Python Threading inside a class

Question

I recently started with python’s threading module. After some trial and error I managed to get basic threading working using the following sample code given in most tutorials.

class SomeThread(threading.Thread):
    def __init__(self, count):
        threading.Thread.__init__(self)

    def run(self):
        print "Do something"

My problem is: I have a Class that has class variables and a function that I want to be run in a separate thread. However the function uses class variables and also writes to class variables. Like so:

class MyClass:
    somevar = 'someval'

    def func_to_be_threaded(self):
        # Uses other class functions
        # Do something with class variables

So how would I essentially ‘put the thread class in MyClass’. So that if MyClass().func_to_threaded() is called it would run in a thread.

Asked By: Niels

||

Source

Answer 1

You can pass class instance to the thread:

class SomeThread(threading.Thread):
    def __init__(self, count, instance):
        threading.Thread.__init__(self)
        self.instance = instance

    def run(self):
        print "Do something"
        self.instance.some_param = data
        self.instance.some_function()

Answered By: ndpu

Answer 2

I’m fairly certain that you can’t make a single function threaded.

The whole class will be threaded (sort of). When you instantiate the object, its __init__ will be called on another thread, and then when you call start() on that object, its run() will be called once, on another thread.

So, if you have a TASK that needs to be on its own thread (disc IO, socket listening, etc), then you need a class to handle that task.

@ndpu’s answer solves your scope/access problems.

Answered By: Matt

Answer 3

If I understand correctly you want to run a function in a separate thread? There are several ways to do that. But basically you wrap your function like this:

class MyClass:
    somevar = 'someval'

    def _func_to_be_threaded(self):
        # main body

    def func_to_be_threaded(self):
        threading.Thread(target=self._func_to_be_threaded).start()

It can be shortened with a decorator:

def threaded(fn):
    def wrapper(*args, **kwargs):
        threading.Thread(target=fn, args=args, kwargs=kwargs).start()
    return wrapper

class MyClass:
    somevar = 'someval'

    @threaded
    def func_to_be_threaded(self):
        # main body

Edit Updated version with a handle:

def threaded(fn):
    def wrapper(*args, **kwargs):
        thread = threading.Thread(target=fn, args=args, kwargs=kwargs)
        thread.start()
        return thread
    return wrapper

class MyClass:
    somevar = 'someval'

    @threaded
    def func_to_be_threaded(self):
        print 'xyz'

This can be used as follows:

>>> my_obj = MyClass()
>>> handle = my_obj.func_to_be_threaded()
>>> handle.join()

Now it is possible to extend it even more if you wish to return a value from the function. Consider this:

from threading import Thread
from concurrent.futures import Future

def call_with_future(fn, future, args, kwargs):
    try:
        result = fn(*args, **kwargs)
        future.set_result(result)
    except Exception as exc:
        future.set_exception(exc)

def threaded(fn):
    def wrapper(*args, **kwargs):
        future = Future()
        Thread(target=call_with_future, args=(fn, future, args, kwargs)).start()
        return future
    return wrapper


class MyClass:
    @threaded
    def get_my_value(self):
        return 1

>>> my_obj = MyClass()
>>> fut = my_obj.get_my_value()  # this will run in a separate thread
>>> fut.result()  # will block until result is computed
1

If you don’t have concurrent.futures.Future class (because for example you are using Python2.7 or older) then you can use this simplified implementation:

from threading import Event

class Future(object):
    def __init__(self):
        self._ev = Event()

    def set_result(self, result):
        self._result = result
        self._ev.set()

    def set_exception(self, exc):
        self._exc = exc
        self._ev.set()

    def result(self):
        self._ev.wait()
        if hasattr(self, '_exc'):
            raise self._exc
        return self._result

I advice reading through concurrent.futures module since it has a lot of neat tools. For example Thread class should be replaced with a ThreadPoolExecutor instance to limit concurrency (e.g. you don’t want to spam 10k threads). Also with ThreadPoolExecutor the code is even simplier (and less error prone):

from concurrent.futures import ThreadPoolExecutor

tp = ThreadPoolExecutor(10)  # max 10 threads

def threaded(fn):
    def wrapper(*args, **kwargs):
        return tp.submit(fn, *args, **kwargs)  # returns Future object
    return wrapper

Just remember you have to tp.shutdown() after you’re done with all parallel work.

Answered By: freakish

Python Threading inside a class

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