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<div class="section" id="dictionaries-and-sets">
<span id="dicts-and-sets"></span><h1>Dictionaries and Sets<a class="headerlink" href="#dictionaries-and-sets" title="Permalink to this headline"></a></h1>
<div class="section" id="dictionary">
<h2>Dictionary<a class="headerlink" href="#dictionary" title="Permalink to this headline"></a></h2>
<p>Python calls it a <code class="docutils literal notranslate"><span class="pre">dict</span></code></p>
<p>Other languages call it:</p>
<blockquote>
<div><ul class="simple">
<li><p>dictionary</p></li>
<li><p>associative array</p></li>
<li><p>map</p></li>
<li><p>hash table</p></li>
<li><p>hash</p></li>
<li><p>key-value pair</p></li>
</ul>
</div></blockquote>
<p>It is also known in Python as a “mapping”, as it “maps” keys to values.</p>
<p>It is like an array, in that it holds a number of items, and you can index into it to get at particular items. But it can use arbitrary indexes, rather than a sequence of numbers.</p>
<p>These indexes are called “keys”, and the items stored are called “values”.</p>
<p>So for any Python sequence, you might do:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">item</span> <span class="o">=</span> <span class="n">stuff</span><span class="p">[</span><span class="mi">3</span><span class="p">]</span>
</pre></div>
</div>
<p>for a dict, you do:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">item</span> <span class="o">=</span> <span class="n">stuff</span><span class="p">[</span><span class="s2">"third"</span><span class="p">]</span>
</pre></div>
</div>
<div class="admonition note">
<p class="admonition-title">Note</p>
<p>It is <em>very</em> common to use strings as keys in a dictionary. So common that virtually all examples you’ll see both here and on the Internet use strings as keys. And many other languages have similar objects that only allow strings (like JavaScript objects, for instance). Python dicts, on the other hand, can use many types as keys, and this can be a very powerful feature to keep in mind.</p>
</div>
<div class="section" id="dictionary-constructors">
<h3>Dictionary Constructors<a class="headerlink" href="#dictionary-constructors" title="Permalink to this headline"></a></h3>
<p>So how does one make a dict? There are a few ways…</p>
<p>The dict “literal”:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># This dict "lies"</span>
<span class="o">>>></span> <span class="p">{</span><span class="s1">'key1'</span><span class="p">:</span> <span class="mi">3</span><span class="p">,</span> <span class="s1">'key2'</span><span class="p">:</span> <span class="mi">5</span><span class="p">}</span>
<span class="p">{</span><span class="s1">'key1'</span><span class="p">:</span> <span class="mi">3</span><span class="p">,</span> <span class="s1">'key2'</span><span class="p">:</span> <span class="mi">5</span><span class="p">}</span>
</pre></div>
</div>
<p>Calling the dict type object constructor:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># passing in a sequence of (key,value) pairs:</span>
<span class="o">>>></span> <span class="nb">dict</span><span class="p">([(</span><span class="s1">'key1'</span><span class="p">,</span> <span class="mi">3</span><span class="p">),</span> <span class="p">(</span><span class="s1">'key2'</span><span class="p">,</span> <span class="mi">5</span><span class="p">)])</span>
<span class="p">{</span><span class="s1">'key1'</span><span class="p">:</span> <span class="mi">3</span><span class="p">,</span> <span class="s1">'key2'</span><span class="p">:</span> <span class="mi">5</span><span class="p">}</span>
<span class="c1"># passing keys and values as keyword arguments:</span>
<span class="o">>>></span> <span class="nb">dict</span><span class="p">(</span><span class="n">key1</span><span class="o">=</span><span class="mi">3</span><span class="p">,</span> <span class="n">key2</span><span class="o">=</span> <span class="mi">5</span><span class="p">)</span>
<span class="p">{</span><span class="s1">'key1'</span><span class="p">:</span> <span class="mi">3</span><span class="p">,</span> <span class="s1">'key2'</span><span class="p">:</span> <span class="mi">5</span><span class="p">}</span>
<span class="c1"># creating an empty dict, and then populating it:</span>
<span class="o">>>></span> <span class="n">d</span> <span class="o">=</span> <span class="p">{}</span>
<span class="o">>>></span> <span class="n">d</span><span class="p">[</span><span class="s1">'key1'</span><span class="p">]</span> <span class="o">=</span> <span class="mi">3</span>
<span class="o">>>></span> <span class="n">d</span><span class="p">[</span><span class="s1">'key2'</span><span class="p">]</span> <span class="o">=</span> <span class="mi">5</span>
<span class="o">>>></span> <span class="n">d</span>
<span class="p">{</span><span class="s1">'key1'</span><span class="p">:</span> <span class="mi">3</span><span class="p">,</span> <span class="s1">'key2'</span><span class="p">:</span> <span class="mi">5</span><span class="p">}</span>
</pre></div>
</div>
</div>
<div class="section" id="dictionary-indexing">
<h3>Dictionary Indexing<a class="headerlink" href="#dictionary-indexing" title="Permalink to this headline"></a></h3>
<p>And how do you get stuff out (index it)?</p>
<p>The same way you index a sequence, except the index (now called a key) can be various types, rather than just an integer:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">d</span> <span class="o">=</span> <span class="p">{</span><span class="s1">'name'</span><span class="p">:</span> <span class="s1">'Brian'</span><span class="p">,</span> <span class="s1">'score'</span><span class="p">:</span> <span class="mi">42</span><span class="p">}</span>
