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 <section id="module-collections">
<h1>8.3. <a class="reference internal" href="#module-collections" title="collections: High-performance datatypes"><code class="xref py py-mod docutils literal notranslate">collections</code></a> — High-performance container datatypes<a class="headerlink" href="#module-collections" title="Permalink to this headline">¶</a></h1>
<div class="versionadded">
New in version 2.4.
</div>
Source code: <a class="reference external" href="https://github.com/python/cpython/tree/2.7/Lib/collections.py">Lib/collections.py</a> and <a class="reference external" href="https://github.com/python/cpython/tree/2.7/Lib/_abcoll.py">Lib/_abcoll.py</a>
<hr class="docutils" />
This module implements specialized container datatypes providing alternatives to
Python’s general purpose built-in containers, <a class="reference internal" href="stdtypes.html#dict" title="dict"><code class="xref py py-class docutils literal notranslate">dict</code></a>, <code class="xref py py-class docutils literal notranslate">list</code>,
<a class="reference internal" href="stdtypes.html#set" title="set"><code class="xref py py-class docutils literal notranslate">set</code></a>, and <a class="reference internal" href="functions.html#tuple" title="tuple"><code class="xref py py-class docutils literal notranslate">tuple</code></a>.
<table class="docutils align-default">
<colgroup>
<col style="width: 18%" />
<col style="width: 59%" />
<col style="width: 23%" />
</colgroup>
<tbody>
<tr class="row-odd"><td><a class="reference internal" href="#collections.namedtuple" title="collections.namedtuple"><code class="xref py py-func docutils literal notranslate">namedtuple()</code></a></td>
<td>factory function for creating tuple subclasses with named fields</td>
<td><div class="versionadded">
New in version 2.6.
</div>
</td>
</tr>
<tr class="row-even"><td><a class="reference internal" href="#collections.deque" title="collections.deque"><code class="xref py py-class docutils literal notranslate">deque</code></a></td>
<td>list-like container with fast appends and pops on either end</td>
<td><div class="versionadded">
New in version 2.4.
</div>
</td>
</tr>
<tr class="row-odd"><td><a class="reference internal" href="#collections.Counter" title="collections.Counter"><code class="xref py py-class docutils literal notranslate">Counter</code></a></td>
<td>dict subclass for counting hashable objects</td>
<td><div class="versionadded">
New in version 2.7.
</div>
</td>
</tr>
<tr class="row-even"><td><a class="reference internal" href="#collections.OrderedDict" title="collections.OrderedDict"><code class="xref py py-class docutils literal notranslate">OrderedDict</code></a></td>
<td>dict subclass that remembers the order entries were added</td>
<td><div class="versionadded">
New in version 2.7.
</div>
</td>
</tr>
<tr class="row-odd"><td><a class="reference internal" href="#collections.defaultdict" title="collections.defaultdict"><code class="xref py py-class docutils literal notranslate">defaultdict</code></a></td>
<td>dict subclass that calls a factory function to supply missing values</td>
<td><div class="versionadded">
New in version 2.5.
</div>
</td>
</tr>
</tbody>
</table>
In addition to the concrete container classes, the collections module provides
<a class="reference internal" href="#collections-abstract-base-classes">abstract base classes</a> that can be
used to test whether a class provides a particular interface, for example,
whether it is hashable or a mapping.
<section id="counter-objects">
<h2>8.3.1. <a class="reference internal" href="#collections.Counter" title="collections.Counter"><code class="xref py py-class docutils literal notranslate">Counter</code></a> objects<a class="headerlink" href="#counter-objects" title="Permalink to this headline">¶</a></h2>
A counter tool is provided to support convenient and rapid tallies.
For example:
<div class="highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; # Tally occurrences of words in a list
&gt;&gt;&gt; cnt = Counter()
&gt;&gt;&gt; for word in [&#39;red&#39;, &#39;blue&#39;, &#39;red&#39;, &#39;green&#39;, &#39;blue&#39;, &#39;blue&#39;]:
... cnt[word] += 1
&gt;&gt;&gt; cnt
Counter({&#39;blue&#39;: 3, &#39;red&#39;: 2, &#39;green&#39;: 1})

&gt;&gt;&gt; # Find the ten most common words in Hamlet
&gt;&gt;&gt; import re
&gt;&gt;&gt; words = re.findall(r&#39;\w+&#39;, open(&#39;hamlet.txt&#39;).read().lower())
&gt;&gt;&gt; Counter(words).most_common(10)
[(&#39;the&#39;, 1143), (&#39;and&#39;, 966), (&#39;to&#39;, 762), (&#39;of&#39;, 669), (&#39;i&#39;, 631),
 (&#39;you&#39;, 554), (&#39;a&#39;, 546), (&#39;my&#39;, 514), (&#39;hamlet&#39;, 471), (&#39;in&#39;, 451)]
</pre></div>
</div>
<dl class="py class">
<dt class="sig sig-object py" id="collections.Counter">
class collections.Counter([iterable-or-mapping])<a class="headerlink" href="#collections.Counter" title="Permalink to this definition">¶</a></dt>
<dd>A <a class="reference internal" href="#collections.Counter" title="collections.Counter"><code class="xref py py-class docutils literal notranslate">Counter</code></a> is a <a class="reference internal" href="stdtypes.html#dict" title="dict"><code class="xref py py-class docutils literal notranslate">dict</code></a> subclass for counting hashable objects.
It is an unordered collection where elements are stored as dictionary keys
and their counts are stored as dictionary values. Counts are allowed to be
any integer value including zero or negative counts. The <a class="reference internal" href="#collections.Counter" title="collections.Counter"><code class="xref py py-class docutils literal notranslate">Counter</code></a>
class is similar to bags or multisets in other languages.
Elements are counted from an iterable or initialized from another
mapping (or counter):
<div class="doctest highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; c = Counter() # a new, empty counter
&gt;&gt;&gt; c = Counter(&#39;gallahad&#39;) # a new counter from an iterable
&gt;&gt;&gt; c = Counter({&#39;red&#39;: 4, &#39;blue&#39;: 2}) # a new counter from a mapping
&gt;&gt;&gt; c = Counter(cats=4, dogs=8) # a new counter from keyword args
</pre></div>
</div>
Counter objects have a dictionary interface except that they return a zero
count for missing items instead of raising a <code class="xref py py-exc docutils literal notranslate">KeyError</code>:
<div class="doctest highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; c = Counter([&#39;eggs&#39;, &#39;ham&#39;])
&gt;&gt;&gt; c[&#39;bacon&#39;] # count of a missing element is zero
0
</pre></div>
</div>
Setting a count to zero does not remove an element from a counter.
Use <code class="docutils literal notranslate">del</code> to remove it entirely:
<div class="doctest highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; c[&#39;sausage&#39;] = 0 # counter entry with a zero count
&gt;&gt;&gt; del c[&#39;sausage&#39;] # del actually removes the entry
</pre></div>
</div>
<div class="versionadded">
New in version 2.7.
</div>
Counter objects support three methods beyond those available for all
dictionaries:
<dl class="py method">
<dt class="sig sig-object py" id="collections.Counter.elements">
elements()<a class="headerlink" href="#collections.Counter.elements" title="Permalink to this definition">¶</a></dt>
<dd>Return an iterator over elements repeating each as many times as its
count. Elements are returned in arbitrary order. If an element’s count
is less than one, <a class="reference internal" href="#collections.Counter.elements" title="collections.Counter.elements"><code class="xref py py-meth docutils literal notranslate">elements()</code></a> will ignore it.
<div class="doctest highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; c = Counter(a=4, b=2, c=0, d=-2)
&gt;&gt;&gt; list(c.elements())
[&#39;a&#39;, &#39;a&#39;, &#39;a&#39;, &#39;a&#39;, &#39;b&#39;, &#39;b&#39;]
</pre></div>
</div>
</dd></dl>

<dl class="py method">
<dt class="sig sig-object py" id="collections.Counter.most_common">
most_common([n])<a class="headerlink" href="#collections.Counter.most_common" title="Permalink to this definition">¶</a></dt>
<dd>Return a list of the n most common elements and their counts from the
most common to the least. If n is omitted or <code class="docutils literal notranslate">None</code>,
<a class="reference internal" href="#collections.Counter.most_common" title="collections.Counter.most_common"><code class="xref py py-func docutils literal notranslate">most_common()</code></a> returns all elements in the counter.
Elements with equal counts are ordered arbitrarily:
<div class="doctest highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; Counter(&#39;abracadabra&#39;).most_common(3)
[(&#39;a&#39;, 5), (&#39;r&#39;, 2), (&#39;b&#39;, 2)]
</pre></div>
</div>
</dd></dl>

<dl class="py method">
<dt class="sig sig-object py" id="collections.Counter.subtract">
subtract([iterable-or-mapping])<a class="headerlink" href="#collections.Counter.subtract" title="Permalink to this definition">¶</a></dt>
<dd>Elements are subtracted from an iterable or from another mapping
(or counter). Like <a class="reference internal" href="stdtypes.html#dict.update" title="dict.update"><code class="xref py py-meth docutils literal notranslate">dict.update()</code></a> but subtracts counts instead
of replacing them. Both inputs and outputs may be zero or negative.
<div class="doctest highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; c = Counter(a=4, b=2, c=0, d=-2)
&gt;&gt;&gt; d = Counter(a=1, b=2, c=3, d=4)
&gt;&gt;&gt; c.subtract(d)
&gt;&gt;&gt; c
Counter({&#39;a&#39;: 3, &#39;b&#39;: 0, &#39;c&#39;: -3, &#39;d&#39;: -6})
</pre></div>
</div>
</dd></dl>

The usual dictionary methods are available for <a class="reference internal" href="#collections.Counter" title="collections.Counter"><code class="xref py py-class docutils literal notranslate">Counter</code></a> objects
except for two which work differently for counters.
<dl class="py method">
<dt class="sig sig-object py" id="collections.Counter.fromkeys">
fromkeys(iterable)<a class="headerlink" href="#collections.Counter.fromkeys" title="Permalink to this definition">¶</a></dt>
<dd>This class method is not implemented for <a class="reference internal" href="#collections.Counter" title="collections.Counter"><code class="xref py py-class docutils literal notranslate">Counter</code></a> objects.
</dd></dl>

