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<div class="section" id="comprehensions">
<h1>Comprehensions</h1>
<p>Comprehensions are a feature of Python which I would really miss if I
ever have to leave it. Comprehensions are constructs that allow
sequences to be built from other sequences. There are three type of
comprehensions in Python:</p>
<ul class="simple">
<li>list comprehensions</li>
<li>dictionary comprehensions</li>
<li>set comprehensions</li>
</ul>
<p><code class="docutils literal"><span class="pre">list</span></code> comprehensions were introduced in Python 2. <code class="docutils literal"><span class="pre">set</span></code> and
<code class="docutils literal"><span class="pre">dictionary</span></code> comprehensions became a part of Python in version 3.0.</p>
<p>We will discuss them one by one. Once you get the hang of using <code class="docutils literal"><span class="pre">list</span></code>
comprehensions then you can use anyone of them easily.</p>
<div class="section" id="list-comprehensions">
<h2><code class="docutils literal"><span class="pre">list</span></code> comprehensions</h2>
<p>List comprehensions provide a short and concise way to create lists. It
consists of square brackets containing an expression followed by a
<code class="docutils literal"><span class="pre">for</span></code> clause, then zero or more <code class="docutils literal"><span class="pre">for</span></code> or <code class="docutils literal"><span class="pre">if</span></code> clauses. The
expressions can be anything, meaning you can put in all kinds of objects
in lists. The result would be a new list made after the evaluation of
the expression in context of the <code class="docutils literal"><span class="pre">if</span></code> and <code class="docutils literal"><span class="pre">for</span></code> clauses.</p>
<p><strong>Blueprint</strong></p>
<div class="code python highlight-python"><div class="highlight"><pre><span class="n">variable</span> <span class="o">=</span> <span class="p">[</span><span class="n">out_exp</span> <span class="k">for</span> <span class="n">out_exp</span> <span class="ow">in</span> <span class="n">input_list</span> <span class="k">if</span> <span class="n">out_exp</span> <span class="o">==</span> <span class="mi">2</span><span class="p">]</span>
</pre></div>
</div>
<p>Here is a short example:</p>
<div class="code python highlight-python"><div class="highlight"><pre><span class="n">multiples</span> <span class="o">=</span> <span class="p">[</span><span class="n">i</span> <span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">30</span><span class="p">)</span> <span class="k">if</span> <span class="n">i</span> <span class="o">%</span> <span class="mi">3</span> <span class="ow">is</span> <span class="mi">0</span><span class="p">]</span>
<span class="k">print</span><span class="p">(</span><span class="n">multiples</span><span class="p">)</span>
<span class="c"># Output: [0, 3, 6, 9, 12, 15, 18, 21, 24, 27]</span>
</pre></div>
</div>
<p>This can be really useful to make lists quickly. It is even preferred by
some instead of <code class="docutils literal"><span class="pre">filter</span></code> function. <code class="docutils literal"><span class="pre">list</span></code> comprehensions really
shine when you want to supply a list to a method or function to make a
new list by appending to it in each iteration of the <code class="docutils literal"><span class="pre">for</span></code> loop. For
instance you would usually do something like this:</p>
<div class="code python highlight-python"><div class="highlight"><pre><span class="n">squared</span> <span class="o">=</span> <span class="p">[]</span>
<span class="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">10</span><span class="p">):</span>
<span class="n">squared</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">x</span><span class="o">**</span><span class="mi">2</span><span class="p">)</span>
</pre></div>
</div>
<p>You can simplify it using <code class="docutils literal"><span class="pre">list</span></code> comprehensions. For example:</p>
<div class="code python highlight-python"><div class="highlight"><pre><span class="n">squared</span> <span class="o">=</span> <span class="p">[</span><span class="n">x</span><span class="o">**</span><span class="mi">2</span> <span class="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">10</span><span class="p">)]</span>
</pre></div>
</div>
</div>
<div class="section" id="dict-comprehensions">
<h2><code class="docutils literal"><span class="pre">dict</span></code> comprehensions</h2>
<p>They are used in a similar way. Here is an example which I found
recently:</p>
<div class="code python highlight-python"><div class="highlight"><pre><span class="n">mcase</span> <span class="o">=</span> <span class="p">{</span><span class="s">'a'</span><span class="p">:</span><span class="mi">10</span><span class="p">,</span> <span class="s">'b'</span><span class="p">:</span> <span class="mi">34</span><span class="p">,</span> <span class="s">'A'</span><span class="p">:</span> <span class="mi">7</span><span class="p">,</span> <span class="s">'Z'</span><span class="p">:</span><span class="mi">3</span><span class="p">}</span>
<span class="n">mcase_frequency</span> <span class="o">=</span> <span class="p">{</span> <span class="n">k</span><span class="o">.</span><span class="n">lower</span><span class="p">()</span> <span class="p">:</span> <span class="n">mcase</span><span class="o">.</span><span class="n">get</span><span class="p">(</span><span class="n">k</span><span class="o">.</span><span class="n">lower</span><span class="p">(),</span> <span class="mi">0</span><span class="p">)</span> <span class="o">+</span> \
<span class="n">mcase</span><span class="o">.</span><span class="n">get</span><span class="p">(</span><span class="n">k</span><span class="o">.</span><span class="n">upper</span><span class="p">(),</span> <span class="mi">0</span><span class="p">)</span> <span class="k">for</span> <span class="n">k</span> <span class="ow">in</span> <span class="n">mcase</span><span class="o">.</span><span class="n">keys</span><span class="p">()</span> <span class="p">}</span>
<span class="c"># mcase_frequency == {'a': 17, 'z': 3, 'b': 34}</span>
</pre></div>
</div>
<p>In the above example we are combining the values of keys which are same
but in different typecase. I personally do not use <code class="docutils literal"><span class="pre">dict</span></code>
comprehensions a lot. You can also quickly reverse a dictionary:</p>
<div class="code python highlight-python"><div class="highlight"><pre><span class="p">{</span><span class="n">v</span><span class="p">:</span> <span class="n">k</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">some_dict</span><span class="o">.</span><span class="n">items</span><span class="p">()}</span>
</pre></div>
</div>
</div>
<div class="section" id="set-comprehensions">
<h2><code class="docutils literal"><span class="pre">set</span></code> comprehensions</h2>
<p>They are also similar to list comprehensions. The only difference is
that they use round brackets <code class="docutils literal"><span class="pre">()</span></code> and return a generator. Here is an
example:</p>
<div class="code python highlight-python"><div class="highlight"><pre><span class="n">squared</span> <span class="o">=</span> <span class="p">(</span><span class="n">x</span><span class="o">**</span><span class="mi">2</span> <span class="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">10</span><span class="p">))</span>
<span class="k">print</span><span class="p">(</span><span class="n">squared</span><span class="p">)</span>
<span class="c"># Output: <generator object <genexpr> at 0x00000000029931B0></span>
<span class="n">squared</span><span class="o">.</span><span class="n">next</span><span class="p">()</span>
<span class="c"># Output: 0</span>
</pre></div>
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