Question

I followed a queue example from the bottom of the Python Queue page. I want to access some global variables within the worker function, however, some globals are accessible and some aren’t. In my simple example below, the ‘rows’ and ‘errors’ variables are accessible but then I get UnboundLocalError for count. If i put “global count” then it works. But I don’t understand why the other globals are accessible.

File "myfile.py", line 184, in dpn_worker
    count += 1
UnboundLocalError: local variable 'count' referenced before assignment

Here is the code example I used:

dpns = [1,2,3,4]
q = Queue.Queue()
rows = []
errors = []
count = 0

def dpn_worker():
    while True:
        item = q.get()
        rows.append(1)
        errors.append(1)
        count += 1
        q.task_done()

def main():
    for d in dpns:
        q.put(d)

    for i in range(NUM_WORKERS):
        t = threading.Thread(target=dpn_worker)
        t.daemon = True
        t.start()

    q.join()

Answer 1

In this case, since count is an integer: –

count += 1

is equivalent to

count = count + 1

So, you are trying to modify the global variable, but are actually creating a local one.. But on the RHS, you are using the local variable before initialization..

Modify your dpn_worker as: –

def dpn_worker():
    global count
    while True:
        item = q.get()
        rows.append(1)
        errors.append(1)
        count += 1
        q.task_done()