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WebSockets

When defining WebSockets, you normally declare a parameter of type WebSocket and with it you can read data from the client and send data to it.

It is provided directly by Starlette, but you can import it from fastapi:

from fastapi import WebSocket

Tip

When you want to define dependencies that should be compatible with both HTTP and WebSockets, you can define a parameter that takes an HTTPConnection instead of a Request or a WebSocket.

fastapi.WebSocket 

WebSocket(scope, receive, send)

Bases: HTTPConnection

Source code in starlette/websockets.py 
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def __init__(self, scope: Scope, receive: Receive, send: Send) -> None:
    super().__init__(scope)
    assert scope["type"] == "websocket"
    self._receive = receive
    self._send = send
    self.client_state = WebSocketState.CONNECTING
    self.application_state = WebSocketState.CONNECTING

scope instance-attribute 

scope = scope

app property 

app

url property 

url

base_url property 

base_url

headers property 

headers

query_params property 

query_params

path_params property 

path_params

cookies property 

cookies

client property 

client

state property 

state

client_state instance-attribute 

client_state = CONNECTING

application_state instance-attribute 

application_state = CONNECTING

url_for 

url_for(name, /, **path_params)
Source code in starlette/requests.py 
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def url_for(self, name: str, /, **path_params: Any) -> URL:
    url_path_provider: Router | Starlette | None = self.scope.get("router") or self.scope.get("app")
    if url_path_provider is None:
        raise RuntimeError("The `url_for` method can only be used inside a Starlette application or with a router.")
    url_path = url_path_provider.url_path_for(name, **path_params)
    return url_path.make_absolute_url(base_url=self.base_url)

receive async 

receive()

Receive ASGI websocket messages, ensuring valid state transitions.

Source code in starlette/websockets.py 
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async def receive(self) -> Message:
    """
    Receive ASGI websocket messages, ensuring valid state transitions.
    """
    if self.client_state == WebSocketState.CONNECTING:
        message = await self._receive()
        message_type = message["type"]
        if message_type != "websocket.connect":
            raise RuntimeError(f'Expected ASGI message "websocket.connect", but got {message_type!r}')
        self.client_state = WebSocketState.CONNECTED
        return message
    elif self.client_state == WebSocketState.CONNECTED:
        message = await self._receive()
        message_type = message["type"]
        if message_type not in {"websocket.receive", "websocket.disconnect"}:
            raise RuntimeError(
                f'Expected ASGI message "websocket.receive" or "websocket.disconnect", but got {message_type!r}'
            )
        if message_type == "websocket.disconnect":
            self.client_state = WebSocketState.DISCONNECTED
        return message
    else:
        raise RuntimeError('Cannot call "receive" once a disconnect message has been received.')

send async 

send(message)

Send ASGI websocket messages, ensuring valid state transitions.

Source code in starlette/websockets.py 
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async def send(self, message: Message) -> None:
    """
    Send ASGI websocket messages, ensuring valid state transitions.
    """
    if self.application_state == WebSocketState.CONNECTING:
        message_type = message["type"]
        if message_type not in {"websocket.accept", "websocket.close", "websocket.http.response.start"}:
            raise RuntimeError(
                'Expected ASGI message "websocket.accept", "websocket.close" or "websocket.http.response.start", '
                f"but got {message_type!r}"
            )
        if message_type == "websocket.close":
            self.application_state = WebSocketState.DISCONNECTED
        elif message_type == "websocket.http.response.start":
            self.application_state = WebSocketState.RESPONSE
        else:
            self.application_state = WebSocketState.CONNECTED
        await self._send(message)
    elif self.application_state == WebSocketState.CONNECTED:
        message_type = message["type"]
        if message_type not in {"websocket.send", "websocket.close"}:
            raise RuntimeError(
                f'Expected ASGI message "websocket.send" or "websocket.close", but got {message_type!r}'
            )
        if message_type == "websocket.close":
            self.application_state = WebSocketState.DISCONNECTED
        try:
            await self._send(message)
        except OSError:
            self.application_state = WebSocketState.DISCONNECTED
            raise WebSocketDisconnect(code=1006)
    elif self.application_state == WebSocketState.RESPONSE:
        message_type = message["type"]
        if message_type != "websocket.http.response.body":
            raise RuntimeError(f'Expected ASGI message "websocket.http.response.body", but got {message_type!r}')
        if not message.get("more_body", False):
            self.application_state = WebSocketState.DISCONNECTED
        await self._send(message)
    else:
        raise RuntimeError('Cannot call "send" once a close message has been sent.')

