Added ellipsis_str function to Objects/sliceobject.c such that str(Ellipsis)=='...'.

What is the rationale of this change?

Thanks for the PR but this seems to be backwards incompatible though I don't know if anyone depends on this and I am curious about the reason to change. Since this was added in e449af7da94 (1996) and I am not sure if this needs to be changed though it's up to Guido to take a call. There seems to be no tests for this in test suite that catches this change too.

$ python2.7
Python 2.7.14 (default, Mar 12 2018, 13:54:56)
[GCC 4.2.1 Compatible Apple LLVM 7.0.2 (clang-700.1.81)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> str(Ellipsis)
'Ellipsis'
>>> repr(Ellipsis)
'Ellipsis'

Thanks

Frankly, because it bothered me that ``...`` evaluates to ``Ellipsis`` but that ``Ellipsis`` could never print as ``...``.

Are you aware that changing just __str__ will not change the result of repr(), so that nested Ellipsis and Ellipsis in REPL will be still represented by name?

>>> print(...)
...
>>> print([...])
[Ellipsis]
>>> ...
Ellipsis

But changing also __repr__ will conflict with using "..." for representing recursive collections.

>>> a = []
>>> a.append(a)
>>> a
[[...]]

Yes. I wanted to change just __str__ because the string value of an Ellipsis should be, well, an ellipsis. The __repr__ representation helps distinguish between recursion and an ellipsis object, so that's fine the way it is.

I'm against this. The str() and repr() of Ellipsis are 'Ellipsis' for a reason: to remind the user that this is "just" an object, not a piece of special syntax.

That's understandable. Thanks for considering it, closing the patch.