Why queue.empty() returns False even after put() is called?

[Cameron Simpson]

On 23Nov2012 11:53, Ian Kelly <ian.g.kelly at gmail.com> wrote:
| On Fri, Nov 23, 2012 at 9:57 AM, Peng Yu <pengyu.ut at gmail.com> wrote:
| > The empty() returns True even after put() has been called. Why it is
| > empty when there some items in it? Could anybody help me understand
| > it? Thanks!
| >
| > ~/linux/test/python/man/library/multiprocessing/Queue/empty$ cat
| > main.py
| > #!/usr/bin/env python
| >
| > import multiprocessing
| >
| > queue = multiprocessing.Queue()
| > print queue.empty()
| > queue.put(['a', 'b'])
| > queue.put(['c', 'd'])
| > print queue.empty()
| 
| According to the docs, the Queue uses a background thread to load data into it:
| 
|     When a process first puts an item on the queue a feeder thread is
|     started which transfers objects from a buffer into the pipe.
| 
| Most likely it still appears to be empty because this thread has not
| had a chance to run yet.  If you try inserting a time.sleep() call,
| you should see the queue become non-empty once the background thread
| has run.

Conversely, might it not appear empty because the objects have been
thrown at the pipe already, appearing to have been consumed? Or is there
end-to-end handshaking controlling what .empty() tests? (Though again,
the far end may have grabbed them already too.)
-- 
Cameron Simpson <cs at zip.com.au>

The ZZR-1100 is not the bike for me, but the day they invent "nerf" roads
and ban radars I'll be the first in line......AMCN