I'm a new Pico-8 user. I feel like this should have some crazy obvious answer right in front of my nose.

This function works fine if called in _draw(), but I cannot get a btnp to call it (or any other function) -

function test_swing() 
print("swing", 100, 100)
end

The btnp code in _update() is

if btnp(4) then
test_swing()
end

I would expect this to just flash up briefly at the 100,100 coordinate on the button press (z). Calling it
in _draw() prints it to the screen continuously as expected. Any ideas? Thanks