trigger event when variable equals x - python

Question

I have the following script which increases the counter every time a button is pressed. When the counter reaches a certain number i.e. 10 lets say I want an event to trigger.

from RPi import GPIO
from time import sleep

clk = 25
dt = 8

GPIO.setmode(GPIO.BCM)
GPIO.setup(clk, GPIO.IN, pull_up_down=GPIO.PUD_DOWN)
GPIO.setup(dt, GPIO.IN, pull_up_down=GPIO.PUD_DOWN)

counter = 0
clkLastState = GPIO.input(clk)

try:
        while True:
                clkState = GPIO.input(clk)
                dtState = GPIO.input(dt)
                if clkState != clkLastState:
                        if dtState != clkState:
                                counter += 1
                        else:
                                counter -= 1
                        print counter
                clkLastState = clkState
                sleep(0.01)
finally:
                GPIO.cleanup()

For the purposes of an example script it might be easiest just to get it to print something on reaching the required number i.e. 'target reached'.

This question relates to an earlier post of mine here - rotary encoder script for raspberry pi using python. Rather than add to or amend that question I thought it better to break down the problem for the purposes of understanding the various components.

Many thanks

Answer 1

If you wanted to perform a (conditional) action on assignment to a value, you'd want it to be an object attribute for which you can hijack the assignment to. Overload __setattr__ method or use a property. For instance:

class C:
    def __init__(self, trigger, init_val=0):
        self._v = init_val
        self.trigger = trigger

    @property
    def v(self):
        return self._v

    @v.setter
    def v(self, value):
        if value == self.trigger:
            print("Trigger {} hit, perform some action.".format(value))
        else:
            print("Nothing to do for {}".format(value))
        self._v = value

c = C(10)
while c.v < 12:  # Go couple turns past the "event" to see its effect
    c.v += 1

I suspect what you really are after though in these two questions is to run both loops so that you can keep counting switch events and to control spinning of the motor until latter is to stop after reaching a threshold in the former. There are more ways of achieving this, for instance threads come to mind. In the following example I've let the counter run in a separate thread and have the motor loop check how far did the counter progress:

from threading import Thread
from time import sleep

class Counter(Thread):
    def __init__(self, limit):
        self.value = 0
        self.limit = limit
        super().__init__()

    def run(self):
        while self.value < self.limit:
            # We'd be acquiring and accumulating actual values here         
            sleep(1)
            self.value += 1
            print("Counter now at {}".format(self.value))

counter = Counter(10)
counter.start()
while counter.value < counter.limit:
    print("Spinning motor")
    sleep(0.5)  # do actual work here
print("Stop motor")

You can combine both examples as well as run also/or motor control in its own thread. However, if both polling switch and spinning motor happen at the same frequency, it would be much simpler just to keep count and compare its value against limit within single loop. In case the motor gets started by one action and needs to be stop by another action when the limit has been reached, Just start, loop over the switch retrieval and stop it once that loop finishes.