Application dependencies are the libraries other than your project code that are required to create and run your application.
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 myriad other tasks. Python's ecosystem facilitates discovery, retrieval and installation so applications are easier for developers to create.
Python libraries are stored in a central location known as the Python Package Index (PyPi). PyPi contains search functionality with results weighted by usage and relevance based on keyword terms.
Dependencies are installed separately from system-level packages to prevent library version conflicts. The most common isolation method is virtualenv. Each virtualenv is its own copy of the Python interpreter and depedencies in the site-packages directory. To use a virtualenv it must first be created with the virtualenv command and then activated.
The recommended way to install Python library dependencies is with the pip command when a virtualenv is activated.
Pip and virtualenv work