Is there are good DRY fix for this painful design pattern?

[Rhodri James]

On 26/02/18 14:41, Steven D'Aprano wrote:
> I have a class with a large number of parameters (about ten) assigned in
> `__init__`. The class then has a number of methods which accept
> *optional* arguments with the same names as the constructor/initialiser
> parameters. If those arguments are None, the defaults are taken from the
> instance attributes.
> 
> An example might be something like this:
> 
> 
> class Foo:
>      def __init__(self, bashful, doc, dopey, grumpy,
>                         happy, sleepy, sneezy):
>          self.bashful = bashful  # etc
> 
>      def spam(self, bashful=None, doc=None, dopey=None,
>                     grumpy=None, happy=None, sleepy=None,
>                     sneezy=None):
>          if bashful is None:
>              bashful = self.bashful
>          if doc is None:
>              doc = self.doc
>          if dopey is None:
>              dopey = self.dopey
>          if grumpy is None:
>              grumpy = self.grumpy
>          if happy is None:
>              happy = self.happy
>          if sleepy is None:
>              sleepy = self.sleepy
>          if sneezy is None:
>              sneezy = self.sneezy
>          # now do the real work...
> 
>      def eggs(self, bashful=None, # etc...
>                     ):
>          if bashful is None:
>              bashful = self.bashful
>          # and so on
>   
> 
> There's a lot of tedious boilerplate repetition in this, and to add
> insult to injury the class is still under active development with an
> unstable API, so every time I change one of the parameters, or add a new
> one, I have to change it in over a dozen places.
> 
> Is there a good fix for this to reduce the amount of boilerplate?

You could use dicts?  Untested code:

class Foo:
     def __init__(self, **kwds):
         self.defaults = kwds

     def spam(self, **kwds):
         vars = self.defaults.copy()
         vars.update(kwds)
	# Do the work with vars['bashful'] etc


-- 
Rhodri James *-* Kynesim Ltd