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<!DOCTYPE html>

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<title>Application Dependencies - Full Stack Python</title>

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<span class="logo-title"><a href="/">Full Stack Python</a></span>

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<h1>Application Dependencies</h1>

<p>Application dependencies are the libraries other than your project code

that are required to create and run your application. </p>

<h2>Why are application dependencies important?</h2>

<p>Python web applications are built upon the work done by thousands of open

source programmers. Application dependencies include not only web frameworks but

also libraries for scraping, parsing, processing, analyzing, visualizing,

and many other tasks. Python's ecosystem facilitates discovery, retrieval and

installation so applications are easier for developers to create.</p>

<h2>Finding libraries</h2>

<p>Python libraries are stored in a central location known as the

<a href="https://pypi.python.org/pypi">Python Package Index</a> (PyPi). PyPi contains

search functionality with results weighted by usage and relevance based on

keyword terms.</p>

<p>Besides PyPi there are numerous resources that list common or "must-have"

libraries. Ultimately the decision for which application dependencies are

necessary for your project is up to you and the functionality you're looking

to build. However, it's useful to browse through these lists in case you come

across a library to solve a problem by reusing the code instead of writing it

all yourself. A few of the best collections of Python libraries are</p>

<ul>

<li>

<p><a href="https://wiki.python.org/moin/UsefulModules">Python.org's useful modules</a>

which groups modules into categories.</p>

</li>

<li>

<p><a href="https://github.com/trending?l=python">GitHub Explore Trending repositories</a>

shows the open source Python projects trending today, this week, and this

month.</p>

</li>

<li>

<p>This list of <a href="http://freepythontips.wordpress.com/2013/07/30/20-python-libraries-you-cant-live-without/">20 Python libraries you can’t live without</a>

is a wide-ranging collection from data analysis to testing tools.</p>

</li>

<li>

<p>Wikipedia actually has an extensive

<a href="http://en.wikipedia.org/wiki/List_of_Python_software">page dedicated to Python libraries</a>

grouped by categories.</p>

</li>

</ul>

<h2>Isolating dependencies</h2>

<p>Dependencies are installed separately from system-level packages to prevent

library version conflicts. The most common isolation method is

<a href="http://www.virtualenv.org/en/latest/">virtualenv</a>. Each virtualenv is its

own copy of the Python interpreter and dependencies in the site-packages

directory. To use a virtualenv it must first be created with the virtualenv

command and then activated.</p>

<p>The virtualenv stores dependencies in an isolated environment. The web

application then relies only on that virtualenv instance which has a separate

copy of the Python interpreter and site-packages directory. A high level of

how a server configured with virtualenv can look is shown in the picture below.</p>

<h2>Installing Python dependencies</h2>

<p>The recommended way to install Python library dependencies is with the

<a href="http://www.pip-installer.org/en/latest/">pip</a> command when a virtualenv

is activated.</p>

<p>Pip and virtualenv work together and have complementary responsibilities.

Pip downloads and installs application dependencies from the central

<a href="https://pypi.python.org/pypi">PyPi</a> repository. </p>

<h2>requirements.txt</h2>

<p>The pip convention for specifying application dependencies is with a

requirements.txt file. When you build a Python web application you

should include a requirements.txt file. </p>

<p>Python projects' dependencies for a web application should be specified in the

requirements.txt with pegged dependencies like the following:</p>

<div class="highlight"><pre>django==1.6

bpython==0.12

django-braces==0.2.1

django-model-utils==1.1.0

logutils==0.3.3

South==0.7.6

requests==1.2.0

stripe==1.9.1

dj-database-url==0.2.1

django-oauth2-provider==0.2.4

djangorestframework==2.3.1

</pre></div>

<p>Pegged dependencies with precise version numbers or Git tags are important

because otherwise the latest version of a dependency will be used. While

it may sound good to stay up to date, there's no telling if your application

actually works with the latest versions of all dependencies. Developers should

deliberately upgrade and test to make sure there were no backwards-incompatible

modifications in newer dependency library versions.</p>

<h2>setup.py</h2>

<p>There is another type of dependency specification for Python libraries

known as

<a href="http://stackoverflow.com/questions/1471994/what-is-setup-py">setup.py</a>.

