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from annotated_doc import Doc
from pydantic import AfterValidator, BaseModel, Field, model_validator
from starlette.responses import StreamingResponse
# Canonical SSE event schema matching the OpenAPI 3.2 spec
# (Section 4.14.4 "Special Considerations for Server-Sent Events")
_SSE_EVENT_SCHEMA: dict[str, Any] = {
"type": "object",
"properties": {
"data": {"type": "string"},
"event": {"type": "string"},
"id": {"type": "string"},
"retry": {"type": "integer", "minimum": 0},
},
}
class EventSourceResponse(StreamingResponse):
"""Streaming response with `text/event-stream` media type.
Use as `response_class=EventSourceResponse` on a *path operation* that uses `yield`
to enable Server Sent Events (SSE) responses.
Works with **any HTTP method** (`GET`, `POST`, etc.), which makes it compatible
with protocols like MCP that stream SSE over `POST`.
The actual encoding logic lives in the FastAPI routing layer. This class
serves mainly as a marker and sets the correct `Content-Type`.
"""
media_type = "text/event-stream"
def _check_single_line(v: str | None, field_name: str) -> str | None:
if v is not None and ("\r" in v or "\n" in v):
raise ValueError(f"SSE '{field_name}' must be a single line")
return v
def _check_event_single_line(v: str | None) -> str | None:
return _check_single_line(v, "event")
def _check_id_valid(v: str | None) -> str | None:
if v is not None and "\0" in v:
raise ValueError("SSE 'id' must not contain null characters")
return _check_single_line(v, "id")
class ServerSentEvent(BaseModel):
"""Represents a single Server-Sent Event.
When `yield`ed from a *path operation function* that uses
`response_class=EventSourceResponse`, each `ServerSentEvent` is encoded
into the [SSE wire format](https://html.spec.whatwg.org/multipage/server-sent-events.html#parsing-an-event-stream)
(`text/event-stream`).
If you yield a plain object (dict, Pydantic model, etc.) instead, it is
automatically JSON-encoded and sent as the `data:` field.
All `data` values **including plain strings** are JSON-serialized.
For example, `data="hello"` produces `data: "hello"` on the wire (with
quotes).
"""
data: Annotated[
Any,
Doc(
"""
The event payload.
Can be any JSON-serializable value: a Pydantic model, dict, list,
string, number, etc. It is **always** serialized to JSON: strings
are quoted (`"hello"` becomes `data: "hello"` on the wire).
Mutually exclusive with `raw_data`.
"""
),
] = None
raw_data: Annotated[
str | None,
Doc(
"""
Raw string to send as the `data:` field **without** JSON encoding.
Use this when you need to send pre-formatted text, HTML fragments,
CSV lines, or any non-JSON payload. The string is placed directly
into the `data:` field as-is.
Mutually exclusive with `data`.
"""
),
] = None
event: Annotated[
str | None,
AfterValidator(_check_event_single_line),
Doc(
"""
Optional event type name.
Maps to `addEventListener(event, ...)` on the browser. When omitted,
the browser dispatches on the generic `message` event. Must be a
single line.
"""
),
] = None
id: Annotated[
str | None,
AfterValidator(_check_id_valid),
Doc(
"""
Optional event ID.
The browser sends this value back as the `Last-Event-ID` header on
automatic reconnection. **Must be a single line** and must not contain
null (`\\0`) characters.
"""
),
] = None
retry: Annotated[
int | None,
Field(ge=0),
Doc(
"""
Optional reconnection time in **milliseconds**.
Tells the browser how long to wait before reconnecting after the
connection is lost. Must be a non-negative integer.
"""
),
] = None
comment: Annotated[
str | None,
Doc(
"""
Optional comment line(s).
Comment lines start with `:` in the SSE wire format and are ignored by
`EventSource` clients. Useful for keep-alive pings to prevent
proxy/load-balancer timeouts.
"""
),
] = None
@model_validator(mode="after")
def _check_data_exclusive(self) -> "ServerSentEvent":
if self.data is not None and self.raw_data is not None:
raise ValueError(
"Cannot set both 'data' and 'raw_data' on the same "
"ServerSentEvent. Use 'data' for JSON-serialized payloads "
"or 'raw_data' for pre-formatted strings."
)
return self
def format_sse_event(
*,
data_str: Annotated[
str | None,
Doc(
"""
Pre-serialized data string to use as the `data:` field.
"""
),
] = None,
event: Annotated[
str | None,
Doc(
"""
Optional event type name (`event:` field).
"""
),
] = None,
id: Annotated[
str | None,
Doc(
"""
Optional event ID (`id:` field).
"""
),
] = None,
retry: Annotated[
int | None,
Doc(
"""
Optional reconnection time in milliseconds (`retry:` field).
"""
),
] = None,
comment: Annotated[
str | None,
Doc(
"""
Optional comment line(s) (`:` prefix).
"""
),
] = None,
) -> bytes:
"""Build SSE wire-format bytes from **pre-serialized** data.
The result always ends with `\n\n` (the event terminator).
"""
lines: list[str] = []
if comment is not None:
for line in comment.splitlines():
lines.append(f": {line}")
if event is not None:
lines.append(f"event: {event}")
if data_str is not None:
for line in data_str.splitlines():
lines.append(f"data: {line}")
if id is not None:
lines.append(f"id: {id}")
if retry is not None:
lines.append(f"retry: {retry}")
lines.append("")
lines.append("")
return "\n".join(lines).encode("utf-8")
# Keep-alive comment, per the SSE spec recommendation
KEEPALIVE_COMMENT = b": ping\n\n"
# Seconds between keep-alive pings when a generator is idle.
# Private but importable so tests can monkeypatch it.
_PING_INTERVAL: float = 15.0
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