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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><a href="http://treyhunner.com/2015/12/python-list-comprehensions-now-in-color/">Python List Comprehensions: Explained Visually</a>
explains how the common idiom for iteration became syntactic sugar in
the language itself and how you can use it in your own programs.</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="https://gist.github.com/bearfrieze/a746c6f12d8bada03589">Comprehensions in Python the Jedi way</a>
shows off comprehensions with a Star Wars theme to walk through the nuts
and bolts. All examples use Python 3.5.</p>
</li>
<li>
<p><a href="https://blogs.msdn.microsoft.com/pythonengineering/2016/03/14/idiomatic-python-comprehensions/">Idiomatic Python: Comprehensions</a>
explains how Python's comprehensions were inspired by Haskell's list
comprehensions. It also provides clear examples that show how comprehensions
are shorthand for common iteration code, such as copying one list into
another while performing some operation on the contained elements.</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>
</ul>
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Development Environments</a><a href="/vim.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",sans-serif; padding-left:27px'>Vim</a><a href="/emacs.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",sans-serif; padding-left:27px'>Emacs</a><a href="/source-control.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",sans-serif; padding-left:27px'>Source Control</a><a href="/git.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",sans-serif; padding-left:27px'>Git</a><a href="/application-dependencies.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",sans-serif; padding-left:27px'>Application Dependencies</a><a href="/python-programming-language.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",sans-serif;'>3. Core Language</a><a href="/generators.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",sans-serif; padding-left:27px'>Generators</a><a href="/comprehensions.html" class="list-group-item smaller-item active" style='font-family: "Helvetica Neue",sans-serif; padding-left:27px'>Comprehensions</a><a href="/testing.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",sans-serif;'>4. Testing</a><a href="/unit-testing.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",sans-serif; padding-left:27px'>Unit Testing</a><a href="/integration-testing.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",sans-serif; padding-left:27px'>Integration Testing</a><a href="/code-metrics.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",sans-serif; padding-left:27px'>Code Metrics</a><a href="/debugging.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",sans-serif; padding-left:27px'>Debugging</a><a href="/logging.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",sans-serif; padding-left:27px'>Logging</a><a href="/markdown.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",sans-serif; padding-left:27px'>Markdown</a><a href="/web-application-security.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",sans-serif;'>6. Security</a><a href="/web-development.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",sans-serif;'>7. Web Development</a><a href="/web-frameworks.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",sans-serif; padding-left:27px'>Web Frameworks</a><a href="/django.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",sans-serif; padding-left:27px'>Django</a><a href="/flask.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",sans-serif; padding-left:27px'>Flask</a><a href="/bottle.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",sans-serif; padding-left:27px'>Bottle</a><a href="/pyramid.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",sans-serif; padding-left:27px'>Pyramid</a><a href="/morepath.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",sans-serif; padding-left:27px'>Morepath</a><a href="/sanic.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",sans-serif; padding-left:27px'>Sanic</a><a href="/other-web-frameworks.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",sans-serif; padding-left:27px'>Other Web Frameworks</a><a href="/template-engines.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",sans-serif; padding-left:27px'>Template Engines</a><a href="/jinja2.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",sans-serif; padding-left:27px'>Jinja2</a><a href="/mako.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",sans-serif; padding-left:27px'>Mako</a><a href="/django-templates.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",sans-serif; padding-left:27px'>Django Templates</a><a href="/web-design.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",sans-serif; padding-left:27px'>Web Design</a><a href="/cascading-style-sheets.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",sans-serif; padding-left:27px'>Cascading Style Sheets (CSS)</a><a href="/javascript.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",sans-serif; padding-left:27px'>JavaScript</a><a href="/task-queues.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",sans-serif; padding-left:27px'>Task Queues</a><a href="/celery.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",sans-serif; padding-left:27px'>Celery</a><a href="/redis-queue-rq.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",sans-serif; padding-left:27px'>Redis Queue (RQ)</a><a href="/microservices.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",sans-serif; padding-left:27px'>Microservices</a><a href="/static-site-generator.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",sans-serif; padding-left:27px'>Static Site Generators</a><a href="/pelican.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",sans-serif; padding-left:27px'>Pelican</a><a href="/lektor.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",sans-serif; padding-left:27px'>Lektor</a><a href="/mkdocs.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",sans-serif; padding-left:27px'>MkDocs</a><a href="/websockets.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",sans-serif; padding-left:27px'>WebSockets</a><a href="/uvloop.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",sans-serif; padding-left:27px'>uvloop</a><a href="/deployment.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",sans-serif;'>8. Deployment</a><a href="/servers.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",sans-serif; padding-left:27px'>Servers</a><a href="/static-content.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",sans-serif; padding-left:27px'>Static Content</a><a href="/platform-as-a-service.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",sans-serif; padding-left:27px'>Platform-as-a-Service</a><a href="/operating-systems.