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<div class="section" id="the-collections-module">

<span id="collections-module"></span><h1><span class="section-number">12.1.2. </span>The Collections Module<a class="headerlink" href="#the-collections-module" title="Permalink to this headline">¶</a></h1>

<p>Python has a very complete set of built in standard types that support most programming tasks. These include strings and numbers, and also types that can be used to hold other objects – or “collection” types.</p>

<ul class="simple">

<li><p>tuples</p></li>

<li><p>lists</p></li>

<li><p>dictionaries</p></li>

<p>You can get pretty darn far with just these basic types – but some problems require (or could be helped by) more complex collection types.</p>

<p>This was recognised by the Python development team, so a number of genreally useful collection types are provided in the collections module.</p>

<div class="section" id="id1">

<h2><span class="section-number">12.1.2.1. </span>The collections module<a class="headerlink" href="#id1" title="Permalink to this headline">¶</a></h2>

<p>The first step is to see what’s there by looking at the documentation:</p>

<p><a class="reference external" href="https://docs.python.org/3/library/collections.html">https://docs.python.org/3/library/collections.html</a></p>

<p>You can see a pretty nice list of options (kind of in order of usefulness)</p>

<ul class="simple">

<li><p><code class="docutils literal notranslate"><span class="pre">namedtuple()</span></code>: factory function for creating tuple subclasses with named fields</p></li>

<li><p><code class="docutils literal notranslate"><span class="pre">deque</span></code>: list-like container with fast appends and pops on either end</p></li>

<li><p><code class="docutils literal notranslate"><span class="pre">Counter</span></code>: dict subclass for counting hashable objects</p></li>

<li><p><code class="docutils literal notranslate"><span class="pre">OrderedDict</span></code>: dict subclass that remembers the order entries were added</p></li>

<li><p><code class="docutils literal notranslate"><span class="pre">defaultdict</span></code>: dict subclass that calls a factory function to supply missing values</p></li>

<li><p><code class="docutils literal notranslate"><span class="pre">ChainMap</span></code>: dict-like class for creating a single view of multiple mappings</p></li>

<p>These are just the regular builtin types, but in a form that they can be subclassed – to make your own custom version.</p>

<ul class="simple">

<li><p><code class="docutils literal notranslate"><span class="pre">UserDict</span></code>: wrapper around dictionary objects for easier dict subclassing</p></li>

<li><p><code class="docutils literal notranslate"><span class="pre">UserList</span></code>: wrapper around list objects for easier list subclassing</p></li>

<li><p><code class="docutils literal notranslate"><span class="pre">UserString</span></code>: wrapper around string objects for easier string subclassing</p></li>

<p>To get an idea what these all are, read the docs, or a nice overview Python Module of the Week:</p>

<p><a class="reference external" href="https://pymotw.com/3/collections/">https://pymotw.com/3/collections/</a></p>

<div class="section" id="using-the-collection-types">

<h2><span class="section-number">12.1.2.2. </span>Using the collection types<a class="headerlink" href="#using-the-collection-types" title="Permalink to this headline">¶</a></h2>

<p>To use these special types, they must be imported:</p>

<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [4]: </span><span class="kn">from</span> <span class="nn">collections</span> <span class="kn">import</span> <span class="n">defaultdict</span>

<p>Then you can use it – creating a <code class="docutils literal notranslate"><span class="pre">defaultdict</span></code> with a empty list as a default:</p>

<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [8]: </span><span class="n">dd</span> <span class="o">=</span> <span class="n">defaultdict</span><span class="p">(</span><span class="nb">list</span><span class="p">)</span>

<span class="gp">In [9]: </span><span class="n">dd</span><span class="p">[</span><span class="s1">&#39;this&#39;</span><span class="p">]</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="mi">23</span><span class="p">)</span>

<span class="gp">In [10]: </span><span class="n">dd</span>

<span class="gh">Out[10]: </span><span class="go">defaultdict(list, {&#39;this&#39;: [23]})</span>

<span class="gp">In [11]: </span><span class="n">dd</span><span class="p">[</span><span class="s1">&#39;this&#39;</span><span class="p">]</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="mi">4</span><span class="p">)</span>

<span class="gp">In [12]: </span><span class="n">dd</span><span class="p">[</span><span class="s1">&#39;this&#39;</span><span class="p">]</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="mi">4</span><span class="p">)</span>

<span class="gp">In [13]: </span><span class="n">dd</span>

<span class="gh">Out[13]: </span><span class="go">defaultdict(list, {&#39;this&#39;: [23, 4, 4]})</span>

<span class="gp">In [14]: </span><span class="n">dd</span><span class="p">[</span><span class="s1">&#39;that&#39;</span><span class="p">]</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="mi">4</span><span class="p">)</span>

<p>And you’ll get a dict that will automatically put an empty list in when the key isn’t there yet. Kind of a handy replacement from having to call <code class="docutils literal notranslate"><span class="pre">dict.setdefault</span></code> each time.</p>

<p>Similarly for the others.</p>

<p>Take a bit of time to try them out – you may find them really useful.</p>

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