Question

This is a little complicated…

Basically, I have this function:

def do_loop(self):
    for line in self.connections[0].iter_lines():
            print line

And, there is a thread constantly running alongside it, which will at arbitrary times change the value of connections[0].

If this happens, if connections[0] is externally changed by the thread, the loop will keep using the old connections[0], I need this to not happen, I need it to immediately use the new connections[0] instead.

For some background, for line in self.connections[0].iter_lines(): is reading data from the Twitter Streaming API using python-requests, hence .iter_lines().

Any ideas? Thanks in advance.

Answer 1

The for loop is going to make an iterator, once, and it won’t keep checking self.connections[0] in the loop. So as @mklauber said, use something like threading.Event.

Assuming that we have a threading.Event instance as self.new_conn and it gets set whenever there is a new connection:

def do_loop(self):
    for line in self.connections[0].iter_lines():
        if self.new_conn.is_set():
            break # or could raise exception here
        print line

If you just need the loop to instantly terminate, you could handle it by making your .iter_lines() method a generator, and making the generator do the check:

def iter_lines(self):
    for line in self.connections[0].private_data_lines:
        if self.new_conn.is_set():
            break
        yield line

This nicely encapsulates the loop. Your original code would work as shown, and it would stop when there was a new connection.