Input Optical Isolation -- Yes I'm a little new..

I was reading the Cpu doc's :~ (Not sure if that's a good thing)

The Atmel328 document says that If the I/O pin is configured as input and internally pulled up, it will source current if externally pulled low.

So what I have drawn in schematic is a short, when I close the switch?

If that's true, is there another way to do the circuit without adding a load resistor, or external pull ups?

Just looking for measurable input signals.

Rich