From Arduino C to Raspberry pi python - volatile variable access inside thread

Question

I was developing small embedded system over arduino, and it was counting events using interrupts. The code was like that:

 volatile float count= 0;
 attachInterrupt(0, increaseCount, CHANGE); 

 void increaseCount(){
 ++count;
 }

the count variable should be volatile to access it inside an interrupt. Now, I am writing it over Raspberry pi using python. But, python has nothing called volatile. So, is there another technique to increase variable during thread/event. python give me that error when the event happens and the variable increases.

  File "MyApp.py", line 5, in my_callback
  count += 1
  UnboundedLocalError: local variable 'count' referenced before assignment

Any help ??

unwind · Accepted Answer · 2014-11-18 08:20:35Z

3

No, there's no such thing as volatile in Python, it's a much too low-level concept.

You should just make sure there is some shared context (like an object instance) in which the variable can reside, so that it can be shared between the two contexts. Python will do the rest.

class MyApp(object):
  def __init__(self):
    self._counter = 0
    registerInterrupt(self.interruptHandler)

  def interruptHandler(self):
    self._counter += 1

  def getCount(self):
    return self._counter

Something like that could be enough, of course you'd have to fill in the details and make the required call(s) to create an instance of MyApp and make sure there's a function registerInterrupt() that does the needed work to set up the callback into the instance.

Also, do note that a counter of events should almost never be float in C, that's incredibly strange design.

edited Nov 18, 2014 at 8:20

answered Nov 18, 2014 at 7:57

unwind

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2 Comments

Mostafa Khattab Over a year ago

Thanks, i will try that solution, now

Mostafa Khattab Over a year ago

But, can u give me an example of shared context in python, i am a beginner

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