<h1>Comprehensions</h1>

<p>Comprehensions are a Python language construct for concisely creating data

in lists, dictionaries and sets. List comprehensions are included in Python 2

while dictionary and set comprehensions were introduced to the language in

Python 3.</p>

<h2>Why are comprehensions important?</h2>

<p>Comprehensions are a more clear syntax for populating conditional data in the

core Python data structures. Creating data without comprehensions often

involves nested loops with conditionals that can be difficult for code

readers to properly evaluate.</p>

<h2>Example code</h2>

<p>List comprehension:</p>

<div class="codehilite"><pre><span class="o">&gt;&gt;&gt;</span> <span class="n">double_digit_evens</span> <span class="o">=</span> <span class="p">[</span><span class="n">e</span><span class="o">*</span><span class="mi">2</span> <span class="k">for</span> <span class="n">e</span> <span class="n">in</span> <span class="n">range</span><span class="p">(</span><span class="mi">5</span><span class="p">,</span> <span class="mi">50</span><span class="p">)]</span>

<span class="o">&gt;&gt;&gt;</span> <span class="n">double_digit_evens</span>

<span class="p">[</span><span class="mi">10</span><span class="p">,</span> <span class="mi">12</span><span class="p">,</span> <span class="mi">14</span><span class="p">,</span> <span class="mi">16</span><span class="p">,</span> <span class="mi">18</span><span class="p">,</span> <span class="mi">20</span><span class="p">,</span> <span class="mi">22</span><span class="p">,</span> <span class="mi">24</span><span class="p">,</span> <span class="mi">26</span><span class="p">,</span> <span class="mi">28</span><span class="p">,</span> <span class="mi">30</span><span class="p">,</span> <span class="mi">32</span><span class="p">,</span> <span class="mi">34</span><span class="p">,</span> <span class="mi">36</span><span class="p">,</span> <span class="mi">38</span><span class="p">,</span> <span class="mi">40</span><span class="p">,</span> <span class="mi">42</span><span class="p">,</span> <span class="mi">44</span><span class="p">,</span> <span class="mi">46</span><span class="p">,</span> <span class="mi">48</span><span class="p">,</span> <span class="mi">50</span><span class="p">,</span> <span class="mi">52</span><span class="p">,</span> <span class="mi">54</span><span class="p">,</span> <span class="mi">56</span><span class="p">,</span> <span class="mi">58</span><span class="p">,</span> <span class="mi">60</span><span class="p">,</span> <span class="mi">62</span><span class="p">,</span> <span class="mi">64</span><span class="p">,</span> <span class="mi">66</span><span class="p">,</span> <span class="mi">68</span><span class="p">,</span> <span class="mi">70</span><span class="p">,</span> <span class="mi">72</span><span class="p">,</span> <span class="mi">74</span><span class="p">,</span> <span class="mi">76</span><span class="p">,</span> <span class="mi">78</span><span class="p">,</span> <span class="mi">80</span><span class="p">,</span> <span class="mi">82</span><span class="p">,</span> <span class="mi">84</span><span class="p">,</span> <span class="mi">86</span><span class="p">,</span> <span class="mi">88</span><span class="p">,</span> <span class="mi">90</span><span class="p">,</span> <span class="mi">92</span><span class="p">,</span> <span class="mi">94</span><span class="p">,</span> <span class="mi">96</span><span class="p">,</span> <span class="mi">98</span><span class="p">]</span>

<p>Set comprehension:</p>

<div class="codehilite"><pre><span class="o">&gt;&gt;&gt;</span> <span class="n">double_digit_odds</span> <span class="o">=</span> <span class="p">{</span><span class="n">e</span><span class="o">*</span><span class="mi">2</span><span class="o">+</span><span class="mi">1</span> <span class="k">for</span> <span class="n">e</span> <span class="n">in</span> <span class="n">range</span><span class="p">(</span><span class="mi">5</span><span class="p">,</span> <span class="mi">50</span><span class="p">)}</span>

