Yes, that worked. But I had to restructure the code
class SUM(Function):
nargs = 2
@classmethod
def eval(cls, arg):
cls.arg=arg
@classmethod
def _eval_evalf(cls, arg):
map = Code("""function () {
emit("sum",{%(field)s:this.%(field)s});
}""" % {'field':cls.arg})
reduce = Code("""
function(key, values) {
var sum = 0;
values.forEach(function(doc) {
if (doc.%(field)s != undefined){
sum += doc.%(field)s;
}
});
return {%(field)s:sum};
};""" % {'field':cls.arg})
result = db.people.map_reduce(map, reduce, "myresults")
return result.find_one()['value'][unicode(cls.arg)]
Because arg in _eval_evalf was strangely enough a float (more
precisely 57, have no clue why :) ).
Thank you all guys!
On Jun 2, 6:07 pm, Ronan Lamy <ronan.l...@gmail.com> wrote:
> Le jeudi 02 juin 2011 à 08:03 -0700, luke a écrit :
>
>
>
>
>
>
>
> > Oh, thanks a lot. Now it's working fine.
> > Now the only problem is that the code is evaluated when calling
> > sympify, and not when calling evalf.
> > Could I ask you what is the way to keep the function lazy? I explain
> > myself better. Since the function SUM has to operate on a database if
> > I have something like that:
>
> > >>> SUM('field') + SUM('field') + SUM('field') #not lazy, computes
> > >>> immediately three queries
> > >>> 1234
>
> > I'm doing the same operation three times and this is not very good in
> > terms of performances, what I expected would have been:
>
> > >>> a = SUM('field') + SUM('field') + SUM('field') -> 3SUM('field') # one
> > >>> single query
> > >>> print a
> > >>> 3*SUM('field')
> > >>> print N(a)
> > >>> 1234
>
> > just like other functions work. e.g.
>
> > >>> log(10)+log(10)+log(10)
> > >>> 3*log(10)
>
> Note that your use case isn't handled very well by sympy (for now).
> Subclasses of Function are supposed to be symbolic representations of
> numeric functions that return numeric results for numeric arguments.
>
> But there are ways to hack around these implicit assumptions. To get
> lazy evaluation, the easiest is probably to use SUM('field',
> evaluate=False), which tells the function to store its argument without
> executing eval(). Then you can evaluate the expression using .doit().
> You should get something like:
>
> >>> a = SUM('field', evaluate=False) + SUM('field', evaluate=False) +
> >>> SUM('field', evaluate=False)
> >>> print a
> 3*SUM('field')
> >>> print a.doit()
>
> 1234
>
> Otherwise, you could implement your class so that the DB lookup and
> calculations are only done when you call evalf() - that should give you
> exactly the behaviour you expected above. For that, you'd have to
> replace your eval() classmethod with an _eval_evalf() method, e.g.
>
> def _eval_evalf(self, prec):
> arg = self.args[0]
> < body of your eval() >
>
>
>
>
>
>
>
> > On Jun 2, 9:38 am, Mateusz Paprocki <matt...@gmail.com> wrote:
> > > Hi,
>
> > > On 2 June 2011 09:07, luke <luca.giaco...@gmail.com> wrote:
>
> > > > Hi eveyone,
> > > > I'm developing an web application which has to interact with "user-
> > > > defined formulas" of some financial kpis.
> > > > I decided to use sympy to have a more solid math engine.
> > > > Basically the input I reiceve is very simple, it might be in the worst
> > > > case something like:
>
> > > > kpi -> "(log(sum('production'))*count('sales')/min('spread')" (this
> > > > formula is totally made-up)
>
> > > > I defined some functions to interact with the database according to
> > > > the official docs and they seem to be working:
>
> > > > For example defining
>
> > > > from sympy.core.function import Function
>
> > > > class SUM(Function):
> > > > nargs = 2
> > > > @classmethod
> > > > def eval(cls, arg):
> > > > map = Code("""function () {
> > > > emit("sum",{%(field)s:this.%(field)s});
> > > > }""" % {'field':arg})
> > > > reduce = Code("""
> > > > function(key, values) {
> > > > var sum = 0;
> > > > values.forEach(function(doc) {
> > > > if (doc.%(field)s != undefined){
> > > > sum += doc.%(field)s;
> > > > }
> > > > });
> > > > return {%(field)s:sum};
> > > > };""" % {'field':arg})
> > > > result = db.people.map_reduce(map, reduce, "myresults")
> > > > return result.find_one()['value'][unicode(arg)]
>
> > > > #EOF
>
> > > > Then from the command line I can type:
>
> > > > >>> print SUM("field")
> > > > >>> 1923
>
> > > Very interesting application. I'm not sure if you are familiar with this,
> > > but staying on the safe side note here that SUM("field") doesn't do
> > > exactly
> > > what you expect, but the outcome is fine. Usually, SymPy's functions don't
> > > accept raw string arguments, but sympify() them:
>
> > > In [1]: class fun(Function):
> > > ...: nargs = 1
> > > ...: @classmethod
> > > ...: def eval(cls, arg):
> > > ...: print type(arg)
> > > ...:
> > > ...:
>
> > > In [2]: fun('abc')
> > > <class 'sympy.core.symbol.Symbol'>
> > > Out[2]: fun(abc)
>
> > > So in eval() you got a symbol not string 'abc', but as str() of a Symbol
> > > is
> > > simply the name of the symbol, then this (and your code) works as
> > > expected:
>
> > > In [3]: print "---%s---" % Symbol('abc')
> > > ---abc---
>
> > > > But when I try to use sympify my function doesn't evaluate..
>
> > > > >>> print sympify("SUM('field')").evalf()
> > > > >>> SUM(field)
> > > > >>> N("SUM('field')")
> > > > >>> SUM(field)
>
> > > The problem here is that SymPy, precisely speaking sympify(), doesn't know
> > > what SUM() is, because SUM() resides in the global namespace of the
> > > interpreter, which is unknown to sympify(), e.g.:
>
> > > In [4]: sympify("whatever(10)")
> > > Out[4]: whatever(10)
>
> > > whatever() is unknown to sympify(), so a new Function object is
> > > constructed
> > > for it. The same for fun() which I defined above:
>
> > > In [6]: sympify("fun(10)")
> > > Out[6]: fun(10)
>
> > > To overcome this, pass globals() to sympify(), e.g.:
>
> > > In [7]: sympify("fun(10)", globals())
> > > <class 'sympy.core.numbers.Integer'>
> > > Out[7]: fun(10)
>
> > > (in your case this will be sympify("SUM('field')", globals())).
>
> > > > Am I doing anything wrong?
> > > > Thanks in advance!
>
> > > > --
> > > > You received this message because you are subscribed to the Google
> > > > Groups
> > > > "sympy" group.
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>
> > > Mateusz
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