Add lifespan documentation (#1759)

Contributes towards https://github.com/spec-first/connexion/issues/1531

connexion/middleware/exceptions.py

@@ -112,6 +112,4 @@ class ExceptionMiddleware(StarletteExceptionMiddleware):
         )
 
     async def __call__(self, scope: Scope, receive: Receive, send: Send) -> None:
-        # Needs to be set so starlette router throws exceptions instead of returning error responses
-        scope["app"] = "connexion"
         await super().__call__(scope, receive, send)

connexion/middleware/main.py

@@ -477,4 +477,7 @@ class ConnexionMiddleware:
     async def __call__(self, scope: Scope, receive: Receive, send: Send) -> None:
         if self.middleware_stack is None:
             self.app, self.middleware_stack = self._build_middleware_stack()
+        # Set so starlette router throws exceptions instead of returning error responses
+        # This instance is also passed to any lifespan handler
+        scope["app"] = self
         await self.app(scope, receive, send)

docs/index.rst

@@ -71,6 +71,7 @@ Documentation
    validation
    security
    context
+   lifespan
    cookbook
    exceptions
    cli

docs/lifespan.rst

@@ -0,0 +1,109 @@
+Lifespan
+========
+
+You can register lifespan handlers to run code before the app starts, or after it shuts down.
+This ideal for setting up and tearing down database connections or machine learning models for
+instance.
+
+.. tab-set::
+
+    .. tab-item:: AsyncApp
+        :sync: AsyncApp
+
+        .. code-block:: python
+
+            import contextlib
+            import typing
+
+            from connexion import AsyncApp, ConnexionMiddleware, request
+
+            @contextlib.asynccontextmanager
+            def lifespan_handler(app: ConnexionMiddleware) -> typing.AsyncIterator:
+                """Called at startup and shutdown, can yield state which will be available on the
+                 request."""
+                client = Client()
+                yield {"client": client}
+                client.close()
+
+            def route():
+                """Endpoint function called when receiving a request, you can access the state
+                on the request here."""
+                client = request.state.client
+                client.call()
+
+            app = AsyncApp(__name__, lifespan=lifespan_handler)
+
+    .. tab-item:: FlaskApp
+        :sync: FlaskApp
+
+        .. code-block:: python
+
+            import contextlib
+            import typing
+
+            from connexion import FlaskApp, ConnexionMiddleware, request
+
+            @contextlib.asynccontextmanager
+            def lifespan_handler(app: ConnexionMiddleware) -> typing.AsyncIterator:
+                """Called at startup and shutdown, can yield state which will be available on the
+                 request."""
+                client = Client()
+                yield {"client": client}
+                client.close()
+
+            def route():
+                """Endpoint function called when receiving a request, you can access the state
+                on the request here."""
+                client = request.state.client
+                client.call()
+
+            app = FlaskApp(__name__, lifespan=lifespan_handler)
+
+    .. tab-item:: ConnexionMiddleware
+        :sync: ConnexionMiddleware
+
+        .. code-block:: python
+
+            import contextlib
+            import typing
+
+            from asgi_framework import App
+            from connexion import ConnexionMiddleware, request
+
+            @contextlib.asynccontextmanager
+            def lifespan_handler(app: ConnexionMiddleware) -> typing.AsyncIterator:
+                """Called at startup and shutdown, can yield state which will be available on the
+                 request."""
+                client = Client()
+                yield {"client": client}
+                client.close()
+
+            def endpoint():
+                """Endpoint function called when receiving a request, you can access the state
+                on the request here."""
+                client = request.state.client
+                client.call()
+
+            app = App(__name__)
+            app = ConnexionMiddleware(app, lifespan=lifespan_handler)
+
+Running lifespan in tests
+-------------------------
+
+If you want lifespan handlers to be called during tests, you can use the ``test_client`` as a
+context manager.
+
+.. code-block:: python
+
+    def test_homepage():
+        app = ...  # Set up app
+        with app.test_client() as client:
+            # Application's lifespan is called on entering the block.
+            response = client.get("/")
+            assert response.status_code == 200
+
+        # And the lifespan's teardown is run when exiting the block.
+
+For more information, please refer to the `Starlette documentation`_.
+
+.. _Starlette documentation: https://www.starlette.io/lifespan/