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From: Benjamin Kaplan on 13 Apr 2010 12:25
All the operators are available as functions in the operator module.
Just use a dict to select the correct function.


import operator

ops = {"and": operator.and_, "or": operator.or_}

op1 = ops[lo1]
op3 = ops[lo3]

if op3( op1( condition1, condition2), condition4) :
#do something



On Tue, Apr 13, 2010 at 11:56 AM, Vishal Rana <ranavishal(a)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
>
> Thanks
>
> Vishal
>
> --
> http://mail.python.org/mailman/listinfo/python-list
>
From: Terry Reedy on 13 Apr 2010 13:58
On 4/13/2010 11:56 AM, Vishal Rana 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

Unless Python on the server is properly sandboxed (not easy), this is
not safe. Consider 'conditions' like

10000**10000
__import__('subprocess').Popen(['format', 'C:']) # don't test this !!!

I may not have the latter exactly correct but you should get the idea.
So sandboxing requires OS supervision and limitation of time and space
consumption as well as removal from Python of dangerous builtins and
modules.

Terry Jan Reedy

From: Chris Rebert on 13 Apr 2010 15:29
On Tue, Apr 13, 2010 at 8:56 AM, Vishal Rana <ranavishal(a)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?

Cheers,
Chris
--
http://blog.rebertia.com
From: Chris Rebert on 13 Apr 2010 16:08
> On Tue, Apr 13, 2010 at 12:29 PM, Chris Rebert <clp2(a)rebertia.com> wrote:
>> On Tue, Apr 13, 2010 at 8:56 AM, Vishal Rana <ranavishal(a)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(a)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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