accept async 

accept(subprotocol=None, headers=None)
Source code in starlette/websockets.py 
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async def accept(
    self,
    subprotocol: str | None = None,
    headers: Iterable[tuple[bytes, bytes]] | None = None,
) -> None:
    headers = headers or []

    if self.client_state == WebSocketState.CONNECTING:  # pragma: no branch
        # If we haven't yet seen the 'connect' message, then wait for it first.
        await self.receive()
    await self.send({"type": "websocket.accept", "subprotocol": subprotocol, "headers": headers})

receive_text async 

receive_text()
Source code in starlette/websockets.py 
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async def receive_text(self) -> str:
    if self.application_state != WebSocketState.CONNECTED:
        raise RuntimeError('WebSocket is not connected. Need to call "accept" first.')
    message = await self.receive()
    self._raise_on_disconnect(message)
    return cast(str, message["text"])

receive_bytes async 

receive_bytes()
Source code in starlette/websockets.py 
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async def receive_bytes(self) -> bytes:
    if self.application_state != WebSocketState.CONNECTED:
        raise RuntimeError('WebSocket is not connected. Need to call "accept" first.')
    message = await self.receive()
    self._raise_on_disconnect(message)
    return cast(bytes, message["bytes"])

receive_json async 

receive_json(mode='text')
Source code in starlette/websockets.py 
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async def receive_json(self, mode: str = "text") -> Any:
    if mode not in {"text", "binary"}:
        raise RuntimeError('The "mode" argument should be "text" or "binary".')
    if self.application_state != WebSocketState.CONNECTED:
        raise RuntimeError('WebSocket is not connected. Need to call "accept" first.')
    message = await self.receive()
    self._raise_on_disconnect(message)

    if mode == "text":
        text = message["text"]
    else:
        text = message["bytes"].decode("utf-8")
    return json.loads(text)

iter_text async 

iter_text()
Source code in starlette/websockets.py 
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async def iter_text(self) -> AsyncIterator[str]:
    try:
        while True:
            yield await self.receive_text()
    except WebSocketDisconnect:
        pass

iter_bytes async 

iter_bytes()
Source code in starlette/websockets.py 
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async def iter_bytes(self) -> AsyncIterator[bytes]:
    try:
        while True:
            yield await self.receive_bytes()
    except WebSocketDisconnect:
        pass

iter_json async 

iter_json()
Source code in starlette/websockets.py 
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async def iter_json(self) -> AsyncIterator[Any]:
    try:
        while True:
            yield await self.receive_json()
    except WebSocketDisconnect:
        pass

send_text async 

send_text(data)
Source code in starlette/websockets.py 
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async def send_text(self, data: str) -> None:
    await self.send({"type": "websocket.send", "text": data})

send_bytes async 

send_bytes(data)
Source code in starlette/websockets.py 
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async def send_bytes(self, data: bytes) -> None:
    await self.send({"type": "websocket.send", "bytes": data})

send_json async 

send_json(data, mode='text')
Source code in starlette/websockets.py 
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async def send_json(self, data: Any, mode: str = "text") -> None:
    if mode not in {"text", "binary"}:
        raise RuntimeError('The "mode" argument should be "text" or "binary".')
    text = json.dumps(data, separators=(",", ":"), ensure_ascii=False)
    if mode == "text":
        await self.send({"type": "websocket.send", "text": text})
    else:
        await self.send({"type": "websocket.send", "bytes": text.encode("utf-8")})

close async 

close(code=1000, reason=None)
Source code in starlette/websockets.py 
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async def close(self, code: int = 1000, reason: str | None = None) -> None:
    await self.send({"type": "websocket.close", "code": code, "reason": reason or ""})

When a client disconnects, a WebSocketDisconnect exception is raised, you can catch it.

You can import it directly form fastapi:

from fastapi import WebSocketDisconnect

fastapi.WebSocketDisconnect 

WebSocketDisconnect(code=1000, reason=None)

Bases: Exception

Source code in starlette/websockets.py 
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def __init__(self, code: int = 1000, reason: str | None = None) -> None:
    self.code = code
    self.reason = reason or ""

code instance-attribute 

code = code

reason instance-attribute 

reason = reason or ''

WebSockets - additional classes

Additional classes for handling WebSockets.

Provided directly by Starlette, but you can import it from fastapi:

from fastapi.websockets import WebSocketDisconnect, WebSocketState

fastapi.websockets.WebSocketDisconnect 

WebSocketDisconnect(code=1000, reason=None)

Bases: Exception

Source code in starlette/websockets.py 
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def __init__(self, code: int = 1000, reason: str | None = None) -> None:
    self.code = code
    self.reason = reason or ""

code instance-attribute 

code = code

reason instance-attribute 

reason = reason or ''

fastapi.websockets.WebSocketState

Bases: Enum

CONNECTING class-attribute instance-attribute 

CONNECTING = 0

CONNECTED class-attribute instance-attribute 

CONNECTED = 1

DISCONNECTED class-attribute instance-attribute 

DISCONNECTED = 2

RESPONSE class-attribute instance-attribute 

RESPONSE = 3