<span class="gp">>>> </span><span class="n">d</span><span class="p">[</span><span class="s1">'score'</span><span class="p">]</span>
<span class="go">42</span>
<span class="gp">>>> </span><span class="n">d</span> <span class="o">=</span> <span class="p">{</span><span class="mi">1</span><span class="p">:</span> <span class="s1">'one'</span><span class="p">,</span> <span class="mi">0</span><span class="p">:</span> <span class="s1">'zero'</span><span class="p">}</span>
<span class="gp">>>> </span><span class="n">d</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span>
<span class="go">'zero'</span>
</pre></div>
</div>
<p>What if the key doesn’t exist in the dict? You get a <code class="docutils literal notranslate"><span class="pre">KeyError</span></code> exception:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">d</span><span class="p">[</span><span class="s1">'non-existing key'</span><span class="p">]</span>
<span class="gt">Traceback (most recent call last):</span>
File <span class="nb">"<stdin>"</span>, line <span class="m">1</span>, in <span class="n"><module></span>
<span class="gr">KeyError</span>: <span class="n">'non-existing key'</span>
</pre></div>
</div>
</div>
<div class="section" id="what-can-keys-be">
<h3>What can keys be?<a class="headerlink" href="#what-can-keys-be" title="Permalink to this headline"></a></h3>
<p>Surely not ANYTHING?</p>
<p>Not quite: keys can be any “immutable”:</p>
<blockquote>
<div><ul class="simple">
<li><p>number</p></li>
<li><p>string</p></li>
<li><p>tuple</p></li>
</ul>
</div></blockquote>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [325]: </span><span class="n">d</span><span class="p">[</span><span class="mi">3</span><span class="p">]</span> <span class="o">=</span> <span class="s1">'string'</span>
<span class="gp">In [326]: </span><span class="n">d</span><span class="p">[</span><span class="mf">3.14</span><span class="p">]</span> <span class="o">=</span> <span class="s1">'pi'</span>
<span class="gp">In [327]: </span><span class="n">d</span><span class="p">[</span><span class="s1">'pi'</span><span class="p">]</span> <span class="o">=</span> <span class="mf">3.14</span>
<span class="gp">In [328]: </span><span class="n">d</span><span class="p">[</span> <span class="p">(</span><span class="mi">1</span><span class="p">,</span><span class="mi">2</span><span class="p">,</span><span class="mi">3</span><span class="p">)</span> <span class="p">]</span> <span class="o">=</span> <span class="s1">'a tuple key'</span>
</pre></div>
</div>
<p>What if you try to use a mutable type?</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [329]: </span><span class="n">d</span><span class="p">[</span> <span class="p">[</span><span class="mi">1</span><span class="p">,</span><span class="mi">2</span><span class="p">,</span><span class="mi">3</span><span class="p">]</span> <span class="p">]</span> <span class="o">=</span> <span class="s1">'a list key'</span>
<span class="go"> TypeError: unhashable type: 'list'</span>
</pre></div>
</div>
<p>Actually – any “hashable” type.</p>
<p>So, technically, it’s not mutability, but hashability that matters.</p>
<p>Although for most intents and purposes, you want to use immutable types as keys in dicts.</p>
</div>
<div class="section" id="hashing">
<h3>Hashing<a class="headerlink" href="#hashing" title="Permalink to this headline"></a></h3>
<p>What does “hashable” mean? (<a class="reference external" href="https://computersciencewiki.org/index.php/Hashing">https://computersciencewiki.org/index.php/Hashing</a>)</p>
<p>Hash functions convert arbitrarily large data to a small proxy (usually an integer).</p>
<p>They always return the same proxy for the same input.</p>
<p>MD5, SHA, etc, are some well known hash algorithms.</p>
<p>Dictionaries hash the key to an integer proxy and use it to find the key and value.</p>
<p>Key lookup is efficient because the hash function leads directly to a bucket with very few keys (often just one).</p>
<p>What would happen if the proxy (hash) changed after storing a key?</p>
<p>(Answer: you wouldn’t be able to find it again!)</p>
<p>Hashability requires immutability.</p>
<p>Key lookup is very efficient:</p>
<p>The access time is constant regardless of the size of the dict.</p>
</div>
<div class="section" id="id1">
<h3>Dictionary indexing<a class="headerlink" href="#id1" title="Permalink to this headline"></a></h3>
<p>Note: cPython name look-ups are implemented with dicts – it’s highly optimized.</p>
<p>Key to value:</p>
<blockquote>
<div><ul class="simple">
<li><p>lookup is one way.</p></li>
</ul>
</div></blockquote>
<p>Value to key:</p>
<blockquote>
<div><ul class="simple">
<li><p>requires visiting the whole dict.</p></li>
</ul>
</div></blockquote>
<p>If you need to check dict values often, create another dict or set.</p>
<p>(But note that it’s then up to you to keep them in sync).</p>
</div>
<div class="section" id="dictionary-ordering-not">
<h3>Dictionary Ordering (not)<a class="headerlink" href="#dictionary-ordering-not" title="Permalink to this headline"></a></h3>
<p>Traditionally, dictionaries have had no defined order. See this example from Python 3.5:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [352]: </span><span class="n">d</span> <span class="o">=</span> <span class="p">{</span><span class="s1">'one'</span><span class="p">:</span><span class="mi">1</span><span class="p">,</span> <span class="s1">'two'</span><span class="p">:</span><span class="mi">2</span><span class="p">,</span> <span class="s1">'three'</span><span class="p">:</span><span class="mi">3</span><span class="p">}</span>
