Python has a very short list of built-in constants. One of them is Ellipsis which is also can be written as .... This constant has no special meaning for the interpreter but is used in places where such syntax looks appropriate. numpy support Ellipsis as a __getitem__ argument, e. g. x[...] returns all elements of x. PEP 484 defines additional meaning: Callable[..., type] is a way to define a type of callables with no argument types specified. Finally, you can use to indicate that function is not yet implemented. This is a completely valid Python code: def x(): ...