<dl class="py method">
<dt class="sig sig-object py" id="collections.Counter.update">
update([iterable-or-mapping])<a class="headerlink" href="#collections.Counter.update" title="Permalink to this definition">¶</a></dt>
<dd>Elements are counted from an iterable or added-in from another
mapping (or counter). Like <a class="reference internal" href="stdtypes.html#dict.update" title="dict.update"><code class="xref py py-meth docutils literal notranslate">dict.update()</code></a> but adds counts
instead of replacing them. Also, the iterable is expected to be a
sequence of elements, not a sequence of <code class="docutils literal notranslate">(key, value)</code> pairs.
</dd></dl>

</dd></dl>

Common patterns for working with <a class="reference internal" href="#collections.Counter" title="collections.Counter"><code class="xref py py-class docutils literal notranslate">Counter</code></a> objects:
<div class="highlight-default notranslate"><div class="highlight"><pre>sum(c.values()) # total of all counts
c.clear() # reset all counts
list(c) # list unique elements
set(c) # convert to a set
dict(c) # convert to a regular dictionary
c.items() # convert to a list of (elem, cnt) pairs
Counter(dict(list_of_pairs)) # convert from a list of (elem, cnt) pairs
c.most_common()[:-n-1:-1] # n least common elements
c += Counter() # remove zero and negative counts
</pre></div>
</div>
Several mathematical operations are provided for combining <a class="reference internal" href="#collections.Counter" title="collections.Counter"><code class="xref py py-class docutils literal notranslate">Counter</code></a>
objects to produce multisets (counters that have counts greater than zero).
Addition and subtraction combine counters by adding or subtracting the counts
of corresponding elements. Intersection and union return the minimum and
maximum of corresponding counts. Each operation can accept inputs with signed
counts, but the output will exclude results with counts of zero or less.
<div class="doctest highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; c = Counter(a=3, b=1)
&gt;&gt;&gt; d = Counter(a=1, b=2)
&gt;&gt;&gt; c + d # add two counters together: c[x] + d[x]
Counter({&#39;a&#39;: 4, &#39;b&#39;: 3})
&gt;&gt;&gt; c - d # subtract (keeping only positive counts)
Counter({&#39;a&#39;: 2})
&gt;&gt;&gt; c &amp; d # intersection: min(c[x], d[x])
Counter({&#39;a&#39;: 1, &#39;b&#39;: 1})
&gt;&gt;&gt; c | d # union: max(c[x], d[x])
Counter({&#39;a&#39;: 3, &#39;b&#39;: 2})
</pre></div>
</div>
<div class="admonition note">
Note
Counters were primarily designed to work with positive integers to represent
running counts; however, care was taken to not unnecessarily preclude use
cases needing other types or negative values. To help with those use cases,
this section documents the minimum range and type restrictions.
<ul class="simple">
<li>The <a class="reference internal" href="#collections.Counter" title="collections.Counter"><code class="xref py py-class docutils literal notranslate">Counter</code></a> class itself is a dictionary subclass with no
restrictions on its keys and values. The values are intended to be numbers
representing counts, but you could store anything in the value field.</li>
<li>The <code class="xref py py-meth docutils literal notranslate">most_common()</code> method requires only that the values be orderable.</li>
<li>For in-place operations such as <code class="docutils literal notranslate">c[key] += 1</code>, the value type need only
support addition and subtraction. So fractions, floats, and decimals would
work and negative values are supported. The same is also true for
<code class="xref py py-meth docutils literal notranslate">update()</code> and <code class="xref py py-meth docutils literal notranslate">subtract()</code> which allow negative and zero values
for both inputs and outputs.</li>
<li>The multiset methods are designed only for use cases with positive values.
The inputs may be negative or zero, but only outputs with positive values
are created. There are no type restrictions, but the value type needs to
support addition, subtraction, and comparison.</li>
<li>The <code class="xref py py-meth docutils literal notranslate">elements()</code> method requires integer counts. It ignores zero and
negative counts.</li>
</ul>
</div>
<div class="admonition seealso">
See also
<ul>
<li><a class="reference external" href="https://code.activestate.com/recipes/576611/">Counter class</a>
adapted for Python 2.5 and an early <a class="reference external" href="https://code.activestate.com/recipes/259174/">Bag recipe</a> for Python 2.4.</li>
<li><a class="reference external" href="https://www.gnu.org/software/smalltalk/manual-base/html_node/Bag.html">Bag class</a>
in Smalltalk.</li>
<li>Wikipedia entry for <a class="reference external" href="https://en.wikipedia.org/wiki/Multiset">Multisets</a>.</li>
<li><a class="reference external" href="http://www.java2s.com/Tutorial/Cpp/0380__set-multiset/Catalog0380__set-multiset.htm">C++ multisets</a>
tutorial with examples.</li>
<li>For mathematical operations on multisets and their use cases, see
Knuth, Donald. The Art of Computer Programming Volume II,
Section 4.6.3, Exercise 19.</li>
<li>To enumerate all distinct multisets of a given size over a given set of
elements, see <a class="reference internal" href="itertools.html#itertools.combinations_with_replacement" title="itertools.combinations_with_replacement"><code class="xref py py-func docutils literal notranslate">itertools.combinations_with_replacement()</code></a>.
<blockquote>
<div>map(Counter, combinations_with_replacement(‘ABC’, 2)) –&gt; AA AB AC BB BC CC
</div></blockquote>
</li>
</ul>
</div>
</section>
<section id="deque-objects">
<h2>8.3.2. <a class="reference internal" href="#collections.deque" title="collections.deque"><code class="xref py py-class docutils literal notranslate">deque</code></a> objects<a class="headerlink" href="#deque-objects" title="Permalink to this headline">¶</a></h2>
<dl class="py class">
<dt class="sig sig-object py" id="collections.deque">
class collections.deque([iterable[, maxlen]])<a class="headerlink" href="#collections.deque" title="Permalink to this definition">¶</a></dt>
<dd>Returns a new deque object initialized left-to-right (using <a class="reference internal" href="#collections.deque.append" title="collections.deque.append"><code class="xref py py-meth docutils literal notranslate">append()</code></a>) with
data from iterable. If iterable is not specified, the new deque is empty.
Deques are a generalization of stacks and queues (the name is pronounced “deck”
and is short for “double-ended queue”). Deques support thread-safe, memory
efficient appends and pops from either side of the deque with approximately the
same O(1) performance in either direction.
Though <code class="xref py py-class docutils literal notranslate">list</code> objects support similar operations, they are optimized for
fast fixed-length operations and incur O(n) memory movement costs for
<code class="docutils literal notranslate">pop(0)</code> and <code class="docutils literal notranslate">insert(0, v)</code> operations which change both the size and
position of the underlying data representation.
<div class="versionadded">
New in version 2.4.
</div>
If maxlen is not specified or is <code class="docutils literal notranslate">None</code>, deques may grow to an
arbitrary length. Otherwise, the deque is bounded to the specified maximum
length. Once a bounded length deque is full, when new items are added, a
corresponding number of items are discarded from the opposite end. Bounded
length deques provide functionality similar to the <code class="docutils literal notranslate">tail</code> filter in
Unix. They are also useful for tracking transactions and other pools of data
where only the most recent activity is of interest.
<div class="versionchanged">
Changed in version 2.6: Added maxlen parameter.
</div>
Deque objects support the following methods:
<dl class="py method">
<dt class="sig sig-object py" id="collections.deque.append">
append(x)<a class="headerlink" href="#collections.deque.append" title="Permalink to this definition">¶</a></dt>
<dd>Add x to the right side of the deque.
</dd></dl>

<dl class="py method">
<dt class="sig sig-object py" id="collections.deque.appendleft">
appendleft(x)<a class="headerlink" href="#collections.deque.appendleft" title="Permalink to this definition">¶</a></dt>
<dd>Add x to the left side of the deque.
</dd></dl>

<dl class="py method">
<dt class="sig sig-object py" id="collections.deque.clear">
clear()<a class="headerlink" href="#collections.deque.clear" title="Permalink to this definition">¶</a></dt>
<dd>Remove all elements from the deque leaving it with length 0.
</dd></dl>

<dl class="py method">
<dt class="sig sig-object py" id="collections.deque.count">
count(x)<a class="headerlink" href="#collections.deque.count" title="Permalink to this definition">¶</a></dt>
<dd>Count the number of deque elements equal to x.
<div class="versionadded">
New in version 2.7.
</div>
</dd></dl>

<dl class="py method">
<dt class="sig sig-object py" id="collections.deque.extend">
extend(iterable)<a class="headerlink" href="#collections.deque.extend" title="Permalink to this definition">¶</a></dt>
<dd>Extend the right side of the deque by appending elements from the iterable
argument.
</dd></dl>

<dl class="py method">
<dt class="sig sig-object py" id="collections.deque.extendleft">
extendleft(iterable)<a class="headerlink" href="#collections.deque.extendleft" title="Permalink to this definition">¶</a></dt>
<dd>Extend the left side of the deque by appending elements from iterable.
Note, the series of left appends results in reversing the order of
elements in the iterable argument.
</dd></dl>

<dl class="py method">
<dt class="sig sig-object py" id="collections.deque.pop">
pop()<a class="headerlink" href="#collections.deque.pop" title="Permalink to this definition">¶</a></dt>
<dd>Remove and return an element from the right side of the deque. If no
elements are present, raises an <code class="xref py py-exc docutils literal notranslate">IndexError</code>.
</dd></dl>

<dl class="py method">
<dt class="sig sig-object py" id="collections.deque.popleft">
popleft()<a class="headerlink" href="#collections.deque.popleft" title="Permalink to this definition">¶</a></dt>
<dd>Remove and return an element from the left side of the deque. If no
elements are present, raises an <code class="xref py py-exc docutils literal notranslate">IndexError</code>.
</dd></dl>

