web.Application and web_urldispatcher.UrlDispatcher can't be used separately

[ikalnytskyi]

Long story short

Since aiohttp 1.1.0 there's no correct way to create web.Application and web_urldispatcher.UrlDispatcher separately, since the latest requires the former as argument and both saves each other as protected properties.

app = web.Application(
    router=web_urldispatcher.UrlDispatcher(app),  # app doesn't exist yet!
)

possible solutions is to patch protected property:

# either on router
app = web.Application(router=web_urldispatcher.UrlDispatcher(None))
app.router._app = app

# or on application
app = web.Application()
app._router = web_urldispatcher.UrlDispatcher(app)

aiohttp users can't rely on protected attributes, since they may changed from version to version.

Expected behaviour

There should be a proper way to resolve this solution.

Actual behaviour

Workaround is needed.

Steps to reproduce

• Create web_urldispatcher.UrlDispatcher independently and pass it to web.Application.

Your environment

• aiohttp 1.1.0
• macOS sierra, Arch Linux

[asvetlov]

What's your use case for deriving from UrlDispatcher?
From my perspective you should either use it as is or reimplement all functionality by supporting AbstractRouter ABC.

[ikalnytskyi]

@asvetlov I don't derive from UrlDispatcher, I just create it explicitly with some factory function. I mean in my case I construct router instance and pass it to application instance, and do not follow pattern used in the docs:

app = web.Application()
app.router.add_route(...)

Since there's a problem with that pattern either - add_route is not in AbstractRouter interface, thus I can't use it if I may have non-standard router one day (and I actually do).

[asvetlov]

Hmm, but why do you need to create UrlDispatcher manually?
Please don't get me wrong: adding Router.setup_app(app) is easy and could solve your issue.
But maybe you just misuse aiohttp Public API?
For example I pretty sure that @toumorokoshi shouldn't derive from UrlDispatcher as I've answered.
At list my suggestion is right from my point of view :)
Honestly I prefer aggregation instead of inheritance and aiohttp is built in this matter: end users shouldn't derive from aiohttp classes unless these ones are not Abstract Base Classes.
It helps to keep API clean and allows to perform very deep aiohttp refactoring without raising unexpected backward compatibility problems.

Thus what is your use case? Would you provide a simple code snippet with problem demonstration?
The only reason that I can imagine is making declarative style for route registering.
If it's true we might work on adding it into aiohttp itself.
At early aiohttp stages I was reluctant for adding declarative style -- I had striven to keep routing API as minimal as possible.
But now this API is stabilized more or less, we could make next step.

[ikalnytskyi]

@asvetlov Who's talking about derivation? :) I'm talking about factory function that somehow gather and return a router that I need to use with my application. Let's say, map endpoints to classes from YAML or create various router classes based on some parameter in the config. To me it sounds like a valid case especially taking into account that web.Application receives router as its parameter. As for router class, I thought UrlDispatcher is a part of public aiohttp API and I can use this class to create UrlDispatcher instance (again, not inherit), can't I? If it's internal API, well, ok, I can close this issue as it's my issue then.

Regarding my particular case. I've decided to use router as a way to do API versioning. In my case I have VersionRouter that inherits from AbstractRouter and implements resolve method. resolve method, on the other side, delegates its work to a concrete UrlDispatcher (i.e. I have multiple UrlDispatcher instances each tracks resources of a certain API version). I've played around with aiohttp in my project and that way to solve versioning problem seemed reasonable to me. Here's an example: https://github.com/xsnippet/xsnippet-api/blob/master/xsnippet_api/router.py#L55-L82

[toumorokoshi]

I closed my ticket #1365, but I think I'll join in on this.

It would be nice to be able to instantiate UrlDispatchers separately, or rely on that behavior. For my project (aiohttp-transmute), my job becomes a lot easier if I can rely on a dispatcher that handles basic mounting of routes resolving to the appropriate path.

I guess if UrlDispatcher isn't supposed to be extended (or public), I would just continue to build my tooling around it's methods anyway, since that's the default router and I'm going to guess that a lot of people will have a painful upgrade if the default router behavior changed.

Just some thoughts. Maybe you can say that you're not supposed to extend from it, but it's pretty "public" already.

[fafhrd91]

actually UrlDispatcher doesnt use _app except in add_subapp and then it uses application's protected method. this method has code smell feeling.
should add_subapp be part of Application object?

[fafhrd91]

@asvetlov ^^^

[ikalnytskyi]

Just some thoughts. Maybe you can say that you're not supposed to extend from it, but it's pretty "public" already.

+1 on this. The thing is, UrlDispatcher is doing a great job resolving routes to views, and implementing a brand new resolver each time I need a slightly different router doesn't seem right. So I bet everyone will try to reuse UrlDispatcher either via inheriting or aggregating.

[asvetlov]

@fafhrd91 add_subapp() cannot be moved to Application -- it creates UrlDispatcher routes.
What I want to do now is collecting all use broken cases.
https://github.com/xsnippet/xsnippet-api/blob/master/xsnippet_api/router.py#L55-L82 is good example, let me sleep on it.

From my perspective UrlDispatcher should be sealed class.

[roadsideseb]

I'm having the same issue that I am overwriting UrlDispatcher and would like to pass it into the Application as router but can't because of the required app argument in UrlDispatcher.

My use case for this is adding route-based validation of incoming request using JSON schema. Being able to subclass UrlDispatcher makes it possible to store route-schema mapping on the dispatcher and validate them in a middleware.

If subclassing UrlDispatcher and passing it into the app at initialization via the router argument, what is the use case for having that argument? Is it to support the sub-app functionality?

[asvetlov]

Fixed by #1458