starlette/docs/routing.md

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Starlette includes a `Router` class which is an ASGI application that
dispatches to other ASGI applications.
```python
from starlette.routing import Router, Path, PathPrefix
from myproject import Homepage, SubMountedApp
app = Router([
Path('/', app=Homepage, methods=['GET']),
PathPrefix('/mount/', app=SubMountedApp)
])
```
Paths can use URI templating style to capture path components.
```python
Path('/users/{username}', app=User, methods=['GET'])
```
Path components are made available in the scope, as `scope["kwargs"]`.
You can also use regular expressions for more complicated matching.
Named capture groups will be included in `scope["kwargs"]`:
```python
Path('/users/(?P<username>[a-zA-Z0-9_]{1,20})', app=User, methods=['GET'])
```
Because each target of the router is an ASGI instance itself, routers
allow for easy composition. For example:
```python
app = Router([
Path('/', app=Homepage, methods=['GET']),
PathPrefix('/users', app=Router([
Path('/', app=Users, methods=['GET', 'POST']),
Path('/{username}', app=User, methods=['GET']),
]))
])
```
The router will respond with "404 Not found" or "405 Method not allowed"
responses for requests which do not match.
### Protocol Routing
You can also route based on the incoming protocol, using the `ProtocolRouter`
class.
```python
from starlette.responses import Response
from starlette.routing import ProtocolRouter
from starlette.websockets import WebSocket
def http_endpoint(scope):
return Response("Hello, world", media_type="text/plain")
def websocket_endpoint(scope):
async def asgi(receive, send):
websocket = WebSocket(scope, receive, send)
await websocket.accept()
await websocket.send_json({"hello": "world"})
await websocket.close()
return asgi
app = ProtocolRouter({
"http": http_endpoint,
"websocket": websocket_endpoint
})
```