<dl class="py method">
<dt class="sig sig-object py" id="collections.deque.remove">
remove(value)<a class="headerlink" href="#collections.deque.remove" title="Permalink to this definition">¶</a></dt>
<dd>Remove the first occurrence of value. If not found, raises a
<code class="xref py py-exc docutils literal notranslate">ValueError</code>.
<div class="versionadded">
New in version 2.5.
</div>
</dd></dl>

<dl class="py method">
<dt class="sig sig-object py" id="collections.deque.reverse">
reverse()<a class="headerlink" href="#collections.deque.reverse" title="Permalink to this definition">¶</a></dt>
<dd>Reverse the elements of the deque in-place and then return <code class="docutils literal notranslate">None</code>.
<div class="versionadded">
New in version 2.7.
</div>
</dd></dl>

<dl class="py method">
<dt class="sig sig-object py" id="collections.deque.rotate">
rotate(n=1)<a class="headerlink" href="#collections.deque.rotate" title="Permalink to this definition">¶</a></dt>
<dd>Rotate the deque n steps to the right. If n is negative, rotate to
the left.
When the deque is not empty, rotating one step to the right is equivalent to
<code class="docutils literal notranslate">d.appendleft(d.pop())</code>, and rotating one step to the left is
equivalent to <code class="docutils literal notranslate">d.append(d.popleft())</code>.
</dd></dl>

Deque objects also provide one read-only attribute:
<dl class="py attribute">
<dt class="sig sig-object py" id="collections.deque.maxlen">
maxlen<a class="headerlink" href="#collections.deque.maxlen" title="Permalink to this definition">¶</a></dt>
<dd>Maximum size of a deque or <code class="docutils literal notranslate">None</code> if unbounded.
<div class="versionadded">
New in version 2.7.
</div>
</dd></dl>

</dd></dl>

In addition to the above, deques support iteration, pickling, <code class="docutils literal notranslate">len(d)</code>,
<code class="docutils literal notranslate">reversed(d)</code>, <code class="docutils literal notranslate">copy.copy(d)</code>, <code class="docutils literal notranslate">copy.deepcopy(d)</code>, membership testing with
the <a class="reference internal" href="../reference/expressions.html#in"><code class="xref std std-keyword docutils literal notranslate">in</code></a> operator, and subscript references such as <code class="docutils literal notranslate">d[-1]</code>. Indexed
access is O(1) at both ends but slows to O(n) in the middle. For fast random
access, use lists instead.
Example:
<div class="highlight-pycon3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; from collections import deque
&gt;&gt;&gt; d = deque(&#39;ghi&#39;) # make a new deque with three items
&gt;&gt;&gt; for elem in d: # iterate over the deque&#39;s elements
... print elem.upper()
G
H
I

&gt;&gt;&gt; d.append(&#39;j&#39;) # add a new entry to the right side
&gt;&gt;&gt; d.appendleft(&#39;f&#39;) # add a new entry to the left side
&gt;&gt;&gt; d # show the representation of the deque
deque([&#39;f&#39;, &#39;g&#39;, &#39;h&#39;, &#39;i&#39;, &#39;j&#39;])

&gt;&gt;&gt; d.pop() # return and remove the rightmost item
&#39;j&#39;
&gt;&gt;&gt; d.popleft() # return and remove the leftmost item
&#39;f&#39;
&gt;&gt;&gt; list(d) # list the contents of the deque
[&#39;g&#39;, &#39;h&#39;, &#39;i&#39;]
&gt;&gt;&gt; d[0] # peek at leftmost item
&#39;g&#39;
&gt;&gt;&gt; d[-1] # peek at rightmost item
&#39;i&#39;

&gt;&gt;&gt; list(reversed(d)) # list the contents of a deque in reverse
[&#39;i&#39;, &#39;h&#39;, &#39;g&#39;]
&gt;&gt;&gt; &#39;h&#39; in d # search the deque
True
&gt;&gt;&gt; d.extend(&#39;jkl&#39;) # add multiple elements at once
&gt;&gt;&gt; d
deque([&#39;g&#39;, &#39;h&#39;, &#39;i&#39;, &#39;j&#39;, &#39;k&#39;, &#39;l&#39;])
&gt;&gt;&gt; d.rotate(1) # right rotation
&gt;&gt;&gt; d
deque([&#39;l&#39;, &#39;g&#39;, &#39;h&#39;, &#39;i&#39;, &#39;j&#39;, &#39;k&#39;])
&gt;&gt;&gt; d.rotate(-1) # left rotation
&gt;&gt;&gt; d
deque([&#39;g&#39;, &#39;h&#39;, &#39;i&#39;, &#39;j&#39;, &#39;k&#39;, &#39;l&#39;])

&gt;&gt;&gt; deque(reversed(d)) # make a new deque in reverse order
deque([&#39;l&#39;, &#39;k&#39;, &#39;j&#39;, &#39;i&#39;, &#39;h&#39;, &#39;g&#39;])
&gt;&gt;&gt; d.clear() # empty the deque
&gt;&gt;&gt; d.pop() # cannot pop from an empty deque
Traceback (most recent call last):
 File &quot;&lt;pyshell#6&gt;&quot;, line 1, in -toplevel-
 d.pop()
IndexError: pop from an empty deque

&gt;&gt;&gt; d.extendleft(&#39;abc&#39;) # extendleft() reverses the input order
&gt;&gt;&gt; d
deque([&#39;c&#39;, &#39;b&#39;, &#39;a&#39;])
</pre></div>
</div>
<section id="deque-recipes">
<h3>8.3.2.1. <a class="reference internal" href="#collections.deque" title="collections.deque"><code class="xref py py-class docutils literal notranslate">deque</code></a> Recipes<a class="headerlink" href="#deque-recipes" title="Permalink to this headline">¶</a></h3>
This section shows various approaches to working with deques.
Bounded length deques provide functionality similar to the <code class="docutils literal notranslate">tail</code> filter
in Unix:
<div class="highlight-default notranslate"><div class="highlight"><pre>def tail(filename, n=10):
 &#39;Return the last n lines of a file&#39;
 return deque(open(filename), n)
</pre></div>
</div>
Another approach to using deques is to maintain a sequence of recently
added elements by appending to the right and popping to the left:
<div class="highlight-default notranslate"><div class="highlight"><pre>def moving_average(iterable, n=3):
 # moving_average([40, 30, 50, 46, 39, 44]) --&gt; 40.0 42.0 45.0 43.0
 # http://en.wikipedia.org/wiki/Moving_average
 it = iter(iterable)
 d = deque(itertools.islice(it, n-1))
 d.appendleft(0)
 s = sum(d)
 for elem in it:
 s += elem - d.popleft()
 d.append(elem)
 yield s / float(n)
</pre></div>
</div>
The <code class="xref py py-meth docutils literal notranslate">rotate()</code> method provides a way to implement <a class="reference internal" href="#collections.deque" title="collections.deque"><code class="xref py py-class docutils literal notranslate">deque</code></a> slicing and
deletion. For example, a pure Python implementation of <code class="docutils literal notranslate">del d[n]</code> relies on
the <code class="xref py py-meth docutils literal notranslate">rotate()</code> method to position elements to be popped:
<div class="highlight-default notranslate"><div class="highlight"><pre>def delete_nth(d, n):
 d.rotate(-n)
 d.popleft()
 d.rotate(n)
</pre></div>
</div>
To implement <a class="reference internal" href="#collections.deque" title="collections.deque"><code class="xref py py-class docutils literal notranslate">deque</code></a> slicing, use a similar approach applying
<code class="xref py py-meth docutils literal notranslate">rotate()</code> to bring a target element to the left side of the deque. Remove
old entries with <code class="xref py py-meth docutils literal notranslate">popleft()</code>, add new entries with <code class="xref py py-meth docutils literal notranslate">extend()</code>, and then
reverse the rotation.
With minor variations on that approach, it is easy to implement Forth style
stack manipulations such as <code class="docutils literal notranslate">dup</code>, <code class="docutils literal notranslate">drop</code>, <code class="docutils literal notranslate">swap</code>, <code class="docutils literal notranslate">over</code>, <code class="docutils literal notranslate">pick</code>,
<code class="docutils literal notranslate">rot</code>, and <code class="docutils literal notranslate">roll</code>.
</section>
</section>
<section id="defaultdict-objects">
<h2>8.3.3. <a class="reference internal" href="#collections.defaultdict" title="collections.defaultdict"><code class="xref py py-class docutils literal notranslate">defaultdict</code></a> objects<a class="headerlink" href="#defaultdict-objects" title="Permalink to this headline">¶</a></h2>
<dl class="py class">
<dt class="sig sig-object py" id="collections.defaultdict">
class collections.defaultdict([default_factory[, ...]])<a class="headerlink" href="#collections.defaultdict" title="Permalink to this definition">¶</a></dt>
<dd>Returns a new dictionary-like object. <a class="reference internal" href="#collections.defaultdict" title="collections.defaultdict"><code class="xref py py-class docutils literal notranslate">defaultdict</code></a> is a subclass of the
built-in <a class="reference internal" href="stdtypes.html#dict" title="dict"><code class="xref py py-class docutils literal notranslate">dict</code></a> class. It overrides one method and adds one writable
instance variable. The remaining functionality is the same as for the
<a class="reference internal" href="stdtypes.html#dict" title="dict"><code class="xref py py-class docutils literal notranslate">dict</code></a> class and is not documented here.
The first argument provides the initial value for the <a class="reference internal" href="#collections.defaultdict.default_factory" title="collections.defaultdict.default_factory"><code class="xref py py-attr docutils literal notranslate">default_factory</code></a>
attribute; it defaults to <code class="docutils literal notranslate">None</code>. All remaining arguments are treated the same
as if they were passed to the <a class="reference internal" href="stdtypes.html#dict" title="dict"><code class="xref py py-class docutils literal notranslate">dict</code></a> constructor, including keyword
arguments.
<div class="versionadded">
New in version 2.5.
</div>
<a class="reference internal" href="#collections.defaultdict" title="collections.defaultdict"><code class="xref py py-class docutils literal notranslate">defaultdict</code></a> objects support the following method in addition to the
standard <a class="reference internal" href="stdtypes.html#dict" title="dict"><code class="xref py py-class docutils literal notranslate">dict</code></a> operations:
<dl class="py method">
<dt class="sig sig-object py" id="collections.defaultdict.__missing__">
__missing__(key)<a class="headerlink" href="#collections.defaultdict.__missing__" title="Permalink to this definition">¶</a></dt>
<dd>If the <a class="reference internal" href="#collections.defaultdict.default_factory" title="collections.defaultdict.default_factory"><code class="xref py py-attr docutils literal notranslate">default_factory</code></a> attribute is <code class="docutils literal notranslate">None</code>, this raises a
<code class="xref py py-exc docutils literal notranslate">KeyError</code> exception with the key as argument.
If <a class="reference internal" href="#collections.defaultdict.default_factory" title="collections.defaultdict.default_factory"><code class="xref py py-attr docutils literal notranslate">default_factory</code></a> is not <code class="docutils literal notranslate">None</code>, it is called without arguments
to provide a default value for the given key, this value is inserted in
the dictionary for the key, and returned.
If calling <a class="reference internal" href="#collections.defaultdict.default_factory" title="collections.defaultdict.default_factory"><code class="xref py py-attr docutils literal notranslate">default_factory</code></a> raises an exception this exception is
propagated unchanged.
This method is called by the <code class="xref py py-meth docutils literal notranslate">__getitem__()</code> method of the
<a class="reference internal" href="stdtypes.html#dict" title="dict"><code class="xref py py-class docutils literal notranslate">dict</code></a> class when the requested key is not found; whatever it
returns or raises is then returned or raised by <code class="xref py py-meth docutils literal notranslate">__getitem__()</code>.
Note that <a class="reference internal" href="#collections.defaultdict.__missing__" title="collections.defaultdict.__missing__"><code class="xref py py-meth docutils literal notranslate">__missing__()</code></a> is not called for any operations besides
<code class="xref py py-meth docutils literal notranslate">__getitem__()</code>. This means that <code class="xref py py-meth docutils literal notranslate">get()</code> will, like normal
dictionaries, return <code class="docutils literal notranslate">None</code> as a default rather than using
<a class="reference internal" href="#collections.defaultdict.default_factory" title="collections.defaultdict.default_factory"><code class="xref py py-attr docutils literal notranslate">default_factory</code></a>.
</dd></dl>

