Python programming question

shadydentist

I have two thread classes, Parent and Child.

The Parent spawns an instance of Child, and needs to know when Child has finished running. Right now, Parent contains a condition variable, is there a way to pass that condition variable to Child?

What I've been trying is

class Child(Thread):
    def __init__(self, Condition c):
        Thread.__init__(self)
        self.monitor=c

    
    def run(self):
        for i in range(0, 5):
            print "hello from child", i

which doesn't work. Any help?

vonPoonBurGer

I'm far from a Python expert, but I've done something similar in other languages by forking the parent, and you can do that in Python.

shadydentist

vonPoonBurGer wrote: »

I'm far from a Python expert, but I've done something similar in other languages by forking the parent, and you can do that in Python.

Hm... that would work, but I don't think I'll be able to do that, because the assignment specifies separate Parent and Child classes.

Doc

shadydentist wrote: »

    def __init__(self, Condition c):

pretty sure that's not legal python.

    def __init__(self, c):

shadydentist

Yea, sorry, I'm a python newb and still used to Java notation.

Doc: I tried this, and it doesn't seem to work. Passing the condition from parent to child seems to create a new instance of the condition.

Lewisham

http://www.dalkescientific.com/writings/NBN/threads.html

This is the best resource on Python threading I've found. Communication between threads is hard. Do it exactly like he says, and it will work (I've done it).

Barrakketh

vonPoonBurGer wrote: »

I'm far from a Python expert, but I've done something similar in other languages by forking the parent, and you can do that in Python.

It depends on what you're trying to do. One of the performance-related problems in Python when you use threads is because of the GIL (global interpreter lock). Only one thread may use the interpreter at a time, and the main practical implication of this is that you can't take advantage of multiple cores. While that link mentions thread pools and queues that isn't a solution for that, and manually managing child processes is cumbersome.

In Python 2.6+ and 3.0+ the multiprocessing module was added which makes dealing with child processes more convenient and lets you do things like create process pools. If you're using the *_async methods of the pool you can specify a callback when the function returns.

OP: I highly recommend reading this for information on using threads in Python.

Doc

What you are asking to do is a pretty complicated task.

shadydentist

Yea, this doesn't seem to be working. I think I might just create a global Condition() variable instead.

How do you create static global variables in Python?

evilmrhenry

shadydentist wrote: »

Yea, this doesn't seem to be working. I think I might just create a global Condition() variable instead.

How do you create static global variables in Python?

#same file:
global variable1
def function():
    global variable1 #Yes, you define it twice. Skip this, and you can still read the variable, but you can't write.
    variable1=3

#different file
import file1
def function2():
    file1.variable1 = 2