Item 39, Page 134: ClosableQueue close() method

[dgolden1]

This isn't an error in your book but more of a gotcha; you define the ClosableQueue class as a subclass for Queue.Queue (I'm using Python 2.7 here); ClosableQueue implements a close() method to inform queue consumers that no more items will be added to the queue. This is totally fine with Queue.Queue, but if you want to take it to the next level with multiprocessing.Queue, you encounter an unexpected problem because multiprocessing.Queue already implements a close() method that does something different:

Indicate that no more data will be put on this queue by the current process. The background thread will quit once it has flushed all buffered data to the pipe. This is called automatically when the queue is garbage collected.

As an unwitting user, I subclassed ClosableQueue from multiprocessing.Queue, which resulted in the base class close() method being overwritten -- not what I wanted!

You may want to add that caveat to the book or consider changing the ClosableQueue name to something like FinalizableQueue.

P.S.
Loved the book!

[bslatkin]

Thanks for the report. Seems like a good suggestion. Let me reproduce the problem and then I'll reply on this thread. Happy to hear you liked the book!

[bslatkin]

I've thought about this. I'd like to keep this simple. I think most users shouldn't touch multiprocessing.Queue, they should use a ProcessPoolExecutor instead. I'm going to leave this as it is in the book.