<a class="reference internal" href="#collections.defaultdict" title="collections.defaultdict"><code class="xref py py-class docutils literal notranslate">defaultdict</code></a> objects support the following instance variable:
<dl class="py attribute">
<dt class="sig sig-object py" id="collections.defaultdict.default_factory">
default_factory<a class="headerlink" href="#collections.defaultdict.default_factory" title="Permalink to this definition">¶</a></dt>
<dd>This attribute is used by the <a class="reference internal" href="#collections.defaultdict.__missing__" title="collections.defaultdict.__missing__"><code class="xref py py-meth docutils literal notranslate">__missing__()</code></a> method; it is
initialized from the first argument to the constructor, if present, or to
<code class="docutils literal notranslate">None</code>, if absent.
</dd></dl>

</dd></dl>

<section id="defaultdict-examples">
<h3>8.3.3.1. <a class="reference internal" href="#collections.defaultdict" title="collections.defaultdict"><code class="xref py py-class docutils literal notranslate">defaultdict</code></a> Examples<a class="headerlink" href="#defaultdict-examples" title="Permalink to this headline">¶</a></h3>
Using <code class="xref py py-class docutils literal notranslate">list</code> as the <code class="xref py py-attr docutils literal notranslate">default_factory</code>, it is easy to group a
sequence of key-value pairs into a dictionary of lists:
<div class="doctest highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; s = [(&#39;yellow&#39;, 1), (&#39;blue&#39;, 2), (&#39;yellow&#39;, 3), (&#39;blue&#39;, 4), (&#39;red&#39;, 1)]
&gt;&gt;&gt; d = defaultdict(list)
&gt;&gt;&gt; for k, v in s:
... d[k].append(v)
...
&gt;&gt;&gt; d.items()
[(&#39;blue&#39;, [2, 4]), (&#39;red&#39;, [1]), (&#39;yellow&#39;, [1, 3])]
</pre></div>
</div>
When each key is encountered for the first time, it is not already in the
mapping; so an entry is automatically created using the <code class="xref py py-attr docutils literal notranslate">default_factory</code>
function which returns an empty <code class="xref py py-class docutils literal notranslate">list</code>. The <code class="xref py py-meth docutils literal notranslate">list.append()</code>
operation then attaches the value to the new list. When keys are encountered
again, the look-up proceeds normally (returning the list for that key) and the
<code class="xref py py-meth docutils literal notranslate">list.append()</code> operation adds another value to the list. This technique is
simpler and faster than an equivalent technique using <a class="reference internal" href="stdtypes.html#dict.setdefault" title="dict.setdefault"><code class="xref py py-meth docutils literal notranslate">dict.setdefault()</code></a>:
<div class="doctest highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; d = {}
&gt;&gt;&gt; for k, v in s:
... d.setdefault(k, []).append(v)
...
&gt;&gt;&gt; d.items()
[(&#39;blue&#39;, [2, 4]), (&#39;red&#39;, [1]), (&#39;yellow&#39;, [1, 3])]
</pre></div>
</div>
Setting the <code class="xref py py-attr docutils literal notranslate">default_factory</code> to <a class="reference internal" href="functions.html#int" title="int"><code class="xref py py-class docutils literal notranslate">int</code></a> makes the
<a class="reference internal" href="#collections.defaultdict" title="collections.defaultdict"><code class="xref py py-class docutils literal notranslate">defaultdict</code></a> useful for counting (like a bag or multiset in other
languages):
<div class="doctest highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; s = &#39;mississippi&#39;
&gt;&gt;&gt; d = defaultdict(int)
&gt;&gt;&gt; for k in s:
... d[k] += 1
...
&gt;&gt;&gt; d.items()
[(&#39;i&#39;, 4), (&#39;p&#39;, 2), (&#39;s&#39;, 4), (&#39;m&#39;, 1)]
</pre></div>
</div>
When a letter is first encountered, it is missing from the mapping, so the
<code class="xref py py-attr docutils literal notranslate">default_factory</code> function calls <a class="reference internal" href="functions.html#int" title="int"><code class="xref py py-func docutils literal notranslate">int()</code></a> to supply a default count of
zero. The increment operation then builds up the count for each letter.
The function <a class="reference internal" href="functions.html#int" title="int"><code class="xref py py-func docutils literal notranslate">int()</code></a> which always returns zero is just a special case of
constant functions. A faster and more flexible way to create constant functions
is to use <a class="reference internal" href="itertools.html#itertools.repeat" title="itertools.repeat"><code class="xref py py-func docutils literal notranslate">itertools.repeat()</code></a> which can supply any constant value (not just
zero):
<div class="doctest highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; def constant_factory(value):
... return itertools.repeat(value).next
&gt;&gt;&gt; d = defaultdict(constant_factory(&#39;&lt;missing&gt;&#39;))
&gt;&gt;&gt; d.update(name=&#39;John&#39;, action=&#39;ran&#39;)
&gt;&gt;&gt; &#39;%(name)s %(action)s to %(object)s&#39; % d
&#39;John ran to &lt;missing&gt;&#39;
</pre></div>
</div>
Setting the <code class="xref py py-attr docutils literal notranslate">default_factory</code> to <a class="reference internal" href="stdtypes.html#set" title="set"><code class="xref py py-class docutils literal notranslate">set</code></a> makes the
<a class="reference internal" href="#collections.defaultdict" title="collections.defaultdict"><code class="xref py py-class docutils literal notranslate">defaultdict</code></a> useful for building a dictionary of sets:
<div class="doctest highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; s = [(&#39;red&#39;, 1), (&#39;blue&#39;, 2), (&#39;red&#39;, 3), (&#39;blue&#39;, 4), (&#39;red&#39;, 1), (&#39;blue&#39;, 4)]
&gt;&gt;&gt; d = defaultdict(set)
&gt;&gt;&gt; for k, v in s:
... d[k].add(v)
...
&gt;&gt;&gt; d.items()
[(&#39;blue&#39;, set([2, 4])), (&#39;red&#39;, set([1, 3]))]
</pre></div>
</div>
</section>
</section>
<section id="namedtuple-factory-function-for-tuples-with-named-fields">
<h2>8.3.4. <a class="reference internal" href="#collections.namedtuple" title="collections.namedtuple"><code class="xref py py-func docutils literal notranslate">namedtuple()</code></a> Factory Function for Tuples with Named Fields<a class="headerlink" href="#namedtuple-factory-function-for-tuples-with-named-fields" title="Permalink to this headline">¶</a></h2>
Named tuples assign meaning to each position in a tuple and allow for more readable,
self-documenting code. They can be used wherever regular tuples are used, and
they add the ability to access fields by name instead of position index.
<dl class="py function">
<dt class="sig sig-object py" id="collections.namedtuple">
collections.namedtuple(typename, field_names[, verbose=False][, rename=False])<a class="headerlink" href="#collections.namedtuple" title="Permalink to this definition">¶</a></dt>
<dd>Returns a new tuple subclass named typename. The new subclass is used to
create tuple-like objects that have fields accessible by attribute lookup as
well as being indexable and iterable. Instances of the subclass also have a
helpful docstring (with typename and field_names) and a helpful <code class="xref py py-meth docutils literal notranslate">__repr__()</code>
method which lists the tuple contents in a <code class="docutils literal notranslate">name=value</code> format.
The field_names are a sequence of strings such as <code class="docutils literal notranslate">['x', 'y']</code>.
Alternatively, field_names can be a single string with each fieldname
separated by whitespace and/or commas, for example <code class="docutils literal notranslate">'x y'</code> or <code class="docutils literal notranslate">'x, y'</code>.
Any valid Python identifier may be used for a fieldname except for names
starting with an underscore. Valid identifiers consist of letters, digits,
and underscores but do not start with a digit or underscore and cannot be
a <a class="reference internal" href="keyword.html#module-keyword" title="keyword: Test whether a string is a keyword in Python."><code class="xref py py-mod docutils literal notranslate">keyword</code></a> such as class, for, return, global, pass, print,
or raise.
If rename is true, invalid fieldnames are automatically replaced
with positional names. For example, <code class="docutils literal notranslate">['abc', 'def', 'ghi', 'abc']</code> is
converted to <code class="docutils literal notranslate">['abc', '_1', 'ghi', '_3']</code>, eliminating the keyword
<code class="docutils literal notranslate">def</code> and the duplicate fieldname <code class="docutils literal notranslate">abc</code>.
If verbose is true, the class definition is printed just before being built.
Named tuple instances do not have per-instance dictionaries, so they are
lightweight and require no more memory than regular tuples.
<div class="versionadded">
New in version 2.6.
</div>
<div class="versionchanged">
Changed in version 2.7: added support for rename.
</div>
</dd></dl>

