Eval (was Re: Question about using python as a scripting language)

[Chris Lambacher]

How is your data stored? (site was not loading for me).

test = 'blah = [1,2,3,4,5]'
>>> var,val = test.split('=')
>>> print var,val
blah   [1,2,3,4,5]
>>> val = val.strip('[] ')
>>> print val
1,2,3,4,5
>>> vals = [int(x) for x in val.split(',')]
>>> print vals
[1, 2, 3, 4, 5]


More sophisiticated situations (like nested lists) may require something like pyparsing.

from pyparsing import Suppress, Regex, delimitedList, Forward, QuotedString

stringValue = QuotedString('"', '\\', multiline=True)

intValue = Regex(r'[+-]?0|([1-9][0-9]*)')
intValue.setParseAction(lambda s,l,toks: int(toks[0]))

floatValue = Regex(r'[+-]?[0-9]+\.[0-9]+([eE][+-]?[0-9]+)?')
floatValue.setParseAction(lambda s,l,toks: ('real', float(toks[0])))
constantValue = stringValue ^ intValue ^ floatValue
arrayInitializer = Forward()
arrayInitializer << (Suppress("[") + delimitedList(constantValue ^ arrayInitializer) \
                        + Suppress("]"))
arrayInitializer.setParseAction(lambda s,l,toks: toks.asList())

test_data = '["a", 1, 5.6, ["b","c",6, 10.0e-19]]'
print arrayInitializer.parseString(test_data)

results in:
['a', 1, ('real', 5.5999999999999996), 'b', 'c', 6, ('real', 1.0000000000000001e-18)]

-Chris
On Wed, Aug 09, 2006 at 10:23:49AM -0400, Brendon Towle wrote:
>    Slawomir Nowaczyk noted:
> 
>      #> Heck, whenever *is* it OK to use eval() then?
>      eval is like optimisation. There are two rules:
>      Rule 1: Do not use it.
>      Rule 2 (for experts only): Do not use it (yet).
> 
>    So, that brings up a question I have. I have some code that goes out to a
>    website, grabs stock data, and sends out some reports based on the data.
>    Turns out that the website in question stores its data in the format of a
>    Python list ([1]http://quotes.nasdaq.com/quote.dll?page=nasdaq100, search
>    the source for "var table_body"). So, the part of my code that extracts
>    the data looks something like this:
>        START_MARKER = 'var table_body = '
>        END_MARKER = '];'
>    def extractStockData(data):
>        pos1 = data.find(START_MARKER)
>        pos2 = data.find(END_MARKER, pos1)
>        return eval(data[pos1+len(START_MARKER):END_MARKER])
>    (I may have an off-by-one error in there somewhere -- this is from memory,
>    and the code actually works.)
>    My question is: what's the safe way to do this?
>    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.
> 
> References
> 
>    Visible links
>    1. http://quotes.nasdaq.com/quote.dll?page=nasdaq100

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