How to Mock a Chunked Encoding Streaming Response in Python

Build a local mock HTTP server with Python's http.server that streams a chunked-encoded response with a 0.5s delay per chunk.

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

http streaming chunked mock-server api-testing httptools

Python code

37 lines

Python 3.9+

from http.server import BaseHTTPRequestHandler, ThreadingHTTPServer
import time

class ChunkedHandler(BaseHTTPRequestHandler):
    protocol_version = "HTTP/1.1"

    def do_GET(self):
        self.send_response(200)
        self.send_header("Content-Type", "text/plain")
        self.send_header("Transfer-Encoding", "chunked")
        self.end_headers()

        for i in range(1, 6):
            chunk = f"Chunk-{i}\n".encode()
            # write hex length + CRLF + data + CRLF
            self.wfile.write(f"{len(chunk):X}\r\n".encode())
            self.wfile.write(chunk + b"\r\n")
            self.wfile.flush()
            time.sleep(0.5)

        # terminating chunk
        self.wfile.write(b"0\r\n\r\n")
        self.wfile.flush()

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

def main():
    server = ThreadingHTTPServer(("127.0.0.1", 8080), ChunkedHandler)
    print("Serving on port 8080 (Press Ctrl+C to stop)")
    try:
        server.serve_forever()
    except KeyboardInterrupt:
        server.shutdown()

if __name__ == "__main__":
    main()

Output

stdout

Serving on port 8080 (Press Ctrl+C to stop)

When a client connects (e.g., with curl):

HTTP/1.1 200 OK
Content-Type: text/plain
Transfer-Encoding: chunked

Chunk-1
Chunk-2
Chunk-3
Chunk-4
Chunk-5

Each chunk appears one every 0.5 seconds.

How it works

The handler sets protocol_version = "HTTP/1.1" so connections stay open and support chunked transfer. send_response(200) sends the status line and headers; then end_headers() finalizes the header block. Each chunk is encoded to bytes, and its hex length is written first, followed by CRLF, the chunk data, and another CRLF as per the HTTP/1.1 chunked transfer protocol. Flushing after each write ensures data is pushed to the client immediately, and sleeping simulates streaming latency. The terminating 0\r\n\r\n chunk signals the end of the stream.

Common mistakes

Forgetting to set `protocol_version = "HTTP/1.1"` — HTTP/1.0 does not support chunked encoding.
Omitting the CRLF after the chunk length or between chunks, breaking the protocol.
Not flushing after each write may buffer all data and defeat streaming.
Sending the terminating chunk as `0\r\n` only — you must include the trailing CRLF.

Variations

Use `http.server.HTTPServer` instead of a threaded server for simpler single-client testing.
Stream binary data (e.g., JSON chunks) by encoding each chunk with `json.dumps`.

Real-world use cases

Locally testing a client that consumes an LLM's token-by-token streaming API without hitting a paid service.
Verifying that a web client handles incremental data rendering in server-sent events or chunked download scenarios.
Prototyping an API endpoint that requires transfer-encoding chunked for real-time data feeds like stock tickers or chat messages.

How to Mock a Chunked Encoding Streaming Response 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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