
> >> > I can think of eval/exec but not sure how safe they are! Any better
> >> > approach
> >> > or alternative? Appreciate your responses :)
> >> >
> >> > PS: Client-side: Flex, Server-side: Python, over internet
> >>
> >> Do you literally get a string, or do/could you get the expression in a
> >> more structured format?
>
> On Tue, Apr 13, 2010 at 12:46 PM, Vishal Rana <ranavishal at gmail.com>
> wrote:
> > Its a form at the client side where you can construct a query
> > using different conditions and logical operators.
> > I can take it in any format!, currently it comes as a parameters to an
> RPC.
>
> Well, then if possible, I'd have the form send it back in a Lisp-like
> format and run it through a simple evaluator:
>
> def and_(conds, context):
>    for cond in conds:
>        if not evaluate(cond, context):
>            return False
>    return True
>
> def or_(conds, context):
>    for cond in conds:
>        if evaluate(cond, context):
>            return True
>    return False
>
> def greater_than(pair, context):
>    left, right = [context[name] for name in pair]
>    return left > right
>
> OPNAME2FUNC = {"and" : and_, "or" : or_, ">" : greater_than}
>
> def evaluate(expr, context):
>    op_name, operands = expr[0], expr[1:]
>    return OPNAME2FUNC[op_name](operands, context)
>
> expression = ["and", [">", "x", "y"], ["or", [">", "y", "z"], [">", "x",
> "z"]]]
> variables = {"x" : 7, "y" : 3, "z" : 5}
> print evaluate(expression, variables)
>
> If it's just ands and ors of bare variables (no '>' or analogous
> operations), the code can be simplified a bit.
>
> Cheers,
> Chris
> --
> http://blog.rebertia.com
>
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