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<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="highlight"><pre>>>> double_digit_evens = [e*2 for e in range(5, 50)]
>>> double_digit_evens
[10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, 32, 34, 36, 38, 40, 42, 44, 46, 48, 50, 52, 54, 56, 58, 60, 62, 64, 66, 68, 70, 72, 74, 76, 78, 80, 82, 84, 86, 88, 90, 92, 94, 96, 98]
</pre></div>
<p>Set comprehension:</p>
<div class="highlight"><pre>>>> double_digit_odds = {e*2+1 for e in range(5, 50)}
{11, 13, 15, 17, 19, 21, 23, 25, 27, 29, 31, 33, 35, 37, 39, 41, 43, 45, 47, 49, 51, 53, 55, 57, 59, 61, 63, 65, 67, 69, 71, 73, 75, 77, 79, 81, 83, 85, 87, 89, 91, 93, 95, 97, 99}
</pre></div>
<p>Dictionary comprehension:</p>
<div class="highlight"><pre>>>> {e: e*10 for e in range(1, 11)}
{1: 10, 2: 20, 3: 30, 4: 40, 5: 50, 6: 60, 7: 70, 8: 80, 9: 90, 10: 100}
</pre></div>
<h2>Comprehension resources</h2>
<ul>
<li>
<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>
</li>
<li>
<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>
</li>
<li>
<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>
</li>
<li>
<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>
</li>
<li>
<p><a href="http://effbot.org/zone/python-list.htm">An Introduction to Python Lists</a>
is a solid overview of Python lists in general and tangentially covers
list comprehensions.</p>
</li>
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