Example:
<div class="highlight-pycon3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; Point = namedtuple(&#39;Point&#39;, [&#39;x&#39;, &#39;y&#39;], verbose=True)
class Point(tuple):
 &#39;Point(x, y)&#39;

__slots__ = ()

_fields = (&#39;x&#39;, &#39;y&#39;)

def __new__(_cls, x, y):
 &#39;Create new instance of Point(x, y)&#39;
 return _tuple.__new__(_cls, (x, y))

@classmethod
 def _make(cls, iterable, new=tuple.__new__, len=len):
 &#39;Make a new Point object from a sequence or iterable&#39;
 result = new(cls, iterable)
 if len(result) != 2:
 raise TypeError(&#39;Expected 2 arguments, got %d&#39; % len(result))
 return result

def __repr__(self):
 &#39;Return a nicely formatted representation string&#39;
 return &#39;Point(x=%r, y=%r)&#39; % self

def _asdict(self):
 &#39;Return a new OrderedDict which maps field names to their values&#39;
 return OrderedDict(zip(self._fields, self))

def _replace(_self, **kwds):
 &#39;Return a new Point object replacing specified fields with new values&#39;
 result = _self._make(map(kwds.pop, (&#39;x&#39;, &#39;y&#39;), _self))
 if kwds:
 raise ValueError(&#39;Got unexpected field names: %r&#39; % kwds.keys())
 return result

def __getnewargs__(self):
 &#39;Return self as a plain tuple. Used by copy and pickle.&#39;
 return tuple(self)

__dict__ = _property(_asdict)

def __getstate__(self):
 &#39;Exclude the OrderedDict from pickling&#39;
 pass

x = _property(_itemgetter(0), doc=&#39;Alias for field number 0&#39;)

y = _property(_itemgetter(1), doc=&#39;Alias for field number 1&#39;)

&gt;&gt;&gt; p = Point(11, y=22) # instantiate with positional or keyword arguments
&gt;&gt;&gt; p[0] + p[1] # indexable like the plain tuple (11, 22)
33
&gt;&gt;&gt; x, y = p # unpack like a regular tuple
&gt;&gt;&gt; x, y
(11, 22)
&gt;&gt;&gt; p.x + p.y # fields also accessible by name
33
&gt;&gt;&gt; p # readable __repr__ with a name=value style
Point(x=11, y=22)
</pre></div>
</div>
Named tuples are especially useful for assigning field names to result tuples returned
by the <a class="reference internal" href="csv.html#module-csv" title="csv: Write and read tabular data to and from delimited files."><code class="xref py py-mod docutils literal notranslate">csv</code></a> or <a class="reference internal" href="sqlite3.html#module-sqlite3" title="sqlite3: A DB-API 2.0 implementation using SQLite 3.x."><code class="xref py py-mod docutils literal notranslate">sqlite3</code></a> modules:
<div class="highlight-default notranslate"><div class="highlight"><pre>EmployeeRecord = namedtuple(&#39;EmployeeRecord&#39;, &#39;name, age, title, department, paygrade&#39;)

import csv
for emp in map(EmployeeRecord._make, csv.reader(open(&quot;employees.csv&quot;, &quot;rb&quot;))):
 print emp.name, emp.title

import sqlite3
conn = sqlite3.connect(&#39;/companydata&#39;)
cursor = conn.cursor()
cursor.execute(&#39;SELECT name, age, title, department, paygrade FROM employees&#39;)
for emp in map(EmployeeRecord._make, cursor.fetchall()):
 print emp.name, emp.title
</pre></div>
</div>
In addition to the methods inherited from tuples, named tuples support
three additional methods and one attribute. To prevent conflicts with
field names, the method and attribute names start with an underscore.
<dl class="py method">
<dt class="sig sig-object py" id="collections.somenamedtuple._make">
classmethod somenamedtuple._make(iterable)<a class="headerlink" href="#collections.somenamedtuple._make" title="Permalink to this definition">¶</a></dt>
<dd>Class method that makes a new instance from an existing sequence or iterable.
<div class="highlight-pycon3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; t = [11, 22]
&gt;&gt;&gt; Point._make(t)
Point(x=11, y=22)
</pre></div>
</div>
</dd></dl>

<dl class="py method">
<dt class="sig sig-object py" id="collections.somenamedtuple._asdict">
somenamedtuple._asdict()<a class="headerlink" href="#collections.somenamedtuple._asdict" title="Permalink to this definition">¶</a></dt>
<dd>Return a new <a class="reference internal" href="#collections.OrderedDict" title="collections.OrderedDict"><code class="xref py py-class docutils literal notranslate">OrderedDict</code></a> which maps field names to their corresponding
values:
<div class="highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; p = Point(x=11, y=22)
&gt;&gt;&gt; p._asdict()
OrderedDict([(&#39;x&#39;, 11), (&#39;y&#39;, 22)])
</pre></div>
</div>
<div class="versionchanged">
Changed in version 2.7: Returns an <a class="reference internal" href="#collections.OrderedDict" title="collections.OrderedDict"><code class="xref py py-class docutils literal notranslate">OrderedDict</code></a> instead of a regular <a class="reference internal" href="stdtypes.html#dict" title="dict"><code class="xref py py-class docutils literal notranslate">dict</code></a>.
</div>
</dd></dl>

<dl class="py method">
<dt class="sig sig-object py" id="collections.somenamedtuple._replace">
somenamedtuple._replace(**kwargs)<a class="headerlink" href="#collections.somenamedtuple._replace" title="Permalink to this definition">¶</a></dt>
<dd>Return a new instance of the named tuple replacing specified fields with new
values:
<div class="highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; p = Point(x=11, y=22)
&gt;&gt;&gt; p._replace(x=33)
Point(x=33, y=22)

&gt;&gt;&gt; for partnum, record in inventory.items():
... inventory[partnum] = record._replace(price=newprices[partnum], timestamp=time.now())
</pre></div>
</div>
</dd></dl>

<dl class="py attribute">
<dt class="sig sig-object py" id="collections.somenamedtuple._fields">
somenamedtuple._fields<a class="headerlink" href="#collections.somenamedtuple._fields" title="Permalink to this definition">¶</a></dt>
<dd>Tuple of strings listing the field names. Useful for introspection
and for creating new named tuple types from existing named tuples.
<div class="highlight-pycon3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; p._fields # view the field names
(&#39;x&#39;, &#39;y&#39;)

&gt;&gt;&gt; Color = namedtuple(&#39;Color&#39;, &#39;red green blue&#39;)
&gt;&gt;&gt; Pixel = namedtuple(&#39;Pixel&#39;, Point._fields + Color._fields)
&gt;&gt;&gt; Pixel(11, 22, 128, 255, 0)
Pixel(x=11, y=22, red=128, green=255, blue=0)
</pre></div>
</div>
</dd></dl>

To retrieve a field whose name is stored in a string, use the <a class="reference internal" href="functions.html#getattr" title="getattr"><code class="xref py py-func docutils literal notranslate">getattr()</code></a>
function:
<div class="doctest highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; getattr(p, &#39;x&#39;)
11
</pre></div>
</div>
To convert a dictionary to a named tuple, use the double-star-operator
(as described in <a class="reference internal" href="../tutorial/controlflow.html#tut-unpacking-arguments">Unpacking Argument Lists</a>):
<div class="doctest highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; d = {&#39;x&#39;: 11, &#39;y&#39;: 22}
&gt;&gt;&gt; Point(**d)
Point(x=11, y=22)
</pre></div>
</div>
Since a named tuple is a regular Python class, it is easy to add or change
functionality with a subclass. Here is how to add a calculated field and
a fixed-width print format:
<div class="doctest highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; class Point(namedtuple(&#39;Point&#39;, &#39;x y&#39;)):
... __slots__ = ()
... @property
... def hypot(self):
... return (self.x ** 2 + self.y ** 2) ** 0.5
... def __str__(self):
... return &#39;Point: x=%6.3f y=%6.3f hypot=%6.3f&#39; % (self.x, self.y, self.hypot)
...
&gt;&gt;&gt; for p in Point(3, 4), Point(14, 5/7.):
... print p
Point: x= 3.000 y= 4.000 hypot= 5.000
Point: x=14.000 y= 0.714 hypot=14.018
</pre></div>
</div>
The subclass shown above sets <code class="docutils literal notranslate">__slots__</code> to an empty tuple. This helps
keep memory requirements low by preventing the creation of instance dictionaries.
Subclassing is not useful for adding new, stored fields. Instead, simply
create a new named tuple type from the <code class="xref py py-attr docutils literal notranslate">_fields</code> attribute:
<div class="doctest highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; Point3D = namedtuple(&#39;Point3D&#39;, Point._fields + (&#39;z&#39;,))
</pre></div>
</div>
Default values can be implemented by using <code class="xref py py-meth docutils literal notranslate">_replace()</code> to
customize a prototype instance:
<div class="doctest highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; Account = namedtuple(&#39;Account&#39;, &#39;owner balance transaction_count&#39;)
&gt;&gt;&gt; default_account = Account(&#39;&lt;owner name&gt;&#39;, 0.0, 0)
&gt;&gt;&gt; johns_account = default_account._replace(owner=&#39;John&#39;)
</pre></div>
</div>
Enumerated constants can be implemented with named tuples, but it is simpler
and more efficient to use a simple class declaration:
<div class="doctest highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; Status = namedtuple(&#39;Status&#39;, &#39;open pending closed&#39;)._make(range(3))
&gt;&gt;&gt; Status.open, Status.pending, Status.closed
(0, 1, 2)
&gt;&gt;&gt; class Status:
... open, pending, closed = range(3)
</pre></div>
</div>
<div class="admonition seealso">
See also
<a class="reference external" href="https://code.activestate.com/recipes/500261/">Named tuple recipe</a>
adapted for Python 2.4.
</div>
</section>
<section id="ordereddict-objects">
<h2>8.3.5. <a class="reference internal" href="#collections.OrderedDict" title="collections.OrderedDict"><code class="xref py py-class docutils literal notranslate">OrderedDict</code></a> objects<a class="headerlink" href="#ordereddict-objects" title="Permalink to this headline">¶</a></h2>
Ordered dictionaries are just like regular dictionaries but they remember the
order that items were inserted. When iterating over an ordered dictionary,
the items are returned in the order their keys were first added.
<dl class="py class">
<dt class="sig sig-object py" id="collections.OrderedDict">
class collections.OrderedDict([items])<a class="headerlink" href="#collections.OrderedDict" title="Permalink to this definition">¶</a></dt>
<dd>Return an instance of a dict subclass, supporting the usual <a class="reference internal" href="stdtypes.html#dict" title="dict"><code class="xref py py-class docutils literal notranslate">dict</code></a>
methods. An OrderedDict is a dict that remembers the order that keys
were first inserted. If a new entry overwrites an existing entry, the
original insertion position is left unchanged. Deleting an entry and
reinserting it will move it to the end.
<div class="versionadded">
New in version 2.7.
</div>
</dd></dl>