<span class="gp">In [353]: </span><span class="nb">str</span><span class="p">(</span><span class="n">d</span><span class="p">)</span>
<span class="gh">Out[353]: </span><span class="go">{'one': 1, 'three': 3, 'two': 2}</span>
<span class="gp">In [354]: </span><span class="n">d</span><span class="o">.</span><span class="n">keys</span><span class="p">()</span>
<span class="gh">Out[354]: </span><span class="go">dict_keys(['three', 'two', 'one'])</span>
</pre></div>
</div>
<p>Note how I defined the dict in a natural order, but when it gets printed, or you display the keys, they are in a different order.</p>
<p>However, In cPython 3.6, the internal implementation was changed, and it <em>does</em> happen to preserve order. In cPython 3.6, that is considered an implementation detail – and you should not count on it! However, as of cPython 3.7, dictionaries preserving order are part of the language specification. This was declared by Guido on the python-dev mailing list on
Dec 15, 2017 <<a class="reference external" href="https://mail.python.org/pipermail/python-dev/2017-December/151283.html">https://mail.python.org/pipermail/python-dev/2017-December/151283.html</a>>.</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="go">In Python 3.6, the above code results in:</span>
<span class="gp">In [9]: </span><span class="n">d</span> <span class="o">=</span> <span class="p">{</span><span class="s1">'one'</span><span class="p">:</span><span class="mi">1</span><span class="p">,</span> <span class="s1">'two'</span><span class="p">:</span><span class="mi">2</span><span class="p">,</span> <span class="s1">'three'</span><span class="p">:</span><span class="mi">3</span><span class="p">}</span>
<span class="gp">In [10]: </span><span class="nb">str</span><span class="p">(</span><span class="n">d</span><span class="p">)</span>
<span class="gh">Out[10]: </span><span class="go">"{'one': 1, 'two': 2, 'three': 3}"</span>
<span class="gp">In [11]: </span><span class="n">d</span><span class="o">.</span><span class="n">keys</span><span class="p">()</span>
<span class="gh">Out[11]: </span><span class="go">dict_keys(['one', 'two', 'three'])</span>
</pre></div>
</div>
<p>When new items are added to a dict, they go on the “end”:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [12]: </span><span class="n">d</span> <span class="o">=</span> <span class="p">{}</span>
<span class="gp">In [13]: </span><span class="n">d</span><span class="p">[</span><span class="s1">'one'</span><span class="p">]</span> <span class="o">=</span> <span class="mi">1</span>
<span class="gp">In [14]: </span><span class="n">d</span><span class="p">[</span><span class="s1">'two'</span><span class="p">]</span> <span class="o">=</span> <span class="mi">2</span>
<span class="gp">In [15]: </span><span class="n">d</span><span class="p">[</span><span class="s1">'three'</span><span class="p">]</span> <span class="o">=</span> <span class="mi">3</span>
<span class="gp">In [16]: </span><span class="nb">str</span><span class="p">(</span><span class="n">d</span><span class="p">)</span>
<span class="gh">Out[16]: </span><span class="go">"{'one': 1, 'two': 2, 'three': 3}"</span>
</pre></div>
</div>
<p>and <code class="docutils literal notranslate"><span class="pre">dict.popitem()</span></code> will remove the “last” item in the dict.</p>
<p><strong>CAUTION</strong> This is new behavior in cPython 3.6 – older versions of Python (notably including Python 2) do not preserve order. In older versions, there is a special version of a dict in the collections module: <code class="docutils literal notranslate"><span class="pre">collections.OrderedDict</span></code> which preserves order in all versions of Python, and has a couple extra features.</p>
<p>Also: while Python dicts now <em>preserve</em> order, they are not really a fully ordered object: there is no direct way to get, say, the “third” item in a dict, or to inset an item at a particular location.</p>
</div>
<div class="section" id="dictionary-iterating">
<h3>Dictionary Iterating<a class="headerlink" href="#dictionary-iterating" title="Permalink to this headline"></a></h3>
<p><code class="docutils literal notranslate"><span class="pre">for</span></code> iterates over the keys</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [23]: </span><span class="n">d</span> <span class="o">=</span> <span class="p">{</span><span class="s1">'name'</span><span class="p">:</span> <span class="s1">'Brian'</span><span class="p">,</span> <span class="s1">'score'</span><span class="p">:</span> <span class="mi">42</span><span class="p">}</span>
<span class="gp">In [24]: </span><span class="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="n">d</span><span class="p">:</span>
<span class="go"> ...: print(x)</span>
<span class="go"> ...:</span>
<span class="go">name</span>
<span class="go">score</span>
</pre></div>
</div>
</div>
<div class="section" id="dict-keys-and-values">
<h3>dict keys and values<a class="headerlink" href="#dict-keys-and-values" title="Permalink to this headline"></a></h3>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [25]: </span><span class="n">d</span> <span class="o">=</span> <span class="p">{</span><span class="s1">'name'</span><span class="p">:</span> <span class="s1">'Brian'</span><span class="p">,</span> <span class="s1">'score'</span><span class="p">:</span> <span class="mi">42</span><span class="p">}</span>
<span class="gp">In [26]: </span><span class="n">d</span><span class="o">.</span><span class="n">keys</span><span class="p">()</span>
<span class="gh">Out[26]: </span><span class="go">dict_keys(['name', 'score'])</span>
