How to Mock a 202 Accepted Long-Running Operation in Python

Build a mock HTTP server that returns a 202 Accepted response immediately and simulates a long-running operation in the background with threading.

Medium Python 3.9+ Aug 9, 2026 API design & gRPC 13 views 0 copies

api mock-server http async 202-accepted

Python code

30 lines

Python 3.9+

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

class MockHandler(BaseHTTPRequestHandler):
    def do_POST(self):
        if self.path == "/long-running":
            self.send_response(202)
            self.send_header("Content-Type", "application/json")
            self.end_headers()
            self.wfile.write(b'{"status": "accepted", "job_id": "12345"}')
            
            # Simulate long-running work in background
            threading.Thread(target=self._process, daemon=True).start()
    
    def _process(self):
        time.sleep(3)  # Simulate 3-second operation
        print("Long-running operation completed")
    
    def log_message(self, format, *args):
        pass  # Silence default logging

def run():
    server = HTTPServer(("127.0.0.1", 8000), MockHandler)
    print("Mock server running on http://127.0.0.1:8000")
    print("POST /long-running returns 202 Accepted immediately")
    server.serve_forever()

if __name__ == "__main__":
    run()

Output

stdout

Mock server running on http://127.0.0.1:8000
POST /long-running returns 202 Accepted immediately
Long-running operation completed

How it works

The do_POST method checks the request path and returns a 202 Accepted status with a JSON body immediately. A background thread is started using threading.Thread with daemon=True so it does not block the server. The time.sleep(3) call simulates real work, and the completion message appears on the server console, not in the HTTP response. Silencing log_message keeps the mock server's output clean for debugging.

Common mistakes

Blocking the request handler with `time.sleep` before returning the response, which defeats the purpose of 202
Forgetting to set `daemon=True` on the background thread, preventing the server from shutting down cleanly
Trying to include the long-running result in the same HTTP response instead of returning it in a separate status endpoint

Variations

Use `concurrent.futures.ThreadPoolExecutor` to manage multiple background jobs instead of raw threads
Add a separate `GET /status/{job_id}` endpoint so clients can poll for completion

Real-world use cases

Mocking an async job queue during integration testing of API clients that expect 202 patterns.
Simulating a report generation service where the API acknowledges the request and processes it in the background.
Creating a fake webhook receiver that accepts events instantly for load testing consumer pipelines.

How to Mock a 202 Accepted Long-Running Operation in Python

Python code

Output

How it works

Common mistakes

Variations

Real-world use cases

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Python code

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How it works

Common mistakes

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Real-world use cases

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