Setup.py is a standard for distributing and installing Python libraries.

If you're building a Python library, such as

<a href="http://www.python-requests.org/en/latest/">requests</a> or

<a href="https://github.com/makaimc/underwear">underwear</a> you must include setup.py

so a dependency manager can correctly install both the library as well as

additional dependencies for the library. There's still quite a bit of

confusion in the Python community over the difference between

requirements.txt and setup.py, so read this

<a href="https://caremad.io/blog/setup-vs-requirement/">well written post</a> for

further clarification.</p>

<h2>Application dependency resources</h2>

<ul>

<li>

<p><a href="https://twitter.com/jonathanchu">Jon Chu</a> wrote a great introduction on

<a href="http://www.jontourage.com/2011/02/09/virtualenv-pip-basics/">virtualenv and pip basics</a>.</p>

</li>

<li>

<p>"<a href="http://dabapps.com/blog/introduction-to-pip-and-virtualenv-python/">A non-magical introduction to virtualenv and pip</a>

breaks down what problems these tools solve and how to use them.</p>

</li>

<li>

<p><a href="http://www.clemesha.org/blog/modern-python-hacker-tools-virtualenv-fabric-pip/">Tools of the modern Python hacker</a>

contains detailed explanations of virtualenv, Fabric, and pip.</p>

</li>

<li>

<p>Occasionally arguments about using Python's dependency manager versus

one of Linux's depenency managers comes up. This provides

<a href="http://notes.pault.ag/debian-python/">one perspective on that debate</a>.</p>

</li>

<li>

<p>This Stack Overflow question details how to set up a

<a href="http://askubuntu.com/questions/244641/how-to-set-up-and-use-a-virtual-python-environment-in-ubuntu">virtual environment for Python development</a>.</p>

</li>

<li>

<p>Another Stack Overflow page answers

<a href="http://stackoverflow.com/questions/9554087/setting-an-environment-variable-in-virtualenv">how to set environment variables when using virtualenv</a>.</p>

</li>

<li>

<p><a href="http://mrcoles.com/tips-using-pip-virtualenv-virtualenvwrapper/">Tips for using pip + virtualenv + virtualenvwrapper</a>

shows how to use shell aliases and postactivate virtualenvwrapper hooks to

make life easier when using these tools.</p>

</li>

<li>

<p>Major speed improvements were made in pip 7 over previous versions.

Read

<a href="https://lincolnloop.com/blog/fast-immutable-python-deployments/">this article about the differences</a>

and be sure to upgrade.</p>

</li>

</ul>

<h2>Open source app dependency projects</h2>

<ul>

<li>

<p><a href="https://github.com/kennethreitz/autoenv">Autoenv</a> is a tool for activating

environment variables stored in a .env file in your projects' home

directories. Environment variables aren't managed by virtualenv and although

virtualenvwrapper has some hooks for handling them, it's often easiest to

use a shell script or .env file to set them in a development environment.</p>

</li>

<li>

<p><a href="https://github.com/bndr/pipreqs">Pipreqs</a> searches through a project for

dependencies based on imports. It then generates a requirements.txt file

based on the libraries necessary to run those dependencies. Note though that

while this could come in handy with a legacy project, the version numbers

for those libraries will not be generated with the output.</p>

</li>

</ul>

<h2>Application dependencies learning checklist</h2>

<ol>

<li>

<p>Ensure the libraries your web application depends on are all captured in

a requirement.txt file with pegged versions. </p>

</li>

<li>

<p>An easy way to capture currently installed dependencies is with the

<code>pip freeze</code> command.</p>

</li>

<li>

<p>Create a fresh virtualenv and install the dependencies from your

requirements.txt file by using the <code>pip install -r requirements.txt</code>

command.</p>

</li>

<li>

<p>Check that your application runs properly with the fresh virtualenv and

only the installed dependencies from the requirements.txt file.</p>

</li>

</ol>

<h3>What's next after installing app dependencies?</h3>

<p class="under-btn">What runs a Python application execute on the server?</p> </div>

</div>

<p class="under-btn">Tell me about standard relational databases.</p> </div>

</div>

<p class="under-btn">What are web application programming interfaces (APIs)?</p> </div>

</div>

</div><div id="mc_embed_signup">

<h4>Sign up here to receive an email with major updates to this site and Python tutorials delivered to your inbox once a month.</h4>

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</form>

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<p style="font-size: .8em; margin-top: 10px;">Searching for a complete, step-by-step deployment walkthrough? Pre-order the <a href="https://gumroad.com/products/WOvyt/">Full Stack Python Guide to Deployments book</a>.

