[Python-Dev] PEP 484 proposal: don't default to Optional if argument default is None

Carl Meyer carl at oddbird.net
Tue May 9 14:03:05 EDT 2017

On 05/09/2017 10:28 AM, Guido van Rossum wrote:
> There's a proposal to change one detail of PEP 484. It currently says:
>     An optional type is also automatically assumed when the default value is
>     |None|, for example::
>     |def handle_employee(e: Employee = None): ... |
>     This is equivalent to::
>     |def handle_employee(e: Optional[Employee] = None) -> None: ... |
> Now that we've got some experience actually using Optional with mypy
> (originally mypy ignored Optional), we're beginning to think that this
> was a bad idea. There's more discussion at
> https://github.com/python/typing/issues/275 and an implementation of the
> change (using a command-line flag) in
> https://github.com/python/mypy/pull/3248.
> Thoughts? Some function declarations will become a bit more verbose, but
> we gain clarity (many users of annotations don't seem to be familiar
> with this feature) and consistency (since this rule doesn't apply to
> variable declarations and class attribute declarations).

I've been code-reviewing a lot of diffs adding type coverage over the
last few months, and implicit-Optional has been among the most common
points of confusion. So I favor this change.

It might be nice to have a less verbose syntax for Optional, but that
can be a separate discussion.


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