Question

I am writing a piece of code where I have one class A and two threads B and C.

I create an instance a of A. I then start both threads, first B then C.

B calls a function func_name in A by a.func_name(). So far so fine.

C on the other hand needs to access the result which is a list, say list_a defined inside func_name() in class A and accessed by instance a.

I have to match a set of string by using a for loop like this,

if self.string_variable in a.list_a:
    print "found"

but it gives me an error:

A instance has no attribute list_a

Can some one please help me?

Answer 1

You will need some kind of synchronization primitive – exactly which one depends on further details of your design and requirements.

Assuming the list a.list_b is to be created once and is not modified later, thread C needs to wait until a.func_name() returns. This can be achieved by adding a threading.Event instance to A. In A.__init__(), add

self.event = threading.Event()

At the end of A.func_name(), add

self.event.set()

Before thread C tries to access a.list_b, add

a.event.wait()

to wait until a.func_name() has finished in thread B.

In general, synchronization between threads is a complex topic and an error-prone task. You should only do this if you really need to.