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<H2><A NAME="SECTION0071210000000000000000"></A><A NAME="deque-recipes"></A>
<BR>
5.12.1 Recipes
</H2>

<P>
This section shows various approaches to working with deques.

<P>
The <tt class="method">rotate()</tt> method provides a way to implement <tt class="class">deque</tt>
slicing and deletion.  For example, a pure python implementation of
<code>del d[n]</code> relies on the <tt class="method">rotate()</tt> method to position
elements to be popped:

<P>
<div class="verbatim"><pre>
def delete_nth(d, n):
    d.rotate(-n)
    d.popleft()
    d.rotate(n)
</pre></div>

<P>
To implement <tt class="class">deque</tt> slicing, use a similar approach applying
<tt class="method">rotate()</tt> to bring a target element to the left side of the deque.
Remove old entries with <tt class="method">popleft()</tt>, add new entries with
<tt class="method">extend()</tt>, and then reverse the rotation.

<P>
With minor variations on that approach, it is easy to implement Forth style
stack manipulations such as <code>dup</code>, <code>drop</code>, <code>swap</code>, <code>over</code>,
<code>pick</code>, <code>rot</code>, and <code>roll</code>.

<P>
A roundrobin task server can be built from a <tt class="class">deque</tt> using
<tt class="method">popleft()</tt> to select the current task and <tt class="method">append()</tt>
to add it back to the tasklist if the input stream is not exhausted:

<P>
<div class="verbatim"><pre>
def roundrobin(*iterables):
    pending = deque(iter(i) for i in iterables)
    while pending:
        task = pending.popleft()
        try:
            yield task.next()
        except StopIteration:
            continue
        pending.append(task)

&gt;&gt;&gt; for value in roundrobin('abc', 'd', 'efgh'):
...     print value

a
d
e
b
f
c
g
h
</pre></div>

<P>
Multi-pass data reduction algorithms can be succinctly expressed and
efficiently coded by extracting elements with multiple calls to
<tt class="method">popleft()</tt>, applying the reduction function, and calling
<tt class="method">append()</tt> to add the result back to the queue.

<P>
For example, building a balanced binary tree of nested lists entails
reducing two adjacent nodes into one by grouping them in a list:

<P>
<div class="verbatim"><pre>
def maketree(iterable):
    d = deque(iterable)
    while len(d) &gt; 1:
        pair = [d.popleft(), d.popleft()]
        d.append(pair)
    return list(d)

&gt;&gt;&gt; print maketree('abcdefgh')
[[[['a', 'b'], ['c', 'd']], [['e', 'f'], ['g', 'h']]]]
</pre></div>

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<span class="release-info">Release 2.4.2, documentation updated on 28 September 2005.</span>
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