How to mock Server-Sent Events (SSE) in Python

A minimal HTTP server that streams Server-Sent Events to clients, perfect for testing and development.

Medium Python 3.9+ Aug 9, 2026 API design & gRPC 14 views 0 copies

sse server-sent-events http mock streaming

Python code

44 lines

Python 3.9+

from http.server import HTTPServer, BaseHTTPRequestHandler
import threading
import time

MESSAGES = iter([
    "data: Hello world\n\n",
    "data: Second message\n\n",
    "event: custom\n",
    "data: Custom event payload\n\n",
    "data: Final message\n\n"
])

class SSEHandler(BaseHTTPRequestHandler):
    def do_GET(self):
        if self.path != "/events":
            self.send_error(404)
            return
        
        self.send_response(200)
        self.send_header("Content-Type", "text/event-stream")
        self.send_header("Cache-Control", "no-cache")
        self.send_header("Connection", "keep-alive")
        self.end_headers()
        
        try:
            for msg in MESSAGES:
                self.wfile.write(msg.encode())
                self.wfile.flush()
                time.sleep(1)
        except BrokenPipeError:
            pass

    def log_message(self, format, *args):
        pass

if __name__ == "__main__":
    server = HTTPServer(("localhost", 8000), SSEHandler)
    print("SSE mock running at http://localhost:8000/events")
    print("Use curl: curl -N http://localhost:8000/events")
    
    # Auto-stop after serving all messages
    timer = threading.Timer(6, server.shutdown)
    timer.start()
    server.serve_forever()

Output

stdout

SSE mock running at http://localhost:8000/events
Use curl: curl -N http://localhost:8000/events

How it works

The handler sets Content-Type: text/event-stream and Cache-Control: no-cache to signal a streaming response. Each message is written with utf-8 encoding and flushed immediately to simulate real-time events. The MESSAGES iterator yields SSE-formatted strings, including named events and data frames. A timer auto-shuts down the server after all messages are sent to avoid hanging. log_message is suppressed to keep the console clean during development.

Common mistakes

Forgetting the trailing blank line after each event, which is required by the SSE spec.
Not flushing the write buffer, causing messages to be delayed or batched on the client.
Using `print` instead of `self.wfile.write`, which doesn't send data over the HTTP connection.

Variations

Use Flask with `Response(stream_with_context(...))` for a more framework-friendly mock.
Make the server loop forever by removing the `Timer` shutdown, useful for interactive testing.

Real-world use cases

Developing and testing a chat or notification UI without a real backend.
Simulating stock price or sports scores stream to validate client-side event handling.
Demoing an AI assistant's token-by-token response streaming to a web frontend.

How to mock Server-Sent Events (SSE) in Python

Python code

Output

How it works

Common mistakes

Variations

Real-world use cases

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Python code

Output

How it works

Common mistakes

Variations

Real-world use cases

More from API design & gRPC

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