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Wed Feb 10 13:15:02 EST 2016 · https://mail.python.org/pipermail/python-dev/2016-February/143252.html

On Wed, Feb 10, 2016 at 10:01 AM, Phil Thompson
<phil at riverbankcomputing.com> wrote:
> On 10 Feb 2016, at 5:52 pm, Guido van Rossum <guido at python.org> wrote:
[...]
> That should do it, thanks. A followup question...
>
> Is...
>
>     def foo(bar: str = Optional[str])
>
> ...valid? In other words, bar can be omitted, but if specified must be a str or None?

The syntax you gave makes no sense (the default value shouldn't be a
type) but to do what your words describe you can do

def foo(bar: Optional[str] = ...): ...

That's literally what you would put in the stub file (the ... are
literal ellipses).

In a .py file you'd have to specify a concrete default value. If your
concrete default is neither str nor None you'd have to use cast(str,
default_value), e.g.

_NO_VALUE = object()  # marker

def foo(bar: Optional[str] = cast(str, _NO_VALUE)):
    ...implementation...

Now the implementation can distinguish between foo(), foo(None) and foo('').

-- 
--Guido van Rossum (python.org/~guido)

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Showing content from https://mail.python.org/pipermail/python-dev/2016-February/143252.html below:

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