aiohttp.web OpenAPI 3 schema first server applications

OpenAPI 3 is a powerful way of describing request / response specifications for API endpoints. There are two ways on using OpenAPI 3 within Python server applications:

While both ways have their pros & cons, rororo library is heavily inspired by pyramid_openapi3 and as result requires valid OpenAPI 3 schema file to be provided.

In total, to build aiohttp.web OpenAPI 3 server applications with rororo you need to:

  1. Provide valid OpenAPI 3 schema file

  2. Map operationId with aiohttp.web view handler via rororo.openapi.OperationTableDef

  3. Call rororo.openapi.setup_openapi() to finish setup process

Below more details provided for all significant parts.

Part 1. Provide OpenAPI 3 schema file

From one point of view, generating OpenAPI 3 schemas from Python data structures is more Pythonic way, but it results in several issues:

  • Which Python data structure use as a basis? For example,

  • Data structure library need to support whole OpenAPI 3 specification on their own

  • Sharing OpenAPI 3 schema with other parts of your application (frontend, mobile application, etc) became a tricky task, which in most cases requires to be handled by specific CI/CD job

In same time, as rororo requires OpenAPI 3 schema file it allows to,

  • Use any Python data structure library for accessing valid request data and for providing valid response data

  • Track changes to OpenAPI specification file directly with source control management system as git or mercurial

  • Use all available OpenAPI 3 specification features

To start with OpenAPI 3 schema it is recommended to,

Supply schema & spec instances instead of schema path

To simplify developer experience rororo expects only on OpenAPI 3 schema path. However it is possible to pass predefined schema dict and spec instance instead. Consult rororo.openapi.setup_openapi() to check how to achieve that.

Part 2. Map operation with view handler

After OpenAPI 3 schema file is valid and ready to be used, it is needed to map OpenAPI operations with aiohttp.web view handlers.

As operationId field for the operation is,

Unique string used to identify the operation. The id MUST be unique among all operations described in the API.

It makes possible to tell aiohttp.web to use specific view as a handler for every given OpenAPI 3 operation.

For example,

  1. OpenAPI 3 specification has hello_world operation

  2. api.views module has hello_world view handler

To connect both of described parts rororo.openapi.OperationTableDef need to be used as (in

from aiohttp import web
from rororo import OperationTableDef

operations = OperationTableDef()

async def hello_world(request: web.Request) -> web.Response:
    return web.json_response("Hello, world!")

In case, when operationId does not match view handler name it is needed to to pass operation_id string as first argument of @operations.register decorator,

async def not_a_hello_world(
    request: web.Request,
) -> web.Response:
    return web.json_response("Hello, world!")

Class Based Views

rororo supports class based views as well.

In basic mode it expects that OpenAPI schema contains operationId, which equals to all view method qualified names. For example, code below expects OpenAPI schema to declare UsersView.get & operation IDs,

class UsersView(web.View):
    async def get(self) -> web.Response:

    async def post(self) -> web.Response:

Next, it might be useful to provide different prefix instead of UsersView. In example below, rororo expects OpenAPI schema to provide users.get & operation IDs,

class UsersView(web.View):
    async def get(self) -> web.Response:

    async def post(self) -> web.Response:

Finally, it might be useful to provide custom operationId instead of guessing it from view or view method name. Example below, illustrates the case, when OpenAPI schema contains list_users & create_user operation IDs,

class UsersView(web.View):
    async def get(self) -> web.Response:

    async def post(self) -> web.Response:

To access in class based views you need to pass self.request into rororo.openapi.openapi_context() or rororo.openapi.get_openapi_context() as done below,

class UserView(web.View):
    async def patch(self) -> web.Response:
        user = get_user_or_404(self.request)
        with openapi_context(self.request) as context:
            next_user = attr.evolve(user, **
        return web.json_response(next_user.to_api_dict())


On registering class based views with multiple view methods (for example with get, patch & put) you need to ensure that all methods could be mapped to operation ID in provided OpenAPI schema file.

Request Validation

Decorating view handler with @operations.register will ensure that it will be executed only with valid request body & parameters according to OpenAPI 3 operation specification.

If any parameters are missed or invalid, as well as if request body does not pass validation it will result in 422 response.

