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

Brett Cannon brett at python.org
Tue May 9 18:51:34 EDT 2017

On Tue, 9 May 2017 at 11:11 Carl Meyer <carl at oddbird.net> wrote:

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 handleemployee(e: Employee = None): ... | > > This is equivalent to:: > > |def handleemployee(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.

I personally like the shorthand, but the type hints I have written are in my own projects so my experience with others is zilch and shouldn't count for much.

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

You should be able to do that today with from typing import Optional as Eh or whatever your preferred optional/maybe name is. :) -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://mail.python.org/pipermail/python-dev/attachments/20170509/7cc4b475/attachment-0001.html>

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