On Sun, May 3, 2009 at 9:30 AM, Robert Kern <[email protected]> wrote:
>
> On Sun, May 3, 2009 at 11:06, Robert Kern <[email protected]> wrote:
>> On Sun, May 3, 2009 at 10:59, Ondrej Certik <[email protected]> wrote:
>>
>>> Google app engine supports eval(), so we may just write a simple
>>> preparser based on the "re" module (also available on the app engine),
>>> that converts things like 1/2 to Integer(1)/Integer(2) and "x" to
>>> Symbol("x") and we should be fine. I don't know if eval() can handle
>>> large expressions (to fix #976), that would have to be tried. Usually
>>> a large expression is one big Add instance, so one solution that just
>>> occured to me is to split it into several smaller Add instances,
>>> evaluate each using eval() and add them together.
>>
>> It's feasible to write and maintain a pyparsing parser for Python's
>> expression subgrammar. This also opens up the possibility for easily
>> extending the grammar to add other operators and such.
>
> Another alternative is Fredrik Lundh's hand-built parser here:
>
> http://effbot.org/zone/simple-top-down-parsing.htm
>
> It has the advantage of already being written.
Thanks for both suggestions. I played with both and it should not be
that difficult as I feared.
I still think that an easier option is to just write a preparse()
function, that takes a string, and returns a string:
>>> preparse("3/4 + x")
'Integer(3)/Integer(4) + Symbol("x")'
And then we just pass it to eval(). That should be easy to debug and
maintain. Just like the lambdify() works.
Then the question is, how to write the preparse() function, it seems
to me a simple regex based substitution should do the job, just like
in preparse_numeric_literals() in sage.misc.preparser.py.
Ondrej
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