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",sans-serif; padding-left:27px'>Operating Systems</a><a href="/ubuntu.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",sans-serif; padding-left:27px'>Ubuntu</a><a href="/web-servers.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",sans-serif; padding-left:27px'>Web Servers</a><a href="/apache-http-server.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",sans-serif; padding-left:27px'>Apache HTTP Server</a><a href="/nginx.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",sans-serif; padding-left:27px'>Nginx</a><a href="/caddy.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",sans-serif; padding-left:27px'>Caddy</a><a href="/wsgi-servers.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",sans-serif; padding-left:27px'>WSGI Servers</a><a href="/green-unicorn-gunicorn.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",sans-serif; padding-left:27px'>Green Unicorn (Gunicorn)</a><a href="/continuous-integration.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",sans-serif; padding-left:27px'>Continuous Integration</a><a href="/jenkins.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",sans-serif; padding-left:27px'>Jenkins</a><a href="/configuration-management.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",sans-serif; padding-left:27px'>Configuration Management</a><a href="/docker.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",sans-serif; padding-left:27px'>Docker</a><a href="/data.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",sans-serif;'>9. Data</a><a href="/databases.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",sans-serif; padding-left:27px'>Relational Databases</a><a href="/postgresql.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",sans-serif; padding-left:27px'>PostgreSQL</a><a href="/mysql.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",sans-serif; padding-left:27px'>MySQL</a><a href="/sqlite.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",sans-serif; padding-left:27px'>SQLite</a><a href="/object-relational-mappers-orms.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",sans-serif; padding-left:27px'>Object-relational Mappers</a><a href="/sqlalchemy.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",sans-serif; padding-left:27px'>SQLAlchemy</a><a href="/peewee.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",sans-serif; padding-left:27px'>Peewee</a><a href="/no-sql-datastore.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",sans-serif; padding-left:27px'>NoSQL Data Stores</a><a href="/redis.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",sans-serif; padding-left:27px'>Redis</a><a href="/mongodb.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",sans-serif; padding-left:27px'>MongoDB</a><a href="/apache-cassandra.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",sans-serif; padding-left:27px'>Apache Cassandra</a><a href="/application-programming-interfaces.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",sans-serif;'>10. Web APIs</a><a href="/bots.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",sans-serif; padding-left:35px'>Bots</a><a href="/api-creation.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",sans-serif; padding-left:35px'>API Creation</a><a href="/api-integration.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",sans-serif; padding-left:35px'>API Integration</a><a href="/twilio.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",sans-serif; padding-left:35px'>Twilio</a><a href="/devops.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",sans-serif;'>11. DevOps</a><a href="/monitoring.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",sans-serif; padding-left:35px'>Monitoring</a><a href="/caching.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",sans-serif; padding-left:35px'>Caching</a><a href="/web-analytics.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",sans-serif; padding-left:35px'>Web Analytics</a><a href="/change-log.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",sans-serif;'>12. Change Log</a><a href="/what-full-stack-means.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",sans-serif; padding-left:35px'>What Full Stack Means</a><a href="/about-author.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",sans-serif; padding-left:35px'>About the Author</a><a href="/future-directions.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",sans-serif; padding-left:35px'>Future Directions</a> <a href="/table-of-contents.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",san-serif;background-color:#22B24C; color: #fff;'>...or <span style="border-bottom: 1px dotted;">view the full table of contents</span>.</a>
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<div class="panel-heading"><h3 class="panel-head">Comprehensions</h3></div>
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Major updates are tweeted via
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<a href="/introduction.html" class="list-group-item smaller-item " style='font-family: "Helvetica Neue",sans-serif;'>1. Introduction</a><a href="/development-environments.html" class="list-group-item smaller-item " style='font-family: "Helvetica Neue",sans-serif;'>2. Development Environments</a><a href="/python-programming-language.html" class="list-group-item smaller-item " style='font-family: "Helvetica Neue",sans-serif;'>3. Core Language</a> <a href="/comprehensions.html" class="list-group-item smaller-item active" style='font-family: "Helvetica Neue",sans-serif;'>» Comprehensions</a>
<a href="/testing.html" class="list-group-item smaller-item " style='font-family: "Helvetica Neue",sans-serif;'>4. Testing</a><a href="/web-application-security.html" class="list-group-item smaller-item " style='font-family: "Helvetica Neue",sans-serif;'>6. Security</a><a href="/web-development.html" class="list-group-item smaller-item " style='font-family: "Helvetica Neue",sans-serif;'>7. Web Development</a><a href="/deployment.html" class="list-group-item smaller-item " style='font-family: "Helvetica Neue",sans-serif;'>8. Deployment</a><a href="/data.html" class="list-group-item smaller-item " style='font-family: "Helvetica Neue",sans-serif;'>9. Data</a><a href="/application-programming-interfaces.html" class="list-group-item smaller-item " style='font-family: "Helvetica Neue",sans-serif;'>10. Web APIs</a><a href="/devops.html" class="list-group-item smaller-item " style='font-family: "Helvetica Neue",sans-serif;'>11. DevOps</a><a href="/change-log.html" class="list-group-item smaller-item " style='font-family: "Helvetica Neue",sans-serif;'>12. Change Log</a> <a href="/table-of-contents.html" class="list-group-item smaller-item" style='font-family: "Helvetica Neue",san-serif;background-color:#22B24C; color: #fff;'>...or <span style="border-bottom: 1px dotted;">view the full table of contents</span>.</a>
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