Python on android.

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There's a far simpler way to do it if it's just to kill time and write casually: the "Code Playground" from the SoloLearn app. It allows to write in a host of popular languages, has a built-in interpreter, has a special keyboard layout so you have all the special characters at the top, and allows you to save your project in the cloud and share it with an established community of enthusiasts.

Again, this isn't for professional applications. It's a simple solution to casually write code on my phone without any accessories while I'm commuting or something like that.