I wanted to use processing to program in python, but I can’t change the mode to python. I’ve got a message box saying: 
. I don’t know what happened. Can somebody help me?
Don’t worry this message box. It’s not on your computer.
Hello @wyqsuy, and welcome to the Processing Foundation Forum!
Did you have a program open that contains code from Java Mode or P5.js Mode? That could be the reason you saw that message box when you attempted to change the mode.
Go to the File menu and open a New file. Then change to Python Mode before writing any code. After that you will also have the option of opening a program that was previously written in Python Mode.
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Thank you it worked 
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Just a thought, I also was confused by this dialog box time to time - can we make a pull request to make it a bit more friendly??
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Yes, indeed, the message should be made more friendly. It was probably intended to help the user avoid inadvertently attempting to execute an open file in an inappropriate mode. One possible solution would be to invite the user to save and close any open files from the previous mode before the change to the requested mode is completed.
Edited on March 23, 2021 to add details to the proposed remedy.
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thanks, I opened an issue here but not sure if I should open one in processing(3) repo as well
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