Ellipsis

[Skaperen]

what is Ellipsis (the type with only one value expressed by the literal ...) used for?

[scidam]

First of all, ellipsis is convenient triple of symbols (dots) to denote "something" when working with indexes.
It is ususally used in NumPy to denote multiple dimensions, e.g.
x = np.array(np.arange(10000)).reshape(10, 10, 10, 10) (x is array of shape 10x10x10x10, dim=4) x[:, :, :, :5] could be replaced with x[..., :5] (x[..., 5].shape = (10, 10, 10, 5)). So, ellipsis is useful when working with high dimensional data.

You can define your own behavior of the Ellipsis symbol, this, I think, its primary purpose.

class Container:
    def __getitem__(self, index):
        if index is Ellipsis:
            print("This is ellipsis")
            return 0
        else:
            return index

c = Container()
c[...]
c[3]

Output:
This is ellipsis

3

[Skaperen]

i have a dictionary of option defaults. i had been using None in past code to indicate the option has no default. but in the current project i need to indicate there is no default along with that parameter must be specified on the command line (or it is an error). i was thinking maybe i could use Ellipsis for that.

[scidam]

I am not sure I can understand you, may be this is because I am not a native English speaker. Are you talking about passing arguments to a function, or overriding dictionary methods, e.g. dict.__getitem__? Could you provide some code: where it was used None and where you want to use Ellipsis instead. If you need to override dict.__getitem__, you need to subclass UserDict instead of dict:

from collections import UserDict

class option(UserDict):
    def __getitem__(self, key):
        if key is Ellipsis:
            print("Do some stuff")
        else:
            return super().__getitem__(key)

z = option()
z.update({'one': 1})
z[...]

Hope that helps...