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      <h1>Application Dependencies</h1>
<p>Application dependencies are the Python libraries and their versions
required for an application to work properly. These dependencies are 
installed separately from system-level packages to prevent library version
conflicts.</p>
<p>The most common way to install Python library dependencies is with 
the <a href="http://www.pip-installer.org/en/latest/">pip</a>
command combined with 
<a href="http://www.virtualenv.org/en/latest/">virtualenv</a> to isolate the
dependencies of individual applications from each other.</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. Virtualenv creates an 
isolated Python installation is where those dependencies are installed into.</p>
<h2>requirements.txt</h2>
<p>The pip convention for specifying application dependencies is with a 
<a href="http://www.pip-installer.org/en/1.4.1/cookbook.html#requirements-files">requirements.txt</a>
file. When you build a Python web application you should include a 
requirements.txt file with 
<a href="https://devcenter.heroku.com/articles/python-pip">pegged dependencies</a>.</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>
</ul>
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