How do I invoke a non-interactive lisp function interactively?

Question

This is dim, but I've forgotten how to do this, and I can't work it out, sorry:

I'm in python mode with elpy, and I want to run the function python-nav-beginning-of-defun in a python buffer, to see what it does

M-x python-nav-beginning-of-defun 

won't work, because the function is not a command (it has no interactive spec).

And putting (python-nav-beginning-of-defun) in a buffer and using C-x C-e will disturb the buffer and move the point.

I seem to remember that there's some way that I can type (python-nav-beginning-of-defun) in a mini-buffer somewhere, and see what it does to the python buffer, without disturbing the buffer.

Can anyone tell me what it's called / what the magic keys are?

Answer 1

You're looking for M-: which runs the command eval-expression

Answer 2

You can execute Lisp code with the command eval-expression, bound to M-: by default (i.e. Alt+: or Esc :).

At the prompt, type the code to evaluate. To call a function, put the function name in parentheses.

M-: (python-nav-beginning-of-defun) RET

If there are arguments, put them after the function name, within the parentheses.

M-: (+ 2 2) RET

The return value of the function is displayed in the echo area. (In your case it isn't interesting, I mention that for the general case.) If it doesn't fit, or you want to see it afterwards or to copy-paste it, switch to the *Messages* buffer. If you'd rather assign the result to a variable, use the setq function:

M-: (setq x (+ 2 2)) RET

If the code triggers an error and a debugger window pops up, press q in the debugger window. If the code prompts for something and you want to cancel, press ESC three times (keyboard-escape-quit). If the code goes into a tight loop, break with C-g as usual.