What does the Ellipsis object do?

This came up in another question recently. I'll elaborate on my answer from there:

Ellipsis is an object that can appear in slice notation. For example:

myList[1:2, ..., 0]

Its interpretation is purely up to whatever implements the __getitem__ function and sees Ellipsis objects there, but its main (and intended) use is in the numpy third-party library, which adds a multidimensional array type. Since there are more than one dimensions, slicing becomes more complex than just a start and stop index; it is useful to be able to slice in multiple dimensions as well. E.g., given a 4 × 4 array, the top left area would be defined by the slice [:2, :2]:

>>> a
array([[ 1,  2,  3,  4],
       [ 5,  6,  7,  8],
       [ 9, 10, 11, 12],
       [13, 14, 15, 16]])

>>> a[:2, :2]  # top left
array([[1, 2],
       [5, 6]])

Extending this further, Ellipsis is used here to indicate a placeholder for the rest of the array dimensions not specified. Think of it as indicating the full slice [:] for all the dimensions in the gap it is placed, so for a 3d array, a[..., 0] is the same as a[:, :, 0] and for 4d a[:, :, :, 0], similarly, a[0, ..., 0] is a[0, :, :, 0] (with however many colons in the middle make up the full number of dimensions in the array).

Interestingly, in python3, the Ellipsis literal (...) is usable outside the slice syntax, so you can actually write:

>>> ...
Ellipsis

EDIT: Ellipsis is also used in the standard library typing module: e.g. Callable[..., int] to indicate a callable that returns an int without specifying the signature, or tuple[str, ...] to indicate a variable-length homogeneous tuple of strings.

Summing up what others have said, as of Python 3, Ellipsis is essentially another singleton constant similar to None, but without a particular intended use. Existing uses include:

• In slice syntax to represent the full slice in remaining dimensions
• In type hinting to indicate only part of a type(Callable[..., int] or Tuple[str, ...])
• In type stub files to indicate there is a default value without specifying it

Possible uses could include:

• As a default value for places where None is a valid option
• As the content for a function you haven't implemented yet

From the Python documentation:

This object is commonly used by slicing (see Slicings). It supports no special operations. There is exactly one ellipsis object, named Ellipsis (a built-in name). type(Ellipsis)() produces the Ellipsis singleton.

It is written as Ellipsis or ....

For anyone coming to this answer from working in a codebase with heavy Pydantic use: this is also how Pydantic indicates a field that is required but can be set to None, which they refer to as "required optional fields". This is why they end up used in FastAPI, too.

You can use Ellipsis yourself, in custom slicing situations like numpy has done, but it has no usage in any builtin class.

I don't know if it was added specifically for use in numpy, but I certainly haven't seen it used elsewhere.

See also: How do you use the ellipsis slicing syntax in Python?

As mentioned by @noɥʇʎԀʎzɐɹƆ and @phoenix - You can indeed use it in stub files. e.g.

class Foo:
    bar: Any = ...
    def __init__(self, name: str=...) -> None: ...

More information and examples of how to use this ellipsis can be discovered here https://www.python.org/dev/peps/pep-0484/#stub-files

This is equivalent.

l=[..., 1,2,3]
l=[Ellipsis, 1,2,3]

... is a constant defined inside built-in constants.

Ellipsis

The same as the ellipsis literal “...”. Special value used mostly in conjunction with extended slicing syntax for user-defined container data types.

In typer ... is used to create required parameters: The Argument class expects a default value, and if you pass the ... it will complain if the user does not pass the particular argument.

You could use None for the same if Ellipsis was not there, but this would remove the opportunity to express that None is the default value, in case that made any sense in your program.

FastAPI makes use of the Ellipsis for creating required Parameters. https://fastapi.tiangolo.com/tutorial/query-params-str-validations/