Changed APIView.headers to Dict[str, str]

[MarcinWieczorek]

closes #76

[sobolevn]

Context:

• https://github.com/encode/django-rest-framework/blob/bb133522efaf6ce3ae8fdf1dec6cd79566cfd166/rest_framework/views.py#L494
• https://github.com/encode/django-rest-framework/blob/bb133522efaf6ce3ae8fdf1dec6cd79566cfd166/rest_framework/views.py#L439

One question: is it Dict[str, str] or Dict[str, Optional[str]]? Is there any special None usage?

[MarcinWieczorek]

That pop assigns None if Vary isn't in the dict and is used only for the if right below, therefore it doesn't count as None usage. I'm not aware of any other uses.

[MarcinWieczorek]

I only found a similar case here: https://github.com/encode/django-rest-framework/blob/master/rest_framework/views.py#L456
None is used for flow control and is nullchecked before assignment. All other uses (as in grep -re headers) are either on strings constants or formatted strings as such: headers['Retry-After'] = '%d' % exc.wait

[sobolevn]

Thanks!

[MarcinWieczorek]

Great, thanks for quick response.
I'll use this moment to remind you of my PR in django-stubs that has been idling since March :)

[sobolevn]

@MarcinWieczorek oh, sorry! I might have missed it. I will check it out right now.