<span class="gp">In [27]: </span><span class="n">d</span><span class="o">.</span><span class="n">values</span><span class="p">()</span>
<span class="gh">Out[27]: </span><span class="go">dict_values(['Brian', 42])</span>
<span class="gp">In [28]: </span><span class="n">d</span><span class="o">.</span><span class="n">items</span><span class="p">()</span>
<span class="gh">Out[28]: </span><span class="go">dict_items([('name', 'Brian'), ('score', 42)])</span>
</pre></div>
</div>
<p>Notice that these are of type <code class="docutils literal notranslate"><span class="pre">dict_keys</span></code> and <code class="docutils literal notranslate"><span class="pre">dict_values</span></code>. These are special types that provide iteration, printing and other features, but are tied to the underlying dict, rather than copies. They are actually Sets (see below), so can be compared to sets, and the <code class="docutils literal notranslate"><span class="pre">in</span></code> operator is efficient.</p>
<p>(Python2 would simply create lists of keys and values – but then you were making a copy when you probably didn’t need one). If you do need a copy, or a proper Sequence, simmply wrap them in a <code class="docutils literal notranslate"><span class="pre">list()</span></code>:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="n">the_values_as_a_list</span> <span class="o">=</span> <span class="nb">list</span><span class="p">(</span><span class="n">a_dict</span><span class="o">.</span><span class="n">values</span><span class="p">())</span>
</pre></div>
</div>
</div>
<div class="section" id="id2">
<h3>dict keys and values<a class="headerlink" href="#id2" title="Permalink to this headline"></a></h3>
<p>Iterating on everything</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [26]: </span><span class="n">d</span> <span class="o">=</span> <span class="p">{</span><span class="s1">'name'</span><span class="p">:</span> <span class="s1">'Brian'</span><span class="p">,</span> <span class="s1">'score'</span><span class="p">:</span> <span class="mi">42</span><span class="p">}</span>
<span class="gp">In [27]: </span><span class="k">for</span> <span class="n">k</span><span class="p">,</span> <span class="n">v</span> <span class="ow">in</span> <span class="n">d</span><span class="o">.</span><span class="n">items</span><span class="p">():</span>
<span class="go"> print("%s: %s" % (k,v))</span>
<span class="gp"> ....:</span>
<span class="go">name: Brian</span>
<span class="go">score: 42</span>
</pre></div>
</div>
</div>
<div class="section" id="dictionary-performance">
<h3>Dictionary Performance<a class="headerlink" href="#dictionary-performance" title="Permalink to this headline"></a></h3>
<blockquote>
<div><blockquote>
<div><ul class="simple">
<li><p>Indexing is fast and constant time: O(1).</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">in</span> <span class="pre">s</span></code> is fast and constant time: O(1).</p></li>
<li><p>Visiting all items is proportional to n: O(n).</p></li>
<li><p>Inserting is constant time: O(1).</p></li>
<li><p>Deleting is constant time: O(1).</p></li>
</ul>
</div></blockquote>
<p><a class="reference external" href="http://wiki.python.org/moin/TimeComplexity">http://wiki.python.org/moin/TimeComplexity</a></p>
</div></blockquote>
</div>
<div class="section" id="other-dict-operations">
<h3>Other dict operations:<a class="headerlink" href="#other-dict-operations" title="Permalink to this headline"></a></h3>
<p>See them all here:</p>
<p><a class="reference external" href="https://docs.python.org/3/library/stdtypes.html#mapping-types-dict">https://docs.python.org/3/library/stdtypes.html#mapping-types-dict</a></p>
<p>Is it in there?</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [5]: </span><span class="n">d</span>
<span class="gh">Out[5]: </span><span class="go">{'that': 7, 'this': 5}</span>
<span class="gp">In [6]: </span><span class="s1">'that'</span> <span class="ow">in</span> <span class="n">d</span>
<span class="gh">Out[6]: </span><span class="go">True</span>
<span class="gp">In [7]: </span><span class="s1">'this'</span> <span class="ow">not</span> <span class="ow">in</span> <span class="n">d</span>
<span class="gh">Out[7]: </span><span class="go">False</span>
</pre></div>
</div>
<p>Containment is on the keys.</p>
<p>Think of it like a “real” dictionary, where the keys are the words, and the values are the definitions.</p>
<p>Is the word “gullible” in the dictionary? is asking if the key is in the dict.</p>
</div>
<div class="section" id="getting-something-like-indexing">
<h3>Getting something: (like indexing)<a class="headerlink" href="#getting-something-like-indexing" title="Permalink to this headline"></a></h3>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [9]: </span><span class="n">d</span><span class="o">.</span><span class="n">get</span><span class="p">(</span><span class="s1">'this'</span><span class="p">)</span>
<span class="gh">Out[9]: </span><span class="go">5</span>
</pre></div>
</div>
<p>But you can specify a default:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [11]: </span><span class="n">d</span><span class="o">.</span><span class="n">get</span><span class="p">(</span><span class="s1">'something'</span><span class="p">,</span> <span class="s1">'a default'</span><span class="p">)</span>
<span class="gh">Out[11]: </span><span class="go">'a default'</span>
</pre></div>
</div>
<p><cite>get()</cite> never raises an Exception (default is None).</p>
</div>
<div class="section" id="iterating">
