how to modify a variable that is passed as parameter

[StefanL38]

Hi everyone,

I have quite some experience with Delphi, Parallax SPIN and Arduino C++

NOw I started to use CircuitPython with a Teensy 4.0-microcontrollerBoard.
The following question is about programming in python in general

On micorcontrollers non-blocking timing is essential. Do multiple things
at the (almost) same time.

From C++ you can pass variables by pointer to be able to modify the variables value
that is used to pass the parameter

So in c++ it would look like this

boolean TimePeriodIsOver (unsigned long &expireTime, unsigned long TimePeriod) {
  unsigned long currentMillis  = millis();
  if ( currentMillis - expireTime >= TimePeriod )
  {
    expireTime = currentMillis; // set new expireTime
    return true;                // more time than TimePeriod) has elapsed since last time if-condition was true
  } 
  else return false;            // not expired
}

the parameter expiretime is passed by pointer so the modifying of the value is stored
inside of the variable that is used by the functioncall

example

unsigned long MyTimer 

if ( TimePeriodIsOver (MyTimer, 500) )

With this call the variable MyTimer gets updated automatically
what makes the call very compact

I tried to realise this in python I have this working solution

import board
import digitalio
import time

def TimePeriodIsOver(p_TimerVar, Period):
    now = time.monotonic()
    Diff = now - p_TimerVar
    if Diff >= (Period / 1000):
       p_TimerVar = now + Period / 1000
       return True , p_TimerVar
       
    else:        
       return False, p_TimerVar


led = digitalio.DigitalInOut(board.D13)

led.direction = digitalio.Direction.OUTPUT
duration = 0.1

print('Hello 4 World!')
now = time.monotonic()
now_ns = time.monotonic_ns()
print ('Now:',now, '  nanosec:', now_ns)

TimerVar = time.monotonic()
print('before while true TimerVar',TimerVar)

while True:
    #print('TimerVar',TimerVar)
    #time.sleep(0.25)
    TPOver, TimerVar, = TimePeriodIsOver(TimerVar, 500)
    if TPOver == True:
      if led.value == True:
        led.value = False

      else:
          led.value = True

But this is not as compact as the c++-Version
So can it be done more compact without the need of another variable like TPOver?

best regards Stefan

[deanhystad]

Numbers are immutable, so you cannot change it. That is why you cannot pass a number to a function and have the function change the number, You can only make a new number and return. If you want to change a value passed as a function argument the value must be mutable. If you put your expireTime in a list and passed the list, the function could change the value and the change would be seen outside the function.

def TimePeriodIsOver (expireTime, TimePeriod):
    currentMillis  = millis()
    if currentMillis - expireTime[0] >= TimePeriod:
        expireTime[0] = currentMillis
        return True
    return False

expireTime = [millis()]
while not TimePeriodIsOver(expireTime, 5.0):
    """do something"""

But there are other mutable types that are better suited. How about a class? If this timer thing is something you use a lot, why not make it a class?

import time

class Timer():
    def __init__(self, period=0):
        self.period = period
        self.start_time = None

    def start(self):
        self.start_time = time.time()

    def period_over(self):
        new_time = time.time()
        if self.start_time is None:
            self.start_time = new_time
        if new_time - self.start_time > self.period:
            self.start_time = new_time
            return True
        return False

timer = Timer(5)
while not timer.period_over():
    print('waiting')
    time.sleep(1)

There are packages in Python designed to periodically execute code. I don't know what is available for your target.

[StefanL38]

Hi deanhystad,

thank you very much for answering and showing how to code it as a class.
Still I have a question:

Do I understand right
in your demo-code
timer = Timer(5) creates an "instance" of class Timer and the period after which
the "method" .period_over returns true is 5 seconds
and if result is true the timer-"object" gets updated to return false until another 5 seconds are over?

So this could be used in an infinite loop for repeated "delayed" execution of code?

best regards Stefan