<span class="p">{</span><span class="mi">11</span><span class="p">,</span> <span class="mi">13</span><span class="p">,</span> <span class="mi">15</span><span class="p">,</span> <span class="mi">17</span><span class="p">,</span> <span class="mi">19</span><span class="p">,</span> <span class="mi">21</span><span class="p">,</span> <span class="mi">23</span><span class="p">,</span> <span class="mi">25</span><span class="p">,</span> <span class="mi">27</span><span class="p">,</span> <span class="mi">29</span><span class="p">,</span> <span class="mi">31</span><span class="p">,</span> <span class="mi">33</span><span class="p">,</span> <span class="mi">35</span><span class="p">,</span> <span class="mi">37</span><span class="p">,</span> <span class="mi">39</span><span class="p">,</span> <span class="mi">41</span><span class="p">,</span> <span class="mi">43</span><span class="p">,</span> <span class="mi">45</span><span class="p">,</span> <span class="mi">47</span><span class="p">,</span> <span class="mi">49</span><span class="p">,</span> <span class="mi">51</span><span class="p">,</span> <span class="mi">53</span><span class="p">,</span> <span class="mi">55</span><span class="p">,</span> <span class="mi">57</span><span class="p">,</span> <span class="mi">59</span><span class="p">,</span> <span class="mi">61</span><span class="p">,</span> <span class="mi">63</span><span class="p">,</span> <span class="mi">65</span><span class="p">,</span> <span class="mi">67</span><span class="p">,</span> <span class="mi">69</span><span class="p">,</span> <span class="mi">71</span><span class="p">,</span> <span class="mi">73</span><span class="p">,</span> <span class="mi">75</span><span class="p">,</span> <span class="mi">77</span><span class="p">,</span> <span class="mi">79</span><span class="p">,</span> <span class="mi">81</span><span class="p">,</span> <span class="mi">83</span><span class="p">,</span> <span class="mi">85</span><span class="p">,</span> <span class="mi">87</span><span class="p">,</span> <span class="mi">89</span><span class="p">,</span> <span class="mi">91</span><span class="p">,</span> <span class="mi">93</span><span class="p">,</span> <span class="mi">95</span><span class="p">,</span> <span class="mi">97</span><span class="p">,</span> <span class="mi">99</span><span class="p">}</span>

<p>Dictionary comprehension:</p>

<div class="codehilite"><pre><span class="o">&gt;&gt;&gt;</span> <span class="p">{</span><span class="n">e</span><span class="o">:</span> <span class="n">e</span><span class="o">*</span><span class="mi">10</span> <span class="k">for</span> <span class="n">e</span> <span class="n">in</span> <span class="n">range</span><span class="p">(</span><span class="mi">1</span><span class="p">,</span> <span class="mi">11</span><span class="p">)}</span>

<span class="p">{</span><span class="mi">1</span><span class="o">:</span> <span class="mi">10</span><span class="p">,</span> <span class="mi">2</span><span class="o">:</span> <span class="mi">20</span><span class="p">,</span> <span class="mi">3</span><span class="o">:</span> <span class="mi">30</span><span class="p">,</span> <span class="mi">4</span><span class="o">:</span> <span class="mi">40</span><span class="p">,</span> <span class="mi">5</span><span class="o">:</span> <span class="mi">50</span><span class="p">,</span> <span class="mi">6</span><span class="o">:</span> <span class="mi">60</span><span class="p">,</span> <span class="mi">7</span><span class="o">:</span> <span class="mi">70</span><span class="p">,</span> <span class="mi">8</span><span class="o">:</span> <span class="mi">80</span><span class="p">,</span> <span class="mi">9</span><span class="o">:</span> <span class="mi">90</span><span class="p">,</span> <span class="mi">10</span><span class="o">:</span> <span class="mi">100</span><span class="p">}</span>

<h2>Comprehension resources</h2>

<p>Intermediate Python's

<a href="http://intermediatepythonista.com/python-comprehensions">Python Comprehensions</a>

post gives a well written overview of comprehensions for the three core

Python data structures.</p>

<p>The Python 3 Patterns and Idioms site has an overview of

<a href="http://python-3-patterns-idioms-test.readthedocs.org/en/latest/Comprehensions.html">comprehensions</a>

including code examples and diagrams to explain how they work.</p>

<p><a href="http://blog.cdleary.com/2010/04/learning-python-by-example-list-comprehensions/">Learning Python by example: list comprehensions</a>

gives an example of an incorrect list comprehension then shows how to

correct its issues.</p>

<p><a href="http://www.pythonforbeginners.com/basics/list-comprehensions-in-python">List comprehensions in Python</a>

covers what the code for list comprehensions looks like and gives some

example code to show how they work.</p>

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