<h3>iterating<a class="headerlink" href="#iterating" title="Permalink to this headline"></a></h3>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [13]: </span><span class="k">for</span> <span class="n">item</span> <span class="ow">in</span> <span class="n">d</span><span class="p">:</span>
<span class="gp"> ....: </span> <span class="nb">print</span><span class="p">(</span><span class="n">item</span><span class="p">)</span>
<span class="gp"> ....:</span>
<span class="go">this</span>
<span class="go">that</span>
</pre></div>
</div>
<p>Which is equivalent to, but a bit faster than:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [15]: </span><span class="k">for</span> <span class="n">key</span> <span class="ow">in</span> <span class="n">d</span><span class="o">.</span><span class="n">keys</span><span class="p">():</span>
<span class="go"> print(key)</span>
<span class="gp"> ....:</span>
<span class="go">this</span>
<span class="go">that</span>
</pre></div>
</div>
<p>In fact, there are very few things you can do with the <code class="docutils literal notranslate"><span class="pre">dict_keys</span></code> that you can’t do directly with the dict.</p>
<p>But to get values, you must specify you want the values:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [16]: </span><span class="k">for</span> <span class="n">val</span> <span class="ow">in</span> <span class="n">d</span><span class="o">.</span><span class="n">values</span><span class="p">():</span>
<span class="go"> print(val)</span>
<span class="gp"> ....:</span>
<span class="go">5</span>
<span class="go">7</span>
</pre></div>
</div>
<p>and to get both, you use <code class="docutils literal notranslate"><span class="pre">.items</span></code>:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [4]: </span><span class="k">for</span> <span class="n">item</span> <span class="ow">in</span> <span class="n">d</span><span class="o">.</span><span class="n">items</span><span class="p">():</span>
<span class="gp"> ...: </span> <span class="nb">print</span><span class="p">(</span><span class="n">item</span><span class="p">)</span>
<span class="gp"> ...:</span>
<span class="go">('this', 5)</span>
<span class="go">('that', 7)</span>
</pre></div>
</div>
</div>
<div class="section" id="pop">
<h3><code class="docutils literal notranslate"><span class="pre">pop()</span></code><a class="headerlink" href="#pop" title="Permalink to this headline"></a></h3>
<p>“Popping”: getting the value while removing it.</p>
<p>Pop out a particular key:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [19]: </span><span class="n">d</span><span class="o">.</span><span class="n">pop</span><span class="p">(</span><span class="s1">'this'</span><span class="p">)</span>
<span class="gh">Out[19]: </span><span class="go">5</span>
<span class="gp">In [20]: </span><span class="n">d</span>
<span class="gh">Out[20]: </span><span class="go">{'that': 7}</span>
</pre></div>
</div>
<p>pop out an arbitrary key, value pair</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [23]: </span><span class="n">d</span><span class="o">.</span><span class="n">popitem</span><span class="p">()</span>
<span class="gh">Out[23]: </span><span class="go">('that', 7)</span>
<span class="gp">In [24]: </span><span class="n">d</span>
<span class="gh">Out[24]: </span><span class="go">{}</span>
</pre></div>
</div>
<p>note that it’s the last item, so not completely arbitrary.</p>
</div>
<div class="section" id="setdefault">
<h3><code class="docutils literal notranslate"><span class="pre">setdefault</span></code><a class="headerlink" href="#setdefault" title="Permalink to this headline"></a></h3>
<p>This one is handy:</p>
<p><code class="docutils literal notranslate"><span class="pre">setdefault(key[,</span> <span class="pre">default])</span></code></p>
<p>gets the value if it’s there, sets it to the specified default if it’s not. Returns the value in either case.</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [4]: </span><span class="n">d</span> <span class="o">=</span> <span class="p">{}</span>
<span class="gp">In [5]: </span><span class="n">d</span><span class="o">.</span><span class="n">setdefault</span><span class="p">(</span><span class="s1">'something'</span><span class="p">,</span> <span class="s1">'a value'</span><span class="p">)</span>
<span class="gh">Out[5]: </span><span class="go">'a value'</span>
<span class="gp">In [6]: </span><span class="n">d</span>
<span class="gh">Out[6]: </span><span class="go">{'something': 'a value'}</span>
</pre></div>
</div>
<p>The next time you call it, it gets the already set value:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [7]: </span><span class="n">d</span><span class="o">.</span><span class="n">setdefault</span><span class="p">(</span><span class="s1">'something'</span><span class="p">,</span> <span class="s1">'a different value'</span><span class="p">)</span>
<span class="gh">Out[7]: </span><span class="go">'a value'</span>
</pre></div>
</div>
</div>
<div class="section" id="assignment">
<h3>Assignment<a class="headerlink" href="#assignment" title="Permalink to this headline"></a></h3>
<p>Assignment (with <code class="docutils literal notranslate"><span class="pre">=</span></code>) is a link to the original dict, just like lists or anything else. Remember that assignment is simply binding a new name to something.</p>
<p>And dicts are mutable – so be careful!</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [47]: </span><span class="n">d</span>
<span class="gh">Out[47]: </span><span class="go">{'something': 'a value'}</span>
<span class="gp">In [48]: </span><span class="n">item_view</span> <span class="o">=</span> <span class="n">d</span>
