Eval (was Re: Question about the use of python as a scripting language)

[Brendon Towle]

Oops -- I missed the subject line on my last post.

On 10 Aug 2006, at 9:41 AM, skip at pobox.com wrote:

>     Brendon> Seems that parsing negative numbers is outside of the  
> scope of
>     Brendon> this routine. Here's the source (which is Frederik's  
> source
>     Brendon> with one minor renaming; I take no credit here);  
> anyone have
>     Brendon> any ideas?
>
> Negative numbers are actually tokenized as a MINUS followed by a  
> NUMBER:
>
>     % python
>     Python 2.5b2 (trunk:50921, Jul 28 2006, 20:21:50)
>     [GCC 4.0.0 (Apple Computer, Inc. build 5026)] on darwin
>     Type "help", "copyright", "credits" or "license" for more  
> information.
>>>> - 47
>     -47
>
> Try changing your atom function to detect a minus sign, remember  
> that fact,
> then require the next token to be a number.


While I appreciate the suggestion, this is exactly the route I don't  
want to take -- it essentially involves rewriting the Python parser/ 
tokenizer, which has already been written and tested by people much  
more qualified than I. In fact, the code I was using (Frederik's  
code) has already been written and tested by people much more  
qualified than I, and it _still_ doesn't work.

A shortcut occurs to me; maybe someone can tell me what's wrong with  
my reasoning here. It seems that any string that is unsafe to pass to  
eval() must involve a function call, and thus must contain an opening  
paren. Given that I know that the data I expect contains no parens,  
would people expect this code to be safe:

START_MARKER = 'var table_body = '
END_MARKER = '];'

def extractStockData(data):
     pos1 = data.find(START_MARKER)
     pos2 = data.find(END_MARKER, pos1)
     parenPos = data.find('(')
     if parenPos >= 0:
         raise "Data format changed -- found a paren"
     else:
         return eval(data[pos1+len(START_MARKER):pos2+1])


B.

-- 
Brendon Towle, PhD
Cognitive Scientist
+1-412-690-2442x127
Carnegie Learning, Inc.
The Cognitive Tutor Company ®
Helping over 375,000 students in 1000 school districts succeed in math.


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