baffling error-handling problem
Scott David Daniels
Scott.Daniels at Acm.Org
Thu Jul 28 18:11:02 CEST 2005
Chris Fonnesbeck wrote:
> I thought I knew how to do error handling in python, but apparently I
> dont. I have a bunch of code to calculate statistical likelihoods, and
> use error handling to catch invalid parameters. For example, for the
> bernoulli distribution, I have:
>
> def bernoulli_like(self, x, p, name='bernoulli'):
> ... if sum(p>=1 or p<=0): raise LikelihoodError ...
>
> where LikelihoodError is simply a subclass of ValueError that I created:
>
> class LikelihoodError(ValueError):
> "Log-likelihood is invalid or negative infinite"
>
> I catch these errors with the following:
>
> try: like = self.calculate_likelihood()
> except LikelihoodError:
> return 0
>
> ... [when using] ...
> like=self.bernoulli_like(x,p)
>
> I get the following when an invalid parameter is passed:
>
> Traceback (most recent call last):
> File "...\model_000.py", line 381, in ?
> model.sample(iterations=iter, burn=burn,plot=False)
> File "...\PyMC\MCMC.py", line 1691, in sample
> self._like = self.calculate_likelihood()
> File "...\model_000.py", line 194, in calculate_likelihood
> like+=self.bernoulli_like(x,p)
> File "...\MCMC.py", line 868, in bernoulli_like
> if sum(p>=1 or p<=0): raise LikelihoodError
> LikelihoodError
>
> I have no idea how this can happen, given how I have coded this.
Might you be referring to a different LikelihoodError in the
try: ... except ... part of your code than in the ... raise ... part?
Similarly defined classes are not the same class. If you didn't
get LikelihoodError in mode4l_000.py with the moral equivalent of
from MCMC import LikelihoodError
then this is what is going wrong.
By the way, if it were I, I'd: raise LikelihoodError(p) just so I
could discover a bit of what went wrong.
--Scott David Daniels
Scott.Daniels at Acm.Org
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