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. > > > > To post to this group, send email to sympy@googlegroups.com. > > > > To unsubscribe from this group, send email to > > > > sympy+unsubscr...@googlegroups.com. > > > > For more options, visit this group at > > > >http://groups.google.com/group/sympy?hl=en. > > > > Mateusz -- You received this message because you are subscribed to the Google Groups "sympy" group. To post to this group, send email to sympy@googlegroups.com. 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