Good question.

In early revisions of PEP 484 you could write

def foo(a: int = None) -> ...

and it would imply that the type of 'a' was really Optional[int].

After several years of practice we decided to *remove* this automatic optionalization because it was widely misunderstood and/or confusing for users. For the whole discussion, see https://github.com/python/typing/issues/275. (Also, PEP 484 still mentions it -- "A past version of this PEP allowed ...")

In fact, mypy still defaults to this behavior, and you have to request the new, stricter PEP 484 behavior with `--no-implied-optional`.

Maybe we could satisfy both sides if the notation for Optional[X] was truly short, like ?X or -X. But that has its own issues -- none of the existing unary operators (+X, -X, ~X) really satisfies, and ?X would require a syntax change (and would forever fix the meaning of one of the last ASCII punctuation characters that has no meaning in Python -- $ being the other one).

Finally, I wish the whole term "optional" would go away, because of the endless confusion with optional parameters and optional dict keys -- in these contexts the word is widely used to mean "may or may not be present", without implying that the type may include None.

But I digress...

--Guido

On Wed, May 6, 2020 at 8:47 AM Luciano Ramalho <luciano@ramalho.org> wrote:

Given recent usability improvements in type hints (PEPs 585, 604), I
thought I'd raise this minor issue which I find relevant as an
instructor.

Given this signature:

def f(a: float, b: Optional[int]) -> float:

The `b` parameter is not optional. The caller must always provide an
actual argument as an integer or None. To make `b` optional, we must
write:

def f(a: float, b: Optional[int] = None) -> float:

PEP 604 syntax avoids the misleading name `Optional`, which is progress:

def f(a: float, b: int | None = None) -> float:

But now another minor issue is highlighted: the repetition of None.
That's unfortunate given that almost all uses of None in parameters
are precisely as default values for optional parameters—that the
caller doesn't need to provide.

I wonder if the authors of the typing PEPs considered these issues, or
if there are ideas to fix them.

I don't have a proposal at this time. I wanted to start the
conversation to see if we can find a fix, or at least learn to live
with the current status—which is not bad, but also not excellent in
terms of readability.

Cheers,

Luciano

--
Luciano Ramalho
| Author of Fluent Python (O'Reilly, 2015)
| http://shop.oreilly.com/product/0636920032519.do
| Technical Principal at ThoughtWorks
| Twitter: @ramalhoorg
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