76 — Network: polling a server

Programs that watch a server keep asking it for the latest state. This is called polling. The loop is simple: fetch, show, wait, repeat — and stop when the server says it is done.

#Steps

1. Create status.json with a state that a worker will change:

   {"status": "working", "step": 1}
   

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2. Serve the folder, as in guide 55:

   python3 -m http.server 8000
   

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3. Fetching the status is the same urlopen call as guide 55:

   from urllib.request import urlopen
   from json import loads
   
   url = "http://localhost:8000/status.json"
   status = loads(urlopen(url).read().decode("utf-8"))
   

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4. The full program polls in a loop, reports each state, and stops when the server reports "done". Save poll.nme:

   # poll.nme — watch status.json until it says done.
   # Run: nme r poll
   # Serve this folder first: python3 -m http.server 8000
   # Change status.json to {"status": "done", "step": 2} while it runs.
   
   from urllib.request import urlopen
   from json import loads
   from time import sleep
   
   url = "http://localhost:8000/status.json"
   
   while True:
       status = loads(urlopen(url).read().decode("utf-8"))
       show f"step {status['step']}: {status['status']}"
       if status["status"] == "done":
           show "worker finished"
           break
       sleep(1)
   

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   Each loop fetches the current file, prints it, and waits one second. To see the change, edit status.json (the HTTP server re-reads the file on every request) and watch the program report the new step before stopping.

5. Run it, then edit status.json while it runs:

   nme r poll
   

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   step 1: working
   step 2: done
   worker finished
   

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#Try it yourself

Add a started timestamp to status.json and report how many seconds the worker ran, or poll two endpoints and report when either one finishes.

#What you learned

• Polling means fetching the latest state in a loop with a wait between tries.
• The HTTP server re-reads the file on every request, so edits appear live.
• A sentinel value like "done" tells the loop when to stop.
• time.sleep controls how often the program asks.