<dl class="py method">
<dt class="sig sig-object py" id="collections.OrderedDict.popitem">
OrderedDict.popitem(last=True)<a class="headerlink" href="#collections.OrderedDict.popitem" title="Permalink to this definition">¶</a></dt>
<dd>The <a class="reference internal" href="#collections.OrderedDict.popitem" title="collections.OrderedDict.popitem"><code class="xref py py-meth docutils literal notranslate">popitem()</code></a> method for ordered dictionaries returns and removes
a (key, value) pair. The pairs are returned in LIFO order if last is
true or FIFO order if false.
</dd></dl>

In addition to the usual mapping methods, ordered dictionaries also support
reverse iteration using <a class="reference internal" href="functions.html#reversed" title="reversed"><code class="xref py py-func docutils literal notranslate">reversed()</code></a>.
Equality tests between <a class="reference internal" href="#collections.OrderedDict" title="collections.OrderedDict"><code class="xref py py-class docutils literal notranslate">OrderedDict</code></a> objects are order-sensitive
and are implemented as <code class="docutils literal notranslate">list(od1.items())==list(od2.items())</code>.
Equality tests between <a class="reference internal" href="#collections.OrderedDict" title="collections.OrderedDict"><code class="xref py py-class docutils literal notranslate">OrderedDict</code></a> objects and other
<a class="reference internal" href="#collections.Mapping" title="collections.Mapping"><code class="xref py py-class docutils literal notranslate">Mapping</code></a> objects are order-insensitive like regular
dictionaries. This allows <a class="reference internal" href="#collections.OrderedDict" title="collections.OrderedDict"><code class="xref py py-class docutils literal notranslate">OrderedDict</code></a> objects to be substituted
anywhere a regular dictionary is used.
The <a class="reference internal" href="#collections.OrderedDict" title="collections.OrderedDict"><code class="xref py py-class docutils literal notranslate">OrderedDict</code></a> constructor and <code class="xref py py-meth docutils literal notranslate">update()</code> method both accept
keyword arguments, but their order is lost because Python’s function call
semantics pass-in keyword arguments using a regular unordered dictionary.
<div class="admonition seealso">
See also
<a class="reference external" href="https://code.activestate.com/recipes/576693/">Equivalent OrderedDict recipe</a>
that runs on Python 2.4 or later.
</div>
<section id="ordereddict-examples-and-recipes">
<h3>8.3.5.1. <a class="reference internal" href="#collections.OrderedDict" title="collections.OrderedDict"><code class="xref py py-class docutils literal notranslate">OrderedDict</code></a> Examples and Recipes<a class="headerlink" href="#ordereddict-examples-and-recipes" title="Permalink to this headline">¶</a></h3>
Since an ordered dictionary remembers its insertion order, it can be used
in conjunction with sorting to make a sorted dictionary:
<div class="highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; # regular unsorted dictionary
&gt;&gt;&gt; d = {&#39;banana&#39;: 3, &#39;apple&#39;: 4, &#39;pear&#39;: 1, &#39;orange&#39;: 2}

&gt;&gt;&gt; # dictionary sorted by key
&gt;&gt;&gt; OrderedDict(sorted(d.items(), key=lambda t: t[0]))
OrderedDict([(&#39;apple&#39;, 4), (&#39;banana&#39;, 3), (&#39;orange&#39;, 2), (&#39;pear&#39;, 1)])

&gt;&gt;&gt; # dictionary sorted by value
&gt;&gt;&gt; OrderedDict(sorted(d.items(), key=lambda t: t[1]))
OrderedDict([(&#39;pear&#39;, 1), (&#39;orange&#39;, 2), (&#39;banana&#39;, 3), (&#39;apple&#39;, 4)])

&gt;&gt;&gt; # dictionary sorted by length of the key string
&gt;&gt;&gt; OrderedDict(sorted(d.items(), key=lambda t: len(t[0])))
OrderedDict([(&#39;pear&#39;, 1), (&#39;apple&#39;, 4), (&#39;orange&#39;, 2), (&#39;banana&#39;, 3)])
</pre></div>
</div>
The new sorted dictionaries maintain their sort order when entries
are deleted. But when new keys are added, the keys are appended
to the end and the sort is not maintained.
It is also straight-forward to create an ordered dictionary variant
that remembers the order the keys were last inserted.
If a new entry overwrites an existing entry, the
original insertion position is changed and moved to the end:
<div class="highlight-default notranslate"><div class="highlight"><pre>class LastUpdatedOrderedDict(OrderedDict):
 &#39;Store items in the order the keys were last added&#39;

def __setitem__(self, key, value):
 if key in self:
 del self[key]
 OrderedDict.__setitem__(self, key, value)
</pre></div>
</div>
An ordered dictionary can be combined with the <a class="reference internal" href="#collections.Counter" title="collections.Counter"><code class="xref py py-class docutils literal notranslate">Counter</code></a> class
so that the counter remembers the order elements are first encountered:
<div class="highlight-default notranslate"><div class="highlight"><pre>class OrderedCounter(Counter, OrderedDict):
 &#39;Counter that remembers the order elements are first encountered&#39;

def __repr__(self):
 return &#39;%s(%r)&#39; % (self.__class__.__name__, OrderedDict(self))

