One serial communication - multiple read and write between Python script and Arduino?

It looks like once index == 19, it may or may not get reset to 0 depending upon later logic. If you enter Comp() a second time and index >= 19 then you'll never read the serial port again.

I think @Zeus is entirely right (and hence I upvoted that answer), but there are also other problems. To reiterate what @Zeus is saying.

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