On Wed, Aug 09, 2017 at 12:06:37PM -0400, C W wrote:
> Dear Python experts,
> What exactly does the three dots do?
> > aList = ...
... is literal syntax for the Ellipsis singleton object.
Ellipsis was added to the language at the express request of the numpy
developers. Although numpy is a third-party project outside of the
standard library, it is big enough and important enough that their
requests carry a LOT of weight with the core Python devs. Other features
Python has that were originally added at the request of numpy include:
- extended slicing with two colons obj[a:b:c]
- the @ operator used by numpy for matrix multiplication.
I don't know what Ellipsis is used for by numpy, but now it makes a
convenient pseudo-pass command:
> It's an ellipsis, a spot holder to later. But what data type is it: vector,
Its a singleton object. Think of it as a sibling to None and
NotImplemented, but with optional funny syntactic sugar ... to refer to
None is a special value used as "no such value", or nil or null;
NotImplemented is a special value used by operator dunder methods like
__add__ and __mul__ to mean "I can't handle this argument";
Ellipsis is a special value used by numpy to mean whatever it is that
numpy uses it to me.
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