[Python-ideas] slice.literal notation

[Terry Reedy]

On 6/10/2015 11:33 AM, Joseph Jevnik wrote:

> I often find that when working with pandas and numpy I want to store
> slice objects in variables to pass around and re-use; however, the
> syntax for constructing a slice literal outside of an indexer is very
> different from the syntax used inside of a subscript. This patch
> proposes the following change:
>
>      slice.literal
>
> This would be a singleton instance of a class that looks like:
>
> class sliceliteral(object):
>      def __getitem__(self, key):
>          return key

Alternate constructors are implemented as class methods.

class slice:
     ...
     @classmethod
     def literal(cls, key):
         if isinstance(key, cls):
             return key
         else:
             else raise ValueError('slice literal mush be slice')

They are typically names fromxyz or from_xyz.

Tal Einat pointed out that not all keys are slices

 > [0] -> int
 > [...] -> Ellipsis
 > [0:1, 2:3] -> 2-tuple of slice object

I think the first two cases should value errors. The third might be 
debated, but if allowed, this would not be a slice constructor.

-- 
Terry Jan Reedy