Is it better to use class variables or pass parameters?

Eric Brunel eric_brunel at
Thu Mar 16 09:25:38 CET 2006

On 15 Mar 2006 15:04:19 -0800, Derek Basch <dbasch at> wrote:

> One more question everybody. Say you have a class that performs a
> series of evaluations on several strings of peptides.

Errr..., no? A class does not perform some operations. A class is a  
template for objects. What are your objects? If you had a class Peptide  
for example (I'm guessing here; I'm not exactly fluent in this domain), I  
would have understood. But what you're describing here is not a class;  
it's just a set of functions.

> Heres the class:
> class PeptideEvaluator:
>     def evaluate(self, peptide):
>         peptide_name = peptide + "Rules!"
>         result1 = self.test1(peptide, peptide_name)
>         result2 = self.test2(peptide, peptide_name)
>         result3 = self.test3(peptide, peptide_name)
>     def test1(self, peptide, peptide_name):
>         f = open(peptide_name + ".txt", "w")
>         f.write(peptide)
>         f.close()
>     def test2(self, peptide, peptide_name):
>         f = open(peptide_name + ".txt", "w")
>         f.write(peptide)
>         f.close()
>     def test3(self, peptide, peptide_name):
>         f = open(peptide_name + ".txt", "w")
>         f.write(peptide)
>         f.close()
> So, you instantiate a class called "PeptideEvaluator" and pass in each
> string to its "evaluate" method. Now you have to repeatedly pass the
> peptide and peptide_name to each function. According to what everyone
> has said declaring them as class variables is bad because they are not
> related to the state of the "PeptideEvaluator". How can I avoid having
> to pass the same parameters all over a class? I can';t quite seem to
> wrap my head around this one.

QED: this "class" has no need at all for attributes, and does not even  
have a constructor. This is usually a strong signal that what you're doing  
is wrong. Just turn this class into a module containing functions. Or put  
these methods into another class (the Peptide class may be a good  
candidate if you have one; "evaluating" a peptide seems to be an operation  
that you do on peptides).

python -c "print ''.join([chr(154 - ord(c)) for c in  

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