Build a Mock REST API with PUT and GET in Python

A minimal mock REST server implementing idempotent PUT for resource replacement and GET for retrieval, built with Python's http.server module.

Medium Python 3.9+ Aug 9, 2026 API design & gRPC 15 views 0 copies

rest-api http-server mock put idempotent

Python code

49 lines
Python 3.9+
Run it 
from http.server import BaseHTTPRequestHandler, HTTPServer
import json
from urllib.parse import urlparse

mock_db = {}

class MockAPIHandler(BaseHTTPRequestHandler):
    def do_PUT(self):
        parsed = urlparse(self.path)
        resource_id = parsed.path.strip("/").split("/")[-1]
        content_length = int(self.headers.get("Content-Length", 0))
        payload = json.loads(self.rfile.read(content_length) or b"{}")

        mock_db[resource_id] = payload

        self.send_response(200)
        self.send_header("Content-Type", "application/json")
        self.end_headers()
        self.wfile.write(json.dumps({
            "id": resource_id,
            "data": payload,
            "message": "Resource replaced successfully (idempotent)"
        }).encode())

    def do_GET(self):
        parsed = urlparse(self.path)
        resource_id = parsed.path.strip("/").split("/")[-1]

        if resource_id in mock_db:
            self.send_response(200)
            self.send_header("Content-Type", "application/json")
            self.end_headers()
            self.wfile.write(json.dumps(mock_db[resource_id]).encode())
        else:
            self.send_response(404)
            self.end_headers()

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


def run_server():
    server = HTTPServer(("localhost", 8000), MockAPIHandler)
    print("Mock REST server running on http://localhost:8000")
    server.serve_forever()


if __name__ == "__main__":
    run_server()

Output

stdout 
Mock REST server running on http://localhost:8000

# Client requests:
curl -X PUT http://localhost:8000/users/42 -d '{"name": "Alice"}'
{"id": "42", "data": {"name": "Alice"}, "message": "Resource replaced successfully (idempotent)"}

curl http://localhost:8000/users/42
{"name": "Alice"}

curl http://localhost:8000/users/99
(404 response)

# Repeating the same PUT returns the same 200 response — idempotent behavior:

How it works

The do_PUT method reads the request body with Content-Length to avoid blocking, then parses it as JSON. Storing the payload in a dictionary keyed by the resource ID makes replacement idempotent — repeating the same PUT yields identical results. The do_GET handler looks up the ID and returns 404 for missing resources. All state lives in-memory, so the mock resets on restart, which is ideal for tests or local prototyping.

Common mistakes

  • Forgetting to handle `Content-Length` when reading the request body, causing hangs or partial reads
  • Assuming the resource ID is always numeric; the URL path parsing may need validation
  • Not setting `Content-Type` headers, leading to clients misinterpreting responses

Variations

  1. Use Flask or FastAPI for more routing features, validation, and auto-generated docs
  2. Add DELETE support by removing the key from the dictionary

Real-world use cases

  • Stubbing a backend API during frontend development so UI work isn't blocked by service availability.
  • Providing a throwaway test double in integration tests to verify client retry and error-handling logic.
  • Simulating a resource store for acceptance tests of PUT idempotency contract requirements.

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