Can I connect an output pin to an input pin to measure it's voltage on a single board? So just bought an arduino Uno and I've been playing around with it. I want to connect one of the input pins to one of the PWM outputs and measure the output from there by sending it to my computer through Serial and then plot it in python.

Sorry if this is a basic question.
Thanks
PWM are digital output pins, and it seems that you wish to measure analog equivalent voltage generated by PWM output. If so, you'd need a filter.

Construct a unity gain op-amp based RC filter with cut-off frequency significantly smaller than PWM clock frequency. This will generate a analog waveform that can drive analog circuits. You could connect this to ADC input pin.

You could Google "Active low pass filter" for a lot of schematics and formula.

BUT

To be able to faithfully reproduce analog signal captured through ADC on a computer screen graph, your ADC sample rate must be more than ~2.5 times analog signal's bandwidth.

If your sample rate is close to Nyquist rates,  you'd need to filter (interpolate) your digitized signal in software to generate a smooth graph. This is a bit involved in maths and software. Otherwise you'd only get a "spikey" waveform.

Or preferably have a sample rate > 10 times analog bandwidth. This will give large enough sample counts to reliably show analog data without much processing.