55 — Network: a mini chat

A chat client reads messages from a server and sends new ones. This guide fetches a JSON message list from a local server, prints it like a chat log, and shows exactly what a new reply would look like. A plain static server can serve the messages; the reply step explains why sending needs a server that accepts it.

#Steps

1. Create messages.json with a short chat log:

   [
     {"who": "Mina", "text": "hello"},
     {"who": "Jun", "text": "hi Mina"},
     {"who": "Mina", "text": "ready to code?"}
   ]
   

   copyrun it →

2. Serve the folder with Python's built-in HTTP server, as in guide 32:

   python3 -m http.server 8000
   

   copy

3. The program fetches the list and prints each message:

   from urllib.request import urlopen
   from json import loads
   
   url = "http://localhost:8000/messages.json"
   messages = loads(urlopen(url).read().decode("utf-8"))
   
   for message in messages:
       show f"{message['who']}: {message['text']}"
   

   copyrun it →

   Each dict holds a who and a text; message["who"] reads one field. Reading a page is a GET — any web server can answer it.

4. A reply is the same shape: a dict that would be added to the list. The full program builds the reply and prints exactly what it would send:

   # chat.nme — fetch a chat log, then show the reply you would post.
   # Run: nme r chat
   # Serve this folder first: python3 -m http.server 8000
   
   from urllib.request import urlopen
   from json import loads, dumps
   
   url = "http://localhost:8000/messages.json"
   messages = loads(urlopen(url).read().decode("utf-8"))
   
   show "chat log:"
   for message in messages:
       show f"  {message['who']}: {message['text']}"
   
   reply = {"who": "you", "text": "nice to meet you"}
   payload = dumps(reply)
   show ""
   show "your reply would be POSTed as:"
   show payload
   

   copyrun it →

   Run it with the server running:

   nme r chat
   

   copy

   chat log:
     Mina: hello
     Jun: hi Mina
     Mina: ready to code?
   your reply would be POSTed as:
   {"who": "you", "text": "nice to meet you"}
   

   copy

5. Sending the reply needs urllib.request.Request(url, data=..., method="POST"). A plain http.server does not accept POST, so the guide prints the payload instead of pretending to send it. A real chat server would accept the POST and append the message — the data format here is exactly that.

#Try it yourself

Add a second message to messages.json and run the chat again — the log grows. Change the reply's text and watch the payload change.

#What you learned

• urlopen(url).read().decode("utf-8") fetches a page as text.
• loads(...) turns JSON text into a Python list of dicts.
• message["who"] reads a field from one dict.
• A reply is a dict; sending it as a POST needs a server that accepts it.