Recursive thread creation in python

Question

I'm trying to implement a recursive Fibonacci series which returns the value at an index. It's a homework and needs to be done using multi-threading. This is what I've done so far. My question is how do I add the results from live_thread1 and live_thread2. The threads have to be created at every level in the recursion.

def Recursive(n):
    if n< 2:
        return n
    else:
        return Recursive(n- 1) + Recursive(n- 2)



def FibonacciThreads(n):
    if n< 2:
        return n
    else:
        thread1        = threading.Thread(target=FibonacciThreads,args=(n-1,))
        thread2        = threading.Thread(target=FibonacciThreads,args=(n-2,))
        thread1.start()
        thread2.start()
        thread1.join()
        thread2.join()
        return live_thread1+live_thread2

Answer 1

You can pass a mutable object to the thread to use for storing the result. If you don't want to introduce a new data type, you can for example just use a single element list:

def fib(n, r):
    if n < 2:
        r[0] = n
    else:
        r1 = [None]
        r2 = [None]
        # Start fib() threads that use r1 and r2 for results.
        ...

        # Sum the results of the threads.
        r[0] = r1[0] + r2[0]

def FibonacciThreads(n):
    r = [None]
    fib(n, r)
    return r[0]

Answer 2

This is not possible, because you cannot retrieve the return-value of a function executed in another thread.

To implement the behavior you want, you have to make FibonacciThreads a callable object that stores the result as a member variable:

class FibonacciThreads(object):
    def __init__(self):
        self.result = None

    def __call__(self, n):
        # implement logic here as above 
        # instead of a return, store the result in self.result

You can use instances of this class like functions:

fib = FibonacciThreads() # create instance
fib(23) # calculate the number
print fib.result # retrieve the result

Note that, as I said in my comment, this is not a very smart use of threads. If this really is your assignment, it is a bad one.