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‎application-dependencies.html‎

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<div class="row">
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<div class="col-md-8">
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<h1>Application Dependencies</h1>
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<p>Application dependencies are the Python libraries and their versions
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required for an application to work properly. These dependencies are
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installed separately from system-level packages to prevent library version
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conflicts.</p>
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<p>The most common way to install Python library dependencies is with
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the <a href="http://www.pip-installer.org/en/latest/">pip</a>
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command combined with
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<a href="http://www.virtualenv.org/en/latest/">virtualenv</a> to isolate the
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dependencies of individual applications from each other.</p>
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<p>Application dependencies are the libraries other than your project code
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that are required to create and run your application. </p>
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<h2>Why are application dependencies important?</h2>
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<p>Python web applications are built upon the work done by thousands of open
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source programmers. Application dependencies include not only web frameworks but
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also libraries for scraping, parsing, processing, analyzing, visualizing,
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and myriad other tasks. Python's ecosystem facilitates discovery, retrieval and
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installation so applications are easier for developers to create.</p>
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<h2>Finding libraries</h2>
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<p>Python libraries are stored in a central location known as the
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<a href="https://pypi.python.org/pypi">Python Package Index</a> (PyPi). PyPi contains
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search functionality with results weighted by usage and relevance based on
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keyword terms.</p>
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<h2>Isolating application dependencies</h2>
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<p>Dependencies are installed separately from system-level packages to prevent
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library version conflicts. The most common isolation method is
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<a href="http://www.virtualenv.org/en/latest/">virtualenv</a>. Each virtualenv is its
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own copy of the Python interpreter and depedencies in the site-packages
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directory. To use a virtualenv it must first be created with the virtualenv
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command and then activated.</p>
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<h2>Downloading and installing Python dependencies</h2>
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<p>The recommended way to install Python library dependencies is with the
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<a href="http://www.pip-installer.org/en/latest/">pip</a> command when a virtualenv
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is activated.</p>
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<p>Pip and virtualenv work together and have complementary responsibilities.
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Pip downloads and installs application dependencies from the central
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<a href="https://pypi.python.org/pypi">PyPi</a> repository. Virtualenv creates an
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isolated Python installation is where those dependencies are installed into.</p>
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<a href="https://pypi.python.org/pypi">PyPi</a> repository. </p>
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<h2>requirements.txt</h2>
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<p>The pip convention for specifying application dependencies is with a
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<a href="http://www.pip-installer.org/en/1.4.1/cookbook.html#requirements-files">requirements.txt</a>
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file. When you build a Python web application you should include a
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requirements.txt file with
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<a href="https://devcenter.heroku.com/articles/python-pip">pegged dependencies</a>.</p>
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requirements.txt file. </p>
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<h3>requirementst.txt example with pegged dependencies</h3>
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<p>Python projects' dependencies for a web application should be specified in the
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requirements.txt with
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<a href="https://devcenter.heroku.com/articles/python-pip">pegged dependencies</a> like
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the following:</p>
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<div class="codehilite"><pre><span class="n">django</span><span class="o">==</span><span class="mf">1.6</span>
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<span class="n">bpython</span><span class="o">==</span><span class="mf">0.12</span>
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<span class="n">django</span><span class="o">-</span><span class="n">braces</span><span class="o">==</span><span class="mf">0.2.1</span>
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<span class="n">django</span><span class="o">-</span><span class="n">model</span><span class="o">-</span><span class="n">utils</span><span class="o">==</span><span class="mf">1.1.0</span>
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<span class="n">logutils</span><span class="o">==</span><span class="mf">0.3.3</span>
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<span class="n">South</span><span class="o">==</span><span class="mf">0.7.6</span>
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<span class="n">requests</span><span class="o">==</span><span class="mf">1.2.0</span>
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<span class="n">stripe</span><span class="o">==</span><span class="mf">1.9.1</span>
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<span class="n">dj</span><span class="o">-</span><span class="n">database</span><span class="o">-</span><span class="n">url</span><span class="o">==</span><span class="mf">0.2.1</span>
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<span class="n">django</span><span class="o">-</span><span class="n">oauth2</span><span class="o">-</span><span class="n">provider</span><span class="o">==</span><span class="mf">0.2.4</span>
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<span class="n">djangorestframework</span><span class="o">==</span><span class="mf">2.3.1</span>
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</pre></div>
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<p>Pegged dependencies with precise version numbers or Git tags are important
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because otherwise the latest version of a dependency will be used. While
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it may sound good to stay up to date, there's no telling if your application
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actually works with the latest versions of all dependencies. Developers should
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deliberately upgrade and test to make sure there were no backwards-incompatible
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modifications in newer dependency library versions.</p>
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<h2>setup.py</h2>
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<p>There is another type of dependency specification for Python libraries
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known as