<span class="gp">In [49]: </span><span class="n">d</span><span class="p">[</span><span class="s1">'something else'</span><span class="p">]</span> <span class="o">=</span> <span class="s1">'another value'</span>
<span class="gp">In [50]: </span><span class="n">item_view</span>
<span class="gh">Out[50]: </span><span class="go">{'something': 'a value', 'something else': 'another value'}</span>
</pre></div>
</div>
<p>If you want a copy, use the explicit copy method to get a copy:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="go">In [51] item_copy = d.copy()</span>
<span class="gp">In [52]: </span><span class="n">d</span><span class="p">[</span><span class="s1">'another thing'</span><span class="p">]</span> <span class="o">=</span> <span class="s1">'different value'</span>
<span class="gp">In [53]: </span><span class="n">d</span>
<span class="gh">Out[53]:</span>
<span class="go">{'another thing': 'different value',</span>
<span class="go"> 'something': 'a value',</span>
<span class="go"> 'something else': 'another value'}</span>
<span class="gp">In [54]: </span><span class="n">item_copy</span>
<span class="gh">Out[54]: </span><span class="go">{'something': 'a value', 'something else': 'another value'}</span>
</pre></div>
</div>
<p>or pass any mapping into the dict constructor:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="o">..</span> <span class="n">code</span><span class="o">-</span><span class="n">block</span><span class="p">::</span> <span class="n">python</span>
</pre></div>
</div>
<blockquote>
<div><p>new_dict = dict(old_dict)</p>
</div></blockquote>
</div>
</div>
<div class="section" id="sets">
<h2>Sets<a class="headerlink" href="#sets" title="Permalink to this headline"></a></h2>
<p>a <code class="docutils literal notranslate"><span class="pre">set</span></code> is an unordered collection of distinct values.</p>
<p>Essentially, a <code class="docutils literal notranslate"><span class="pre">set</span></code> is a dict with only keys.</p>
<p><a class="reference external" href="https://docs.python.org/3.8/library/stdtypes.html#set-types-set-frozenset">https://docs.python.org/3.8/library/stdtypes.html#set-types-set-frozenset</a></p>
<div class="section" id="set-constructors">
<h3>Set Constructors:<a class="headerlink" href="#set-constructors" title="Permalink to this headline"></a></h3>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="o">>>></span> <span class="nb">set</span><span class="p">()</span>
<span class="nb">set</span><span class="p">()</span>
<span class="o">>>></span> <span class="nb">set</span><span class="p">([</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">])</span>
<span class="p">{</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">}</span>
<span class="o">>>></span> <span class="p">{</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">}</span>
<span class="p">{</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">}</span>
<span class="o">>>></span> <span class="n">s</span> <span class="o">=</span> <span class="nb">set</span><span class="p">()</span>
<span class="o">>>></span> <span class="n">s</span><span class="o">.</span><span class="n">update</span><span class="p">([</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">])</span>
<span class="o">>>></span> <span class="n">s</span>
<span class="p">{</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">}</span>
</pre></div>
</div>
</div>
<div class="section" id="set-properties">
<h3>Set Properties<a class="headerlink" href="#set-properties" title="Permalink to this headline"></a></h3>
<p><code class="docutils literal notranslate"><span class="pre">Set</span></code> members must be hashable, like dictionary keys – and for same reason (efficient lookup).</p>
<p>No indexing (unordered).</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="o">>>></span> <span class="n">s</span><span class="p">[</span><span class="mi">1</span><span class="p">]</span>
<span class="n">Traceback</span> <span class="p">(</span><span class="n">most</span> <span class="n">recent</span> <span class="n">call</span> <span class="n">last</span><span class="p">):</span>
<span class="n">File</span> <span class="s2">"<stdin>"</span><span class="p">,</span> <span class="n">line</span> <span class="mi">1</span><span class="p">,</span> <span class="ow">in</span> <span class="o"><</span><span class="n">module</span><span class="o">></span>
<span class="ne">TypeError</span><span class="p">:</span> <span class="s1">'set'</span> <span class="nb">object</span> <span class="n">does</span> <span class="ow">not</span> <span class="n">support</span> <span class="n">indexing</span>
</pre></div>
</div>
</div>
<div class="section" id="set-methods">
<h3>Set Methods<a class="headerlink" href="#set-methods" title="Permalink to this headline"></a></h3>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="o">>></span> <span class="n">s</span> <span class="o">=</span> <span class="nb">set</span><span class="p">([</span><span class="mi">1</span><span class="p">])</span>
<span class="o">>>></span> <span class="n">s</span><span class="o">.</span><span class="n">pop</span><span class="p">()</span> <span class="c1"># an arbitrary member</span>
<span class="mi">1</span>
<span class="o">>>></span> <span class="n">s</span><span class="o">.</span><span class="n">pop</span><span class="p">()</span>
<span class="n">Traceback</span> <span class="p">(</span><span class="n">most</span> <span class="n">recent</span> <span class="n">call</span> <span class="n">last</span><span class="p">):</span>