def __reduce__(self):
 return self.__class__, (OrderedDict(self),)
</pre></div>
</div>
</section>
</section>
<section id="collections-abstract-base-classes">
<h2>8.3.6. Collections Abstract Base Classes<a class="headerlink" href="#collections-abstract-base-classes" title="Permalink to this headline">¶</a></h2>
The collections module offers the following <a class="reference internal" href="../glossary.html#term-abstract-base-class">ABCs</a>:
<table class="docutils align-default">
<colgroup>
<col style="width: 21%" />
<col style="width: 18%" />
<col style="width: 18%" />
<col style="width: 43%" />
</colgroup>
<thead>
<tr class="row-odd"><th class="head">ABC</th>
<th class="head">Inherits from</th>
<th class="head">Abstract Methods</th>
<th class="head">Mixin Methods</th>
</tr>
</thead>
<tbody>
<tr class="row-even"><td><a class="reference internal" href="#collections.Container" title="collections.Container"><code class="xref py py-class docutils literal notranslate">Container</code></a></td>
<td></td>
<td><code class="docutils literal notranslate">__contains__</code></td>
<td></td>
</tr>
<tr class="row-odd"><td><a class="reference internal" href="#collections.Hashable" title="collections.Hashable"><code class="xref py py-class docutils literal notranslate">Hashable</code></a></td>
<td></td>
<td><code class="docutils literal notranslate">__hash__</code></td>
<td></td>
</tr>
<tr class="row-even"><td><a class="reference internal" href="#collections.Iterable" title="collections.Iterable"><code class="xref py py-class docutils literal notranslate">Iterable</code></a></td>
<td></td>
<td><code class="docutils literal notranslate">__iter__</code></td>
<td></td>
</tr>
<tr class="row-odd"><td><a class="reference internal" href="#collections.Iterator" title="collections.Iterator"><code class="xref py py-class docutils literal notranslate">Iterator</code></a></td>
<td><a class="reference internal" href="#collections.Iterable" title="collections.Iterable"><code class="xref py py-class docutils literal notranslate">Iterable</code></a></td>
<td><code class="docutils literal notranslate">next</code></td>
<td><code class="docutils literal notranslate">__iter__</code></td>
</tr>
<tr class="row-even"><td><a class="reference internal" href="#collections.Sized" title="collections.Sized"><code class="xref py py-class docutils literal notranslate">Sized</code></a></td>
<td></td>
<td><code class="docutils literal notranslate">__len__</code></td>
<td></td>
</tr>
<tr class="row-odd"><td><a class="reference internal" href="#collections.Callable" title="collections.Callable"><code class="xref py py-class docutils literal notranslate">Callable</code></a></td>
<td></td>
<td><code class="docutils literal notranslate">__call__</code></td>
<td></td>
</tr>
<tr class="row-even"><td><a class="reference internal" href="#collections.Sequence" title="collections.Sequence"><code class="xref py py-class docutils literal notranslate">Sequence</code></a></td>
<td><a class="reference internal" href="#collections.Sized" title="collections.Sized"><code class="xref py py-class docutils literal notranslate">Sized</code></a>,
<a class="reference internal" href="#collections.Iterable" title="collections.Iterable"><code class="xref py py-class docutils literal notranslate">Iterable</code></a>,
<a class="reference internal" href="#collections.Container" title="collections.Container"><code class="xref py py-class docutils literal notranslate">Container</code></a></td>
<td><code class="docutils literal notranslate">__getitem__</code>,
<code class="docutils literal notranslate">__len__</code></td>
<td><code class="docutils literal notranslate">__contains__</code>, <code class="docutils literal notranslate">__iter__</code>, <code class="docutils literal notranslate">__reversed__</code>,
<code class="docutils literal notranslate">index</code>, and <code class="docutils literal notranslate">count</code></td>
</tr>
<tr class="row-odd"><td><a class="reference internal" href="#collections.MutableSequence" title="collections.MutableSequence"><code class="xref py py-class docutils literal notranslate">MutableSequence</code></a></td>
<td><a class="reference internal" href="#collections.Sequence" title="collections.Sequence"><code class="xref py py-class docutils literal notranslate">Sequence</code></a></td>
<td><code class="docutils literal notranslate">__getitem__</code>,
<code class="docutils literal notranslate">__setitem__</code>,
<code class="docutils literal notranslate">__delitem__</code>,
<code class="docutils literal notranslate">__len__</code>,
<code class="docutils literal notranslate">insert</code></td>
<td>Inherited <a class="reference internal" href="#collections.Sequence" title="collections.Sequence"><code class="xref py py-class docutils literal notranslate">Sequence</code></a> methods and
<code class="docutils literal notranslate">append</code>, <code class="docutils literal notranslate">reverse</code>, <code class="docutils literal notranslate">extend</code>, <code class="docutils literal notranslate">pop</code>,
<code class="docutils literal notranslate">remove</code>, and <code class="docutils literal notranslate">__iadd__</code></td>
</tr>
<tr class="row-even"><td><a class="reference internal" href="#collections.Set" title="collections.Set"><code class="xref py py-class docutils literal notranslate">Set</code></a></td>
<td><a class="reference internal" href="#collections.Sized" title="collections.Sized"><code class="xref py py-class docutils literal notranslate">Sized</code></a>,
<a class="reference internal" href="#collections.Iterable" title="collections.Iterable"><code class="xref py py-class docutils literal notranslate">Iterable</code></a>,
<a class="reference internal" href="#collections.Container" title="collections.Container"><code class="xref py py-class docutils literal notranslate">Container</code></a></td>
<td><code class="docutils literal notranslate">__contains__</code>,
<code class="docutils literal notranslate">__iter__</code>,
<code class="docutils literal notranslate">__len__</code></td>
<td><code class="docutils literal notranslate">__le__</code>, <code class="docutils literal notranslate">__lt__</code>, <code class="docutils literal notranslate">__eq__</code>, <code class="docutils literal notranslate">__ne__</code>,
<code class="docutils literal notranslate">__gt__</code>, <code class="docutils literal notranslate">__ge__</code>, <code class="docutils literal notranslate">__and__</code>, <code class="docutils literal notranslate">__or__</code>,
<code class="docutils literal notranslate">__sub__</code>, <code class="docutils literal notranslate">__xor__</code>, and <code class="docutils literal notranslate">isdisjoint</code></td>
</tr>
<tr class="row-odd"><td><a class="reference internal" href="#collections.MutableSet" title="collections.MutableSet"><code class="xref py py-class docutils literal notranslate">MutableSet</code></a></td>
<td><a class="reference internal" href="#collections.Set" title="collections.Set"><code class="xref py py-class docutils literal notranslate">Set</code></a></td>
<td><code class="docutils literal notranslate">__contains__</code>,
<code class="docutils literal notranslate">__iter__</code>,
<code class="docutils literal notranslate">__len__</code>,
<code class="docutils literal notranslate">add</code>,
<code class="docutils literal notranslate">discard</code></td>
<td>Inherited <a class="reference internal" href="#collections.Set" title="collections.Set"><code class="xref py py-class docutils literal notranslate">Set</code></a> methods and
<code class="docutils literal notranslate">clear</code>, <code class="docutils literal notranslate">pop</code>, <code class="docutils literal notranslate">remove</code>, <code class="docutils literal notranslate">__ior__</code>,
<code class="docutils literal notranslate">__iand__</code>, <code class="docutils literal notranslate">__ixor__</code>, and <code class="docutils literal notranslate">__isub__</code></td>
</tr>
<tr class="row-even"><td><a class="reference internal" href="#collections.Mapping" title="collections.Mapping"><code class="xref py py-class docutils literal notranslate">Mapping</code></a></td>
<td><a class="reference internal" href="#collections.Sized" title="collections.Sized"><code class="xref py py-class docutils literal notranslate">Sized</code></a>,
<a class="reference internal" href="#collections.Iterable" title="collections.Iterable"><code class="xref py py-class docutils literal notranslate">Iterable</code></a>,
<a class="reference internal" href="#collections.Container" title="collections.Container"><code class="xref py py-class docutils literal notranslate">Container</code></a></td>
<td><code class="docutils literal notranslate">__getitem__</code>,
<code class="docutils literal notranslate">__iter__</code>,
<code class="docutils literal notranslate">__len__</code></td>
<td><code class="docutils literal notranslate">__contains__</code>, <code class="docutils literal notranslate">keys</code>, <code class="docutils literal notranslate">items</code>, <code class="docutils literal notranslate">values</code>,
<code class="docutils literal notranslate">get</code>, <code class="docutils literal notranslate">__eq__</code>, and <code class="docutils literal notranslate">__ne__</code></td>
</tr>
<tr class="row-odd"><td><a class="reference internal" href="#collections.MutableMapping" title="collections.MutableMapping"><code class="xref py py-class docutils literal notranslate">MutableMapping</code></a></td>
<td><a class="reference internal" href="#collections.Mapping" title="collections.Mapping"><code class="xref py py-class docutils literal notranslate">Mapping</code></a></td>
<td><code class="docutils literal notranslate">__getitem__</code>,
<code class="docutils literal notranslate">__setitem__</code>,
<code class="docutils literal notranslate">__delitem__</code>,
<code class="docutils literal notranslate">__iter__</code>,
<code class="docutils literal notranslate">__len__</code></td>
<td>Inherited <a class="reference internal" href="#collections.Mapping" title="collections.Mapping"><code class="xref py py-class docutils literal notranslate">Mapping</code></a> methods and
<code class="docutils literal notranslate">pop</code>, <code class="docutils literal notranslate">popitem</code>, <code class="docutils literal notranslate">clear</code>, <code class="docutils literal notranslate">update</code>,
and <code class="docutils literal notranslate">setdefault</code></td>
</tr>
<tr class="row-even"><td><a class="reference internal" href="#collections.MappingView" title="collections.MappingView"><code class="xref py py-class docutils literal notranslate">MappingView</code></a></td>
<td><a class="reference internal" href="#collections.Sized" title="collections.Sized"><code class="xref py py-class docutils literal notranslate">Sized</code></a></td>
<td></td>
<td><code class="docutils literal notranslate">__len__</code></td>
</tr>
<tr class="row-odd"><td><a class="reference internal" href="#collections.ItemsView" title="collections.ItemsView"><code class="xref py py-class docutils literal notranslate">ItemsView</code></a></td>
<td><a class="reference internal" href="#collections.MappingView" title="collections.MappingView"><code class="xref py py-class docutils literal notranslate">MappingView</code></a>,
<a class="reference internal" href="#collections.Set" title="collections.Set"><code class="xref py py-class docutils literal notranslate">Set</code></a></td>
<td></td>
<td><code class="docutils literal notranslate">__contains__</code>,
<code class="docutils literal notranslate">__iter__</code></td>
</tr>
<tr class="row-even"><td><a class="reference internal" href="#collections.KeysView" title="collections.KeysView"><code class="xref py py-class docutils literal notranslate">KeysView</code></a></td>
<td><a class="reference internal" href="#collections.MappingView" title="collections.MappingView"><code class="xref py py-class docutils literal notranslate">MappingView</code></a>,
<a class="reference internal" href="#collections.Set" title="collections.Set"><code class="xref py py-class docutils literal notranslate">Set</code></a></td>
<td></td>
<td><code class="docutils literal notranslate">__contains__</code>,
<code class="docutils literal notranslate">__iter__</code></td>
</tr>
<tr class="row-odd"><td><a class="reference internal" href="#collections.ValuesView" title="collections.ValuesView"><code class="xref py py-class docutils literal notranslate">ValuesView</code></a></td>
<td><a class="reference internal" href="#collections.MappingView" title="collections.MappingView"><code class="xref py py-class docutils literal notranslate">MappingView</code></a></td>
<td></td>
<td><code class="docutils literal notranslate">__contains__</code>, <code class="docutils literal notranslate">__iter__</code></td>
</tr>
</tbody>
</table>
<dl class="py class">
<dt class="sig sig-object py" id="collections.Container">
class collections.Container<a class="headerlink" href="#collections.Container" title="Permalink to this definition">¶</a></dt>
<dt class="sig sig-object py" id="collections.Hashable">
class collections.Hashable<a class="headerlink" href="#collections.Hashable" title="Permalink to this definition">¶</a></dt>
<dt class="sig sig-object py" id="collections.Sized">
class collections.Sized<a class="headerlink" href="#collections.Sized" title="Permalink to this definition">¶</a></dt>
<dt class="sig sig-object py" id="collections.Callable">
class collections.Callable<a class="headerlink" href="#collections.Callable" title="Permalink to this definition">¶</a></dt>
<dd>ABCs for classes that provide respectively the methods <code class="xref py py-meth docutils literal notranslate">__contains__()</code>,
<code class="xref py py-meth docutils literal notranslate">__hash__()</code>, <code class="xref py py-meth docutils literal notranslate">__len__()</code>, and <code class="xref py py-meth docutils literal notranslate">__call__()</code>.
</dd></dl>

