Assign None as default for Optional Type

[dmfigol]

Hi, I think it would be awesome if the default value for Optional types would be set to None. Here is a snippet I hoped would work:

import attr

@attr.s(auto_attribs=True)
class Book:
    name: str
    author: Optional[str]


data = {'name': 'Fluent Python'}
Book(**data)

TypeError: __init__() missing 1 required positional argument: 'author'

[euresti]

You can very easily add = None to the class definition. Also default args don't play nice if they are in the middle. e.g.

@attr.s(auto_attribs=True)
class Book:
    name: str
    author: Optional[str]
    isbn: str

[dmfigol]

This is what I am currently doing - assigning None to every field in 20-field schema and it is looking ugly. I believe this is extra information which is already indicated by the Optional type.

[wsanchez]

I'm not sure I agree that the optional type indicates that the default should be None; it only indicates that the value may be None.

That said, it does seem a reasonable default-default. But I'm a little wary of doing that implicitly, and @euresti's point about defaults in the middle would have to be addressed.

[RonnyPfannschmidt]

it would be viable if one could as choose a class creation policy that implied convenient defaults

having it as implicit default limits the control of a class creator over expressing behaviour

[hynek]

If technically viable, this could be part of our new attrs namespace.