Building a Simple HTTP Server

The built-in http.server

Python includes a simple HTTP server in the standard library.

Great for:

• local demos
• serving static files

Not ideal for production.

Serve a folder

If you run:

python -m http.server 8000

Then open:

• http://localhost:8000

Custom handler

from http.server import BaseHTTPRequestHandler, HTTPServer
 
 
class Handler(BaseHTTPRequestHandler):
    def do_GET(self):
        self.send_response(200)
        self.send_header("Content-Type", "text/plain")
        self.end_headers()
        self.wfile.write(b"Hello from Python HTTP server")
 
 
if __name__ == "__main__":
    server = HTTPServer(("127.0.0.1", 8000), Handler)
    print("Serving on http://127.0.0.1:8000")
    server.serve_forever()

What’s happening?

• do_GET handles HTTP GET requests
• send_response sets status code
• headers must be sent before the body

  flowchart TD
  A["GET /hello.txt"] --> B{"does it exist under the CWD?"}
  B -->|yes| C["200 + the file, Content-Type guessed from the suffix"]
  B -->|no| D["404 File not found"]
  E["GET /"] --> F["200 + a GENERATED directory listing"]
  G["POST anything"] --> H["501 Unsupported method"]
  C --> I["the document root is os.getcwd()"]
  F --> I
  I --> J["so it must never be exposed beyond localhost"]

Requests and responses

HTTP is a conversation made of text. The client sends a request: a method (GET, POST), a path (/hello), headers and sometimes a body. The server sends back a response: a status code (200 for success, 404 for not found), headers, and a body. BaseHTTPRequestHandler parses the incoming request for you and calls a method named after the HTTP verb: do_GET for GET, do_POST for POST, and so on. Your job is to write the response in the right order: status line, headers, blank line, body.

Reading the custom handler

In the example, send_response(200) writes the status line. send_header("Content-Type", "text/plain") tells the browser how to read the body. end_headers() writes the blank line that separates headers from the body, and self.wfile.write(...) sends the body. The body must be bytes, which is why the string is written as b"...". If you skip end_headers() the browser sees a broken response.

HTTPServer(("127.0.0.1", 8000), Handler) binds to the local machine only, which is what you want for experiments. serve_forever() then loops, handling one request at a time. Stop it with Ctrl+C.

Reacting to the path

Inside do_GET, self.path holds what the client asked for. This example returns JSON for /api and a plain message otherwise:

import json
from http.server import BaseHTTPRequestHandler, HTTPServer
 
 
class Handler(BaseHTTPRequestHandler):
    def do_GET(self):
        if self.path == "/api":
            body = json.dumps({"status": "ok"}).encode("utf-8")
            content_type = "application/json"
            code = 200
        else:
            body = b"Not found"
            content_type = "text/plain"
            code = 404
        self.send_response(code)
        self.send_header("Content-Type", content_type)
        self.send_header("Content-Length", str(len(body)))
        self.end_headers()
        self.wfile.write(body)
 
 
if __name__ == "__main__":
    HTTPServer(("127.0.0.1", 8000), Handler).serve_forever()

Visit http://127.0.0.1:8000/api to see the JSON and any other path to see the 404. Sending Content-Length lets the client know when the body ends.

Common mistakes

• Writing a str to wfile. It needs bytes; use .encode("utf-8").
• Sending the body before the headers. Headers always come first and end with end_headers().
• Serving the wrong folder. python -m http.server shares the current directory, including files you did not mean to expose.
• Binding to 0.0.0.0 on a shared network and exposing it to others.
• Forgetting other methods. A handler with only do_GET answers a POST with an error.

When to use it

Use http.server for local previews of static sites, quick demos, and learning how HTTP works. For real applications, use a framework such as Flask or FastAPI behind a production server.

Check yourself

🧪 Try It Yourself

Exercise 1 – HTTP GET with urllib

Press Run to see what your code prints.

Exercise 2 – Socket Connection

Press Run to see what your code prints.

Exercise 3 – Resolve a Hostname

Press Run to see what your code prints.