Passing Python slice syntax around to functions

Question

In Python, is it possible to encapsulate exactly the common slice syntax and pass it around? I know that I can use slice or __slice__ to emulate sl

Answer 1

In short, no. That syntax is only valid in the context of the [] operator. I might suggest accepting a tuple as input and then pass that tuple to slice(). Alternatively, maybe you could redesign whatever you're doing so that get_important_values() is somehow implemented as a sliceable object.

For example, you could do something like:

class ImportantValueGetter(object):
    def __init__(self, some_list, some_condition):
        self.some_list = some_list
        self.some_condition = some_condition

    def __getitem__(self, key):
        # Here key could be an int or a slice; you can do some type checking if necessary
        return filter(self.some_condition, self.some_list)[key]

You can probably do one better by turning this into a Container ABC of some sort but that's the general idea.

Answer 2

One way (for simple slices) would be to have the slice argument either be a dict or an int,

ie

get_important_values([1, 2, 3, 4], lambda x: (x%2) == 0, {0: -1})

or

get_important_values([1, 2, 3, 4], lambda x: (x%2) == 0, 1)

then the syntax would stay more or less the same.

This wouldn't work though, for when you want to do things like

some_list[0:6:10..]

Answer 3

Perhaps something along the following lines would work for you:

class SliceMaker(object):
  def __getitem__(self, item):
    return item

make_slice = SliceMaker()

print make_slice[3]
print make_slice[0:]
print make_slice[:-1]
print make_slice[1:10:2,...]

The idea is that you use make_slice[] instead of manually creating instances of slice. By doing this you'll be able to use the familiar square brackets syntax in all its glory.