<span class="n">File</span> <span class="s2">"<stdin>"</span><span class="p">,</span> <span class="n">line</span> <span class="mi">1</span><span class="p">,</span> <span class="ow">in</span> <span class="o"><</span><span class="n">module</span><span class="o">></span>
<span class="ne">KeyError</span><span class="p">:</span> <span class="s1">'pop from an empty set'</span>
<span class="o">>>></span> <span class="n">s</span> <span class="o">=</span> <span class="nb">set</span><span class="p">([</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">])</span>
<span class="o">>>></span> <span class="n">s</span><span class="o">.</span><span class="n">remove</span><span class="p">(</span><span class="mi">2</span><span class="p">)</span>
<span class="o">>>></span> <span class="n">s</span><span class="o">.</span><span class="n">remove</span><span class="p">(</span><span class="mi">2</span><span class="p">)</span>
<span class="n">Traceback</span> <span class="p">(</span><span class="n">most</span> <span class="n">recent</span> <span class="n">call</span> <span class="n">last</span><span class="p">):</span>
<span class="n">File</span> <span class="s2">"<stdin>"</span><span class="p">,</span> <span class="n">line</span> <span class="mi">1</span><span class="p">,</span> <span class="ow">in</span> <span class="o"><</span><span class="n">module</span><span class="o">></span>
<span class="ne">KeyError</span><span class="p">:</span> <span class="mi">2</span>
</pre></div>
</div>
<p>All the “set” operations from math class…</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="n">s</span><span class="o">.</span><span class="n">isdisjoint</span><span class="p">(</span><span class="n">other</span><span class="p">)</span>
<span class="n">s</span><span class="o">.</span><span class="n">issubset</span><span class="p">(</span><span class="n">other</span><span class="p">)</span>
<span class="n">s</span><span class="o">.</span><span class="n">union</span><span class="p">(</span><span class="n">other</span><span class="p">,</span> <span class="o">...</span><span class="p">)</span>
<span class="n">s</span><span class="o">.</span><span class="n">intersection</span><span class="p">(</span><span class="n">other</span><span class="p">,</span> <span class="o">...</span><span class="p">)</span>
<span class="n">s</span><span class="o">.</span><span class="n">difference</span><span class="p">(</span><span class="n">other</span><span class="p">,</span> <span class="o">...</span><span class="p">)</span>
<span class="n">s</span><span class="o">.</span><span class="n">symmetric_difference</span><span class="p">(</span> <span class="n">other</span><span class="p">,</span> <span class="o">...</span><span class="p">)</span>
</pre></div>
</div>
</div>
<div class="section" id="set-operators">
<h3>Set Operators<a class="headerlink" href="#set-operators" title="Permalink to this headline"></a></h3>
<p>All the basic set operations are support with math-class like operators:</p>
<p>Test whether every element in the set is in other:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="nb">set</span> <span class="o"><=</span> <span class="n">other</span>
</pre></div>
</div>
<p>Test whether the set is a proper subset of other, that is, set <= other and set != other:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="nb">set</span> <span class="o"><</span> <span class="n">other</span>
</pre></div>
</div>
<p>Test whether every element in other is in the set:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="nb">set</span> <span class="o">>=</span> <span class="n">other</span>
</pre></div>
</div>
<p>Test whether the set is a proper superset of other, that is, <code class="docutils literal notranslate"><span class="pre">set</span> <span class="pre">>=</span> <span class="pre">other</span> <span class="pre">and</span> <span class="pre">set</span> <span class="pre">!=</span> <span class="pre">other</span></code>:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="nb">set</span> <span class="o">></span> <span class="n">other</span>
</pre></div>
</div>
<p>Union: Return a new set with elements from the set and all others:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="nb">set</span> <span class="o">|</span> <span class="n">other</span> <span class="o">|</span> <span class="o">...</span>
</pre></div>
</div>
<p>Intersection: Return a new set with elements common to the set and all others:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="nb">set</span> <span class="o">&</span> <span class="n">other</span> <span class="o">&</span> <span class="o">...</span>
</pre></div>
</div>
<p>Difference: Return a new set with elements in the set that are not in the others:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="nb">set</span> <span class="o">-</span> <span class="n">other</span> <span class="o">-</span> <span class="o">...</span>
</pre></div>
</div>
<p>Symmetric difference: return a new set with elements in either the set or other but not both:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="nb">set</span> <span class="o">^</span> <span class="n">other</span>
</pre></div>
</div>
<p>In fact, it is the operator versions that make the <code class="docutils literal notranslate"><span class="pre">set</span></code> object “officially” a Set: (<a class="reference external" href="https://docs.python.org/3.8/library/collections.abc.html?highlight=set%20abc#collections.abc.Set">Set ABC</a>)</p>
</div>
<div class="section" id="frozen-set">
<h3>Frozen Set<a class="headerlink" href="#frozen-set" title="Permalink to this headline"></a></h3>
<p>Another kind of set: <code class="docutils literal notranslate"><span class="pre">frozenset</span></code></p>
