The built-in calc sucks: when the keyboard is displayed to edit the current expression, the app can only display ONE previous expression at a time. Sometimes I calculate things and need to refer to 2 or more previous results. I would have to copy paste them one at a time (cumbersome) or memorize them.

If only it could display about a dozen previous results while the keyboard is active... With a Python REPL I can just assign intermediate results to variables.