<dl class="py class">
<dt class="sig sig-object py" id="collections.Iterable">
class collections.Iterable<a class="headerlink" href="#collections.Iterable" title="Permalink to this definition">¶</a></dt>
<dd>ABC for classes that provide the <code class="xref py py-meth docutils literal notranslate">__iter__()</code> method.
See also the definition of <a class="reference internal" href="../glossary.html#term-iterable">iterable</a>.
</dd></dl>

<dl class="py class">
<dt class="sig sig-object py" id="collections.Iterator">
class collections.Iterator<a class="headerlink" href="#collections.Iterator" title="Permalink to this definition">¶</a></dt>
<dd>ABC for classes that provide the <a class="reference internal" href="stdtypes.html#iterator.__iter__" title="iterator.__iter__"><code class="xref py py-meth docutils literal notranslate">__iter__()</code></a> and
<a class="reference internal" href="stdtypes.html#iterator.next" title="iterator.next"><code class="xref py py-meth docutils literal notranslate">next()</code></a> methods. See also the definition of <a class="reference internal" href="../glossary.html#term-iterator">iterator</a>.
</dd></dl>

<dl class="py class">
<dt class="sig sig-object py" id="collections.Sequence">
class collections.Sequence<a class="headerlink" href="#collections.Sequence" title="Permalink to this definition">¶</a></dt>
<dt class="sig sig-object py" id="collections.MutableSequence">
class collections.MutableSequence<a class="headerlink" href="#collections.MutableSequence" title="Permalink to this definition">¶</a></dt>
<dd>ABCs for read-only and mutable <a class="reference internal" href="../glossary.html#term-sequence">sequences</a>.
</dd></dl>

<dl class="py class">
<dt class="sig sig-object py" id="collections.Set">
class collections.Set<a class="headerlink" href="#collections.Set" title="Permalink to this definition">¶</a></dt>
<dt class="sig sig-object py" id="collections.MutableSet">
class collections.MutableSet<a class="headerlink" href="#collections.MutableSet" title="Permalink to this definition">¶</a></dt>
<dd>ABCs for read-only and mutable sets.
</dd></dl>

<dl class="py class">
<dt class="sig sig-object py" id="collections.Mapping">
class collections.Mapping<a class="headerlink" href="#collections.Mapping" title="Permalink to this definition">¶</a></dt>
<dt class="sig sig-object py" id="collections.MutableMapping">
class collections.MutableMapping<a class="headerlink" href="#collections.MutableMapping" title="Permalink to this definition">¶</a></dt>
<dd>ABCs for read-only and mutable <a class="reference internal" href="../glossary.html#term-mapping">mappings</a>.
</dd></dl>

<dl class="py class">
<dt class="sig sig-object py" id="collections.MappingView">
class collections.MappingView<a class="headerlink" href="#collections.MappingView" title="Permalink to this definition">¶</a></dt>
<dt class="sig sig-object py" id="collections.ItemsView">
class collections.ItemsView<a class="headerlink" href="#collections.ItemsView" title="Permalink to this definition">¶</a></dt>
<dt class="sig sig-object py" id="collections.KeysView">
class collections.KeysView<a class="headerlink" href="#collections.KeysView" title="Permalink to this definition">¶</a></dt>
<dt class="sig sig-object py" id="collections.ValuesView">
class collections.ValuesView<a class="headerlink" href="#collections.ValuesView" title="Permalink to this definition">¶</a></dt>
<dd>ABCs for mapping, items, keys, and values <a class="reference internal" href="../glossary.html#term-dictionary-view">views</a>.
</dd></dl>

These ABCs allow us to ask classes or instances if they provide
particular functionality, for example:
<div class="highlight-default notranslate"><div class="highlight"><pre>size = None
if isinstance(myvar, collections.Sized):
 size = len(myvar)
</pre></div>
</div>
Several of the ABCs are also useful as mixins that make it easier to develop
classes supporting container APIs. For example, to write a class supporting
the full <a class="reference internal" href="#collections.Set" title="collections.Set"><code class="xref py py-class docutils literal notranslate">Set</code></a> API, it only necessary to supply the three underlying
abstract methods: <code class="xref py py-meth docutils literal notranslate">__contains__()</code>, <code class="xref py py-meth docutils literal notranslate">__iter__()</code>, and <code class="xref py py-meth docutils literal notranslate">__len__()</code>.
The ABC supplies the remaining methods such as <code class="xref py py-meth docutils literal notranslate">__and__()</code> and
<code class="xref py py-meth docutils literal notranslate">isdisjoint()</code>
<div class="highlight-default notranslate"><div class="highlight"><pre>class ListBasedSet(collections.Set):
 &#39;&#39;&#39; Alternate set implementation favoring space over speed
 and not requiring the set elements to be hashable. &#39;&#39;&#39;
 def __init__(self, iterable):
 self.elements = lst = []
 for value in iterable:
 if value not in lst:
 lst.append(value)

def __iter__(self):
 return iter(self.elements)

def __contains__(self, value):
 return value in self.elements

def __len__(self):
 return len(self.elements)

s1 = ListBasedSet(&#39;abcdef&#39;)
s2 = ListBasedSet(&#39;defghi&#39;)
overlap = s1 &amp; s2 # The __and__() method is supported automatically
</pre></div>
</div>
Notes on using <a class="reference internal" href="#collections.Set" title="collections.Set"><code class="xref py py-class docutils literal notranslate">Set</code></a> and <a class="reference internal" href="#collections.MutableSet" title="collections.MutableSet"><code class="xref py py-class docutils literal notranslate">MutableSet</code></a> as a mixin:
<ol class="arabic simple">
<li>Since some set operations create new sets, the default mixin methods need
a way to create new instances from an iterable. The class constructor is
assumed to have a signature in the form <code class="docutils literal notranslate">ClassName(iterable)</code>.
That assumption is factored-out to an internal classmethod called
<code class="xref py py-meth docutils literal notranslate">_from_iterable()</code> which calls <code class="docutils literal notranslate">cls(iterable)</code> to produce a new set.
If the <a class="reference internal" href="#collections.Set" title="collections.Set"><code class="xref py py-class docutils literal notranslate">Set</code></a> mixin is being used in a class with a different
constructor signature, you will need to override <code class="xref py py-meth docutils literal notranslate">_from_iterable()</code>
with a classmethod that can construct new instances from
an iterable argument.</li>
<li>To override the comparisons (presumably for speed, as the
semantics are fixed), redefine <code class="xref py py-meth docutils literal notranslate">__le__()</code> and <code class="xref py py-meth docutils literal notranslate">__ge__()</code>,
then the other operations will automatically follow suit.</li>
<li>The <a class="reference internal" href="#collections.Set" title="collections.Set"><code class="xref py py-class docutils literal notranslate">Set</code></a> mixin provides a <code class="xref py py-meth docutils literal notranslate">_hash()</code> method to compute a hash value
for the set; however, <code class="xref py py-meth docutils literal notranslate">__hash__()</code> is not defined because not all sets
are hashable or immutable. To add set hashability using mixins,
inherit from both <a class="reference internal" href="#collections.Set" title="collections.Set"><code class="xref py py-meth docutils literal notranslate">Set()</code></a> and <a class="reference internal" href="#collections.Hashable" title="collections.Hashable"><code class="xref py py-meth docutils literal notranslate">Hashable()</code></a>, then define
<code class="docutils literal notranslate">__hash__ = Set._hash</code>.</li>
</ol>
<div class="admonition seealso">
See also
<ul class="simple">
<li><a class="reference external" href="https://code.activestate.com/recipes/576694/">OrderedSet recipe</a> for an
example built on <a class="reference internal" href="#collections.MutableSet" title="collections.MutableSet"><code class="xref py py-class docutils literal notranslate">MutableSet</code></a>.</li>
<li>For more about ABCs, see the <a class="reference internal" href="abc.html#module-abc" title="abc: Abstract base classes according to PEP 3119."><code class="xref py py-mod docutils literal notranslate">abc</code></a> module and <a class="pep reference external" href="https://www.python.org/dev/peps/pep-3119">PEP 3119</a>.</li>
</ul>
</div>
</section>
</section>

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 <ul>
<li><a class="reference internal" href="#">8.3. <code class="xref py py-mod docutils literal notranslate">collections</code> — High-performance container datatypes</a><ul>
<li><a class="reference internal" href="#counter-objects">8.3.1. <code class="xref py py-class docutils literal notranslate">Counter</code> objects</a></li>
<li><a class="reference internal" href="#deque-objects">8.3.2. <code class="xref py py-class docutils literal notranslate">deque</code> objects</a><ul>
<li><a class="reference internal" href="#deque-recipes">8.3.2.1. <code class="xref py py-class docutils literal notranslate">deque</code> Recipes</a></li>
</ul>
</li>
<li><a class="reference internal" href="#defaultdict-objects">8.3.3. <code class="xref py py-class docutils literal notranslate">defaultdict</code> objects</a><ul>
<li><a class="reference internal" href="#defaultdict-examples">8.3.3.1. <code class="xref py py-class docutils literal notranslate">defaultdict</code> Examples</a></li>
</ul>
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<li><a class="reference internal" href="#namedtuple-factory-function-for-tuples-with-named-fields">8.3.4. <code class="xref py py-func docutils literal notranslate">namedtuple()</code> Factory Function for Tuples with Named Fields</a></li>
<li><a class="reference internal" href="#ordereddict-objects">8.3.5. <code class="xref py py-class docutils literal notranslate">OrderedDict</code> objects</a><ul>
<li><a class="reference internal" href="#ordereddict-examples-and-recipes">8.3.5.1. <code class="xref py py-class docutils literal notranslate">OrderedDict</code> Examples and Recipes</a></li>
</ul>
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<li><a class="reference internal" href="#collections-abstract-base-classes">8.3.6. Collections Abstract Base Classes</a></li>
</ul>
</li>
</ul>

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