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<h3 class="panel-head"><a href="/table-of-contents.html" style="color: #fff;">Table of Contents</a></h3>

</div>

<a href="/introduction.html" class="list-group-item smaller-item " style='font-family: "Helvetica Neue",sans-serif;'>Introduction</a>

<a href="/learning-programming.html" class="list-group-item smaller-item " style='font-family: "Helvetica Neue",sans-serif;'>Learning Programming</a>

<a href="/why-use-python.html" class="list-group-item smaller-item " style='font-family: "Helvetica Neue",sans-serif;'>Why Use Python?</a>

<a href="/best-python-resources.html" class="list-group-item smaller-item " style='font-family: "Helvetica Neue",sans-serif;'>Best Python Resources</a>

<a href="/best-python-videos.html" class="list-group-item smaller-item " style='font-family: "Helvetica Neue",sans-serif;'>Best Python Videos</a>

<a href="/enterprise-python.html" class="list-group-item smaller-item " style='font-family: "Helvetica Neue",sans-serif;'>Enterprise Python</a>

<a href="/development-environments.html" class="list-group-item smaller-item " style='font-family: "Helvetica Neue",sans-serif;'>Development Environments</a>

<a href="/emacs.html" class="list-group-item smaller-item " style='font-family: "Helvetica Neue",sans-serif;'>Emacs</a>

<a href="/generators.html" class="list-group-item smaller-item " style='font-family: "Helvetica Neue",sans-serif;'>Generators</a>

<a href="/comprehensions.html" class="list-group-item smaller-item " style='font-family: "Helvetica Neue",sans-serif;'>Comprehensions</a>

<a href="/web-frameworks.html" class="list-group-item smaller-item " style='font-family: "Helvetica Neue",sans-serif;'>Web Frameworks</a>

<a href="/django.html" class="list-group-item smaller-item " style='font-family: "Helvetica Neue",sans-serif;'>Django</a>

<a href="/flask.html" class="list-group-item smaller-item " style='font-family: "Helvetica Neue",sans-serif;'>Flask</a>

<a href="/bottle.html" class="list-group-item smaller-item " style='font-family: "Helvetica Neue",sans-serif;'>Bottle</a>

<a href="/pyramid.html" class="list-group-item smaller-item " style='font-family: "Helvetica Neue",sans-serif;'>Pyramid</a>

<a href="/morepath.html" class="list-group-item smaller-item " style='font-family: "Helvetica Neue",sans-serif;'>Morepath</a>

<a href="/other-web-frameworks.html" class="list-group-item smaller-item " style='font-family: "Helvetica Neue",sans-serif;'>Other Web Frameworks</a>

<a href="/web-design.html" class="list-group-item smaller-item " style='font-family: "Helvetica Neue",sans-serif;'>Web Design</a>

<a href="/cascading-style-sheets.html" class="list-group-item smaller-item " style='font-family: "Helvetica Neue",sans-serif;'>Cascading Style Sheets</a>

<a href="/javascript.html" class="list-group-item smaller-item " style='font-family: "Helvetica Neue",sans-serif;'>JavaScript</a>

<a href="/websockets.html" class="list-group-item smaller-item " style='font-family: "Helvetica Neue",sans-serif;'>WebSockets</a>

<a href="/web-application-security.html" class="list-group-item smaller-item " style='font-family: "Helvetica Neue",sans-serif;'>Web Application Security</a>

<a href="/databases.html" class="list-group-item smaller-item " style='font-family: "Helvetica Neue",sans-serif;'>Databases</a>

<a href="/no-sql-datastore.html" class="list-group-item smaller-item " style='font-family: "Helvetica Neue",sans-serif;'>NoSQL Data Stores</a>

<a href="/object-relational-mappers-orms.html" class="list-group-item smaller-item " style='font-family: "Helvetica Neue",sans-serif;'>Object-relational Mappers (ORMs)</a>

