Demonstration of how Application typing can work with a library. This is a stricter approach, where users (who use strict typing) would need to inherit from aiohttp_jinja2.AppState.

For example, a user application might look like this:

class AppState(aiohttp_jinja2.AppState):
    foo: str

MyRequest = web.Request[AppState]

app: web.Application[AppState] = web.Application()
app.state["foo"] = "bar"
aiohttp_jinja2.setup(app, loader=jinja2.FileSystemLoader('/path/to/templates/folder'))

@aiohttp_jinja2.template('tmpl.jinja2')
async def handler(request: MyRequest) -> Dict[str ,str]:
    return {''foo': request.app.state['foo']}

Breaking change

For typing to work correctly, we need to have hardcoded keys. This means deprecating the customisable app_key argument.
I think this only exists to help with clashes in key names. I suspect almost nobody actually uses this and am fine removing it in a future release. I've also prefixed the private aiohttp-jinja2 names with _ to further reduce the likelihood that it will conflict with someone's object.

Plus, with better static typing, it's much easier for someone to tell if they used the same name. For example, this will give an error in mypy:

class AppState(aiohttp_jinja2.AppState):
    foo: str
    _aiohttp_jinja2_environment: Dict[str, str]