> From: Raymond Hettinger <raymond.hettin...@gmail.com>
>
> On Jan 25, 2010, at 12:36 PM, Steve Howell wrote:
>
> >
> > Deque does not support all the operations that list
> does. It is also roughly twice as slow for accessing
> elements (I've measured it).
>
>
> ISTM that apps that want to insert or pop from the front of
> list are also apps that don't care about accessing arbitrary
> elements in the middle using the position index. When
> lists are growing or shrinking from the front, the meaning
> of the i-th element changes. So, it doesn't
> make sense for an application to track indices of objects in
> such a list.
>
> i = s.find('abc')
> s.pop(0)
> print s[i] # i no longer
> points at 'abc'
>
I am not going to directly address your point, but I'd like to give a examples
of code that uses pop(0) from the standard library.
If you look at the code for multiprocessing/connection.py, you will see that
PipeListener creates _handle_queue as an ordinary Python list, and in line 317
it uses pop(0) to pop the first handle off the top of the queue.
Why does that code not use a deque? In hindsight, it probably should. But to
make the change now, it's not a simple matter of fixing just PipeListener,
because PipeListener passes off _handle_queue to Finalize, which also expects a
list.
In order to understand why Finalize expects a list, you need to look at how it
uses args, and here is one example usage:
res = self._callback(*self._args, **self._kwargs)
Ok, so now you need to know what self._callback is doing, so now you have to
trace through all callers of Finalize are passing in for their args.
So what seems like a trivial matter--switching over a list to a deque--actually
requires a lot of thinking.
It turns out that the callback for PipeListener just iterates through the
remaining handles and closes them. So a deque would not break that code.
If you look at difflib.py, it also does pop(0) in a loop. Why doesn't it use a
deque? Simplicity, maybe?
codecs.py also deletes from the top of the list:
del self.linebuffer[0]
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