<a href="/application-programming-interfaces.html" class="list-group-item smaller-item " style='font-family: "Helvetica Neue",sans-serif;'>Application Programming Interfaces</a>

<a href="/api-integration.html" class="list-group-item smaller-item " style='font-family: "Helvetica Neue",sans-serif;'>API Integration</a>

<a href="/api-creation.html" class="list-group-item smaller-item " style='font-family: "Helvetica Neue",sans-serif;'>API Creation</a>

<a href="/deployment.html" class="list-group-item smaller-item " style='font-family: "Helvetica Neue",sans-serif;'>Deployment</a>

<a href="/servers.html" class="list-group-item smaller-item " style='font-family: "Helvetica Neue",sans-serif;'>Servers</a>

<a href="/platform-as-a-service.html" class="list-group-item smaller-item " style='font-family: "Helvetica Neue",sans-serif;'>Platform-as-a-service</a>

<a href="/operating-systems.html" class="list-group-item smaller-item " style='font-family: "Helvetica Neue",sans-serif;'>Operating Systems</a>

<a href="/web-servers.html" class="list-group-item smaller-item " style='font-family: "Helvetica Neue",sans-serif;'>Web Servers</a>

<a href="/wsgi-servers.html" class="list-group-item smaller-item " style='font-family: "Helvetica Neue",sans-serif;'>WSGI Servers</a>

<a href="/source-control.html" class="list-group-item smaller-item " style='font-family: "Helvetica Neue",sans-serif;'>Source Control</a>

<a href="/application-dependencies.html" class="list-group-item smaller-item active" style='font-family: "Helvetica Neue",sans-serif;'>Application Dependencies</a>

<a href="/static-content.html" class="list-group-item smaller-item " style='font-family: "Helvetica Neue",sans-serif;'>Static Content</a>

<a href="/task-queues.html" class="list-group-item smaller-item " style='font-family: "Helvetica Neue",sans-serif;'>Task Queues</a>

<a href="/configuration-management.html" class="list-group-item smaller-item " style='font-family: "Helvetica Neue",sans-serif;'>Configuration Management</a>

<a href="/continuous-integration.html" class="list-group-item smaller-item " style='font-family: "Helvetica Neue",sans-serif;'>Continuous Integration</a>

<a href="/logging.html" class="list-group-item smaller-item " style='font-family: "Helvetica Neue",sans-serif;'>Logging</a>

<a href="/monitoring.html" class="list-group-item smaller-item " style='font-family: "Helvetica Neue",sans-serif;'>Monitoring</a>

<a href="/web-analytics.html" class="list-group-item smaller-item " style='font-family: "Helvetica Neue",sans-serif;'>Web Analytics</a>

<a href="/docker.html" class="list-group-item smaller-item " style='font-family: "Helvetica Neue",sans-serif;'>Docker</a>

<a href="/caching.html" class="list-group-item smaller-item " style='font-family: "Helvetica Neue",sans-serif;'>Caching</a>

<a href="/code-metrics.html" class="list-group-item smaller-item " style='font-family: "Helvetica Neue",sans-serif;'>Code Metrics</a>

<a href="/what-full-stack-means.html" class="list-group-item smaller-item " style='font-family: "Helvetica Neue",sans-serif;'>What "Full Stack" Means</a>

<a href="/change-log.html" class="list-group-item smaller-item " style='font-family: "Helvetica Neue",sans-serif;'>Change Log</a>

<a href="/future-directions.html" class="list-group-item smaller-item " style='font-family: "Helvetica Neue",sans-serif;'>Future Directions</a>

<a href="/about-author.html" class="list-group-item smaller-item " style='font-family: "Helvetica Neue",sans-serif;'>About the Author</a>

</div>

</p>

</div>

</div> <div class="panel panel-success">

<div class="panel-heading"><h3 class="panel-head">Application Dependencies</h3></div>

Major updates are tweeted via

<a href="https://twitter.com/fullstackpython">@fullstackpython</a>.

<hr/>

Need more detailed tutorials than you see here?

<a href="https://gumroad.com/products/WOvyt/">Pre-order the Full Stack Python Guide to Deployments ebook.</a>

</div>

</div></div>

<hr/>

<a href="http://www.mattmakai.com/" class="underline">Matt Makai</a>

2015

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