[Python-ideas] Mitigating 'self.' Method Pollution

Terry Reedy tjreedy at udel.edu
Sun Jul 12 03:21:57 CEST 2015

On 7/11/2015 3:41 PM, David Mertz wrote:
> On Sat, Jul 11, 2015 at 12:10 PM, Nikolaus Rath
> <Nikolaus at rath.org
> <mailto:Nikolaus at rath.org>> wrote:
>     > class Vector:
>     >     def abs(self):
>     >         return sqrt(x**2 + y**2 + z**2)
>     >         # versus
>     >         return sqrt(self.x**2 + self.y**2 + self.z**2)

     def abs(v):
         return sqrt(v.x**2 + v.y**2 + v.z**2)

> But I also don't want to have to guess about WHICH 'x' or 'y' or 'z' is
> the one being used in calculation of Cartesian distance.  Sure, an
> obvious implementation of a Vector class probably has x, y, z attributes
> of self.  The example was rigged to make that seem obvious.  But even
> then, I don't really know.  Maybe the z direction of the vector is
> stored as a class attribute.  Maybe the class expects to find a global
> definition of the z direction.
> Sure, keeping the z direction somewhere other than in self is probably
> foolish if you assume "Vector" means "generic vector".  What if the
> class was called "RestrictedVector"? Would you know without reading the
> full class and docstring exactly in what respect it is "restricted"?
> E.g. are you sure it's not "restricted in the z dimension"?
> As someone else points out, if you WANT local variables in a method, you
> are welcome to use them.  E.g.:
>      def times_matrix(self, matrix):
>          x, y, z = self.x, self.y, self.z
>          # Many more than one line of implementation
>          # ... stuff with matrix indexing and referencing x
>          # etc.
>          return result_matrix

Localization, which is not limited to self attributes, does two things: 
it removes the need to continually type 'object.x'; it makes the 
remaining code run faster.  I would be interested to know how many 
faster local references are needed to make up for the overhead of 

Terry Jan Reedy

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