‎feeds/all.atom.xml‎

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<?xml version="1.0" encoding="utf-8"?>
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<feed xmlns="http://www.w3.org/2005/Atom"><title>Matt Makai</title><link href="http://www.fullstackpython.com/" rel="alternate"></link><link href="http://www.fullstackpython.com/feeds/all.atom.xml" rel="self"></link><id>http://www.fullstackpython.com/</id><updated>2014-03-19T07:16:32Z</updated></feed>
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<feed xmlns="http://www.w3.org/2005/Atom"><title>Matt Makai</title><link href="http://www.fullstackpython.com/" rel="alternate"></link><link href="http://www.fullstackpython.com/feeds/all.atom.xml" rel="self"></link><id>http://www.fullstackpython.com/</id><updated>2014-03-20T09:27:56Z</updated></feed>

‎source/content/pages/09-application-dependencies/0901-application-dependencies.markdown‎

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55

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# Application Dependencies
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Application dependencies are the Python libraries and their versions
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required for an application to work properly. These dependencies are
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installed separately from system-level packages to prevent library version
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conflicts.
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Application dependencies are the libraries other than your project code
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that are required to create and run your application.
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The most common way to install Python library dependencies is with
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the [pip](http://www.pip-installer.org/en/latest/)
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command combined with
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[virtualenv](http://www.virtualenv.org/en/latest/) to isolate the
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dependencies of individual applications from each other.
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## Why are application dependencies important?
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Python web applications are built upon the work done by thousands of open
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source programmers. Application dependencies include not only web frameworks but
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also libraries for scraping, parsing, processing, analyzing, visualizing,
16+
and myriad other tasks. Python's ecosystem facilitates discovery, retrieval and
17+
installation so applications are easier for developers to create.
18+
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## Finding libraries
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Python libraries are stored in a central location known as the
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[Python Package Index](https://pypi.python.org/pypi) (PyPi). PyPi contains
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search functionality with results weighted by usage and relevance based on
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keyword terms.
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25+
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## Isolating application dependencies
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Dependencies are installed separately from system-level packages to prevent
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library version conflicts. The most common isolation method is
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[virtualenv](http://www.virtualenv.org/en/latest/). Each virtualenv is its
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own copy of the Python interpreter and depedencies in the site-packages
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directory. To use a virtualenv it must first be created with the virtualenv
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command and then activated.
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34+
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## Downloading and installing Python dependencies
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The recommended way to install Python library dependencies is with the
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[pip](http://www.pip-installer.org/en/latest/) command when a virtualenv
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is activated.
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Pip and virtualenv work together and have complementary responsibilities.
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Pip downloads and installs application dependencies from the central
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[PyPi](https://pypi.python.org/pypi) repository. Virtualenv creates an
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isolated Python installation is where those dependencies are installed into.
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[PyPi](https://pypi.python.org/pypi) repository.
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## requirements.txt
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The pip convention for specifying application dependencies is with a
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[requirements.txt](http://www.pip-installer.org/en/1.4.1/cookbook.html#requirements-files)
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file. When you build a Python web application you should include a
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requirements.txt file with
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[pegged dependencies](https://devcenter.heroku.com/articles/python-pip).
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requirements.txt file.
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### requirementst.txt example with pegged dependencies
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Python projects' dependencies for a web application should be specified in the
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requirements.txt with
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[pegged dependencies](https://devcenter.heroku.com/articles/python-pip) like
56+
the following:
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django==1.6
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bpython==0.12
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django-braces==0.2.1
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django-model-utils==1.1.0
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logutils==0.3.3
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South==0.7.6
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requests==1.2.0
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stripe==1.9.1
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dj-database-url==0.2.1
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django-oauth2-provider==0.2.4
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djangorestframework==2.3.1
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Pegged dependencies with precise version numbers or Git tags are important
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because otherwise the latest version of a dependency will be used. While
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it may sound good to stay up to date, there's no telling if your application
73+
actually works with the latest versions of all dependencies. Developers should
74+
deliberately upgrade and test to make sure there were no backwards-incompatible
75+
modifications in newer dependency library versions.
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## setup.py
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##setup.py
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There is another type of dependency specification for Python libraries
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known as
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[setup.py](http://stackoverflow.com/questions/1471994/what-is-setup-py).

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