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Tue Feb 9 18:56:41 EST 2016 · https://mail.python.org/pipermail/python-dev/2016-February/143229.html

On 02/09/2016 03:48 PM, Guido van Rossum wrote:

 > (Assuming you meant Option*al*.) There seems to be an utter confusion
 > of the two uses of the term "optional" here. An "optional argument"
 > (outside PEP 484) is one that has a default value. The "Optional[T]"
 > notation in PEP 484 means "Union[T, None]". They mean different
 > things.

In an effort to be (crystal) clear:

option argument in Python: has a default value, so may be omitted when 
the function is called.

Optional[T] in MyPy: the argument has no default value, and must be 
supplied when the function is called, but the argument can be None.

--
~Ethan~

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[Python-Dev] Experiences with Creating PEP 484 Stub Files