<p>immutable – for use as a key in a dict (or another set…):</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">fs</span> <span class="o">=</span> <span class="nb">frozenset</span><span class="p">((</span><span class="mi">3</span><span class="p">,</span><span class="mi">8</span><span class="p">,</span><span class="mi">5</span><span class="p">))</span>
<span class="gp">>>> </span><span class="n">fs</span><span class="o">.</span><span class="n">add</span><span class="p">(</span><span class="mi">9</span><span class="p">)</span>
<span class="gt">Traceback (most recent call last):</span>
File <span class="nb">"<stdin>"</span>, line <span class="m">1</span>, in <span class="n"><module></span>
<span class="gr">AttributeError</span>: <span class="n">'frozenset' object has no attribute 'add'</span>
</pre></div>
</div>
</div>
</div>
<div class="section" id="a-few-added-notes">
<h2>A few added notes:<a class="headerlink" href="#a-few-added-notes" title="Permalink to this headline"></a></h2>
<div class="section" id="the-count-method">
<h3>The count() method<a class="headerlink" href="#the-count-method" title="Permalink to this headline"></a></h3>
<p>All Python sequences (including strings) have a <code class="docutils literal notranslate"><span class="pre">count()</span></code> method:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [1]: </span><span class="n">s</span> <span class="o">=</span> <span class="s2">"This is an arbitrary string"</span>
<span class="gp">In [2]: </span><span class="n">s</span><span class="o">.</span><span class="n">count</span><span class="p">(</span><span class="s1">'t'</span><span class="p">)</span>
<span class="gh">Out[2]: </span><span class="go">2</span>
</pre></div>
</div>
<p>What if you want a case-insensitive count?</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [3]: </span><span class="n">s</span><span class="o">.</span><span class="n">lower</span><span class="p">()</span><span class="o">.</span><span class="n">count</span><span class="p">(</span><span class="s1">'t'</span><span class="p">)</span>
<span class="gh">Out[3]: </span><span class="go">3</span>
</pre></div>
</div>
</div>
<div class="section" id="set-update">
<h3>set.update()<a class="headerlink" href="#set-update" title="Permalink to this headline"></a></h3>
<p>If you want to add a bunch of stuff to a set, you can use update:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [1]: </span><span class="n">s</span> <span class="o">=</span> <span class="nb">set</span><span class="p">()</span>
<span class="gp">In [2]: </span><span class="n">s</span><span class="o">.</span><span class="n">update</span>
<span class="gh">Out[2]: </span><span class="go"><function set.update></span>
<span class="gp">In [3]: </span><span class="n">s</span><span class="o">.</span><span class="n">update</span><span class="p">([</span><span class="s1">'this'</span><span class="p">,</span> <span class="s1">'that'</span><span class="p">])</span>
<span class="gp">In [4]: </span><span class="n">s</span>
<span class="gh">Out[4]: </span><span class="go">{'that', 'this'}</span>
<span class="gp">In [5]: </span><span class="n">s</span><span class="o">.</span><span class="n">update</span><span class="p">([</span><span class="s1">'this'</span><span class="p">,</span> <span class="s1">'thatthing'</span><span class="p">])</span>
<span class="gp">In [6]: </span><span class="n">s</span>
<span class="gh">Out[6]: </span><span class="go">{'that', 'thatthing', 'this'}</span>
</pre></div>
</div>
<p><strong>NOTE:</strong> It’s VERY often the case that when you find yourself writing a trivial loop – there is a way to do it with a built in method!</p>
</div>
<div class="section" id="sorting-stuff-in-dictionaries">
<h3>Sorting stuff in dictionaries:<a class="headerlink" href="#sorting-stuff-in-dictionaries" title="Permalink to this headline"></a></h3>
<p>dicts aren’t sorted, so what if you want to do something in a sorted way?</p>
<p>The “standard” way:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="k">for</span> <span class="n">key</span> <span class="ow">in</span> <span class="nb">sorted</span><span class="p">(</span><span class="n">d</span><span class="o">.</span><span class="n">keys</span><span class="p">()):</span>
<span class="o">...</span>
</pre></div>
</div>
<p>As dicts DO preserve order, you can make a sorted version of a dict:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="n">sorted_dict</span> <span class="o">=</span> <span class="nb">dict</span><span class="p">(</span><span class="nb">sorted</span><span class="p">(</span><span class="nb">dict</span><span class="o">.</span><span class="n">items</span><span class="p">(),</span> <span class="n">key</span><span class="o">=</span><span class="n">sort_key</span><span class="p">))</span>
</pre></div>
</div>
<p>where sort_key is a function that takes the (key, value) tuple and returns a value to sort on.</p>
<p>Another option:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="n">collections</span><span class="o">.</span><span class="n">OrderedDict</span>
</pre></div>
</div>
<p>Also other nifty stuff in the <code class="docutils literal notranslate"><span class="pre">collections</span></code> module:</p>
<p><a class="reference external" href="https://docs.python.org/3.6/library/collections.html">https://docs.python.org/3.6/library/collections.html</a></p>
<p><strong>NOTE:</strong> As of Python 3.6, dicts do preserve order. But they are not full featured ordered objects. If you want a “properly” ordered object, use <code class="docutils literal notranslate"><span class="pre">OrderedDict</span></code>.</p>
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