Add template context processors.

[alex-oleshkevich]

This PR adds template context processors.

import typing

from starlette.requests import Request
from starlette.templating import Jinja2Templates

def app_context(request: Request) -> typing.Dict[str, typing.Any]:
    return {'app': request.app}

templates = Jinja2Templates(directory='templates', context_processors=[
    app_context,
])

Async processors are not supported.

[Kludex]

What's the motivation for it? You can still perform an analogous logic without introducing the context_processors, correct?

[alex-oleshkevich]

Yes, by copypasting. If you share variables with all templates (for example, teams in sidebar, account balance, etc) you have to generate and pass these variables to the template in every view. Context processors solve this problem. You define login in one place and template's context get automatically updated.

[Kludex]

Flask has it: https://flask.palletsprojects.com/en/2.2.x/templating/#context-processors
Django has it: https://docs.djangoproject.com/en/4.1/ref/templates/api/#built-in-template-context-processors

Let's solve the documentation, and I'm fine with this. 👍

[Kludex]

I wrote a suggestion to simplify a bit the test, and the documentation.

Everything cool here. 👍

hold

[alex-oleshkevich]

Are you waiting for anything from my side, @Kludex ?

[Kludex]

I was kind a waiting for you to press merge 👀