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

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  Why queue.empty() returns False even after put() is called? Peng Yu <pengyu.ut@gmail.com> - 2012-11-23 08:57 -0800
    Re: Why queue.empty() returns False even after put() is called? MRAB <python@mrabarnett.plus.com> - 2012-11-23 18:53 +0000
    Re: Why queue.empty() returns False even after put() is called? Ian Kelly <ian.g.kelly@gmail.com> - 2012-11-23 11:53 -0700

#33854 — Why queue.empty() returns False even after put() is called?

Hi,

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()


Regards,
Peng

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#33862

On 2012-11-23 16:57, Peng Yu wrote:
> Hi,
>
> 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()
>
It works correctly for me.

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#33863

On Fri, Nov 23, 2012 at 9:57 AM, Peng Yu <pengyu.ut@gmail.com> wrote:
> Hi,
>
> 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.

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