[Numpy-discussion] Strange exp

Robert Kern robert.kern at gmail.com
Thu Aug 31 13:35:11 CDT 2006

Angelo Secchi wrote:
> Hi,
> I have the following script 
> import fileinput
> import string
> from math import *
> from scipy import *
> from rpy import *
> import Numeric
> import shelve
> import sys
> def dpolya1(n,N,b,a):
>     a=float(a)
>     b=float(b)
>     L=784
>     probs=((special.gammaln(N+1)+special.gammaln(L*(a/b))+special.gammaln((a/b)+n)+special.gammaln((a/b)*(L-1)+N-n))-(special.gammaln(L*(a/b)+N)+special.gammaln(a/b)+special.gammaln(n+1)+special.gammaln(N-n+1)+special.gammaln(L*(a/b)-(a/b))))#)
>     return probs
> and I observe the following "strange" (for me of course) behaviour
>>>> dpolya1(1,2,0.5,0.4)
> -5.9741312822170585
>>>> type(dpolya1(1,2,0.5,0.4))
> <type 'float64scalar'>
>>>> exp(dpolya1(1,2,0.5,0.4))
> Traceback (most recent call last):
>   File "<stdin>", line 1, in ?
> AttributeError: 'numpy.ndarray' object has no attribute 'exp'
> I do not understand what's wrong. Any help?

Probably rpy (which still uses Numeric, right?) is exposing Numeric's exp() 
implementation and overriding the one that you got from scipy (which is numpy's, 
I presume). When Numeric's exp() is confronted with an object that it doesn't 
recognize, it looks for a .exp() method to call.

If you want to avoid this situation in the future, don't use the "from foo 
import *" form. It makes debugging problems like this difficult.

Robert Kern

"I have come to believe that the whole world is an enigma, a harmless enigma
  that is made terrible by our own mad attempt to interpret it as though it had
  an underlying truth."
   -- Umberto Eco

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