Accessing Valid Request Data

To access valid data for given request it is recommended to use rororo.openapi.openapi_context() context manager as follows,

async def add_pet(request: web.Request) -> web.Response:
    with openapi_context(request) as context:

Resulted context instance will contain,

  • request - untouched aiohttp.web.Request instance

  • app - aiohttp.web.Application instance

  • config_dict

  • parameters - valid parameters mappings (path, query, header, cookie)

  • security - security data, if operation is secured

  • data - valid data from request body

Part 3. Finish setup process

After the OpenAPI 3 schema is provided and view handlers is mapped to OpenAPI operations it is a time to tell an aiohttp.web.Application to use given schema file and operations mapping(s) via rororo.openapi.setup_openapi().

In most cases this setup should be done in application factory function as follows,

from pathlib import Path
from typing import List

from aiohttp import web
from rororo import setup_openapi

from .views import operations

OPENAPI_YAML_PATH = Path(__file__).parent / "openapi.yaml"

def create_app(argv: List[str] = None) -> web.Application:
    app = web.Application()
    setup_openapi(app, OPENAPI_YAML_PATH, opeartions)
    return app


It is recommended to store OpenAPI 3 schema file next to main application module, which semantically will mean: this is an OpenAPI 3 schema file for current application.

But it is not mandatory, and you might want to specify any accessible file path, you want.


By default, OpenAPI schema, which is used for the application will be available via GET requests to {server_url}/openapi.(json|yaml), but it is possible to not serve the schema by passing has_openapi_schema_handler falsy flag to rororo.openapi.setup_openapi()

Configuration & Operation Errors

Setting up OpenAPI for aiohttp.web applicaitons via rororo.openapi.setup_openapi() may result in numerous errors as it relies on many things. While most of the errors designed to be self-descriptive below more information added about most possible cases.

OpenAPI 3 Schema file does not exist or not readable

rororo expects that schema_path is a path to a readable file with OpenAPI schema. To fix the error, pass proper path.

Unable to read OpenAPI 3 Schema from the file

rororo supports reading OpenAPI 3 schema from JSON & YAML files with extensions: .json, .yml, .yaml. If the schema_path file contains valid OpenAPI 3 schema, but has different extension, consider rename it. Also, in same time rororo expects that .json files contain valid JSON, while .yml / .yaml files contain valid YAML data.

OpenAPI 3 Schema is not valid

rororo requires your OpenAPI 3 schema file to be a valid one. If the file is not valid consider running openapi-spec-validator against your file to find the issues.


rororo depends on openapi-spec-validator (via openapi-core), which means after installing rororo, virtual environment (or system) will have openapi-spec-validator script available

Operation not found

Please, use valid operationId while mapping OpenAPI operation to aiohttp.web view handler.

Using invalid operationId will result in runtime error, which doesn’t allow aiohttp.web application to start up.

Accessing OpenAPI Schema & Spec

After OpenAPI setting up for aiohttp.web.Application it is possible to access OpenAPI Schema & Spec inside of any view handler as follows,

from rororo import get_openapi_schema, get_openapi_spec

async def something(request: web.Request) -> web.Response:
    # `Dict[str, Any]` with OpenAPI schema
    schema = get_openapi_schema(

    # `openapi_core.schemas.specs.models.Spec` instance
    spec = get_openapi_spec(request.config_dict)


How it Works?

Under the hood rororo heavily relies on openapi-core library.

  1. rororo.openapi.setup_openapi()

    • Creates the Spec instance from OpenAPI schema source

    • Connects previously registered handlers and views to the application router (aiohttp.web.UrlDispatcher)

    • Registers hidden openapi_middleware to handle request to registered handlers and views

  2. On handling each OpenAPI request RequestValidator.validate(…) method called. Result of validation as supplied to current aiohttp.web.Request instance

  3. If enabled, ResponseValidator.validate(…) method called for each OpenAPI response

Swagger 2.0 Support

While rororo designed to support only OpenAPI 3 Schemas due to openapi-core dependency it is technically able to support Swagger 2.0 for aiohttp.web applications in same manner as well.


Swagger 2.0 support is not tested at all and rororo is not intended to provide it.

With that in mind please consider rororo only as a library to bring OpenAPI 3 Schemas support for aiohttp.web applications.