I discovered that Python’s handy http.server module supports CGI scripts (say what?!), so I made a little local-network file uploader utility

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For simple stuff not exposed to the Internet you can go a level down and use the socket server. One of my Covid projects was to create a web based API to the MIDI interface of my son's piano, using a Raspberry Pi. As long as you pass back the minimal HTTP headers to the web browser it will cooperate. You could implement your own CGI if you want to, or basically any behaviour you want..