Custom large LED speedometer - need help counting rpm sensor pulses

I'm a newbie like yourself, but I did watch a video where someone mentioned you could measure RPM using a hall sensor, e.g.:

https://www.instructables.com/RPM-Measurement-Using-Hall-Sensor-and-Arduino/

Note that I don't know what a PNP sensor is.

A PNP is a type of magnetic proximity sensor.  I just need to know how to accurately count the GPIO incoming voltage pulses over say, a 1/2 second. 
I had a global variable called "pulses" in my interrupt code that I planned to increment every interrupt.  Unfortunately, I get the error: "local variable referenced before assignment" whenever the interrupt occurred.

If you can post some code, we might be able to help. If the variable is global and is being used in the interrupt function, then you have to declare it as global in the function:

my_var = 3

def my_func():
    global my_var;
    print(my_var)

Thanks YurkshireLad - I got it working using your suggestion.  Is there a cleaner way to do this?

from machine import Pin 
import utime

pulse_in = Pin(28, Pin.IN, Pin.PULL_DOWN) #set up sensor input
scan_time = 1 #secs between display updates

def add_count():
    global pulses
    pulses += 1
    print(pulses)

def int_handler(pin):
    pulse_in.irq(handler=None)
    print("input pulse occurred")
    add_count()
    pulse_in.irq(handler=int_handler)

pulse_in.irq(trigger=Pin.IRQ_RISING, handler=int_handler)

while True:
    pulses = 0
    utime.sleep(scan_time)
    print("total pulses: " + str(pulses))
    MPH = str(round(50 * pulses / 381 / scan_time, 1)) + " mph"
    print(MPH)
    break

Here's an image of the led assembly - the two large digits are 2.5" tall.  There will be a colored plastic "window" covering the leds - colors tbd.  I took one of the leds and ground a couple 45° flats on the end - this seemed to effectively disperse the light across the length of the segment pocket.  We'll see how well it works out once I get the parts printed.