C-style static variables in Python?

Duncan Booth duncan.booth at invalid.invalid
Fri Apr 2 19:53:18 CEST 2010


kj <no.email at please.post> wrote:

>  I suppose one could refactor this:
> 
><procedural>
> def spam(x, y, z):
>     try:
>         mongo = spam.mongo
>     except AttributeError:
>         mongo = spam.mongo = heavy_lifting_at_runtime()
>     return frobnicate(x, y, z, mongo)
> 
> ham = spam(3, 4, 5)
></procedural>
> 
> into this:
> 
><OO>
> class _Spam(object):
>     @classmethod
>     def _(cls, x, y, z):
>         try:
>             mongo = cls.mongo
>         except AttributeError:
>             mongo = cls.mongo = heavy_lifting_at_runtime()
>         return frobnicate(x, y, z, mongo)
> 
> ham = _Spam._(1, 2, 3)
></OO>
> 
> 
> Is this really more natural or more readable?  Hmmm.

No, but that's because it is needlessly obfuscated. What's with the weird _ 
method? Why use a class method? Why not just create an instance?

class Spam(object):
   mongo = None
   def __call__(self, x, y, z):
       if self.mongo is None:
           self.mongo = heavy_lifting_at_runtime()
       return frobnicate(x, y, z, self.mongo)
spam = Spam()

ham = spam(1, 2, 3)


That's natural and readable.

There's also another good reason why the class is better than the static 
variable: you can construct multiple different instances with different 
calls to 'heavy_lifting_at_runtime'. e.g. You could write a unit test where 
mongo is initialised to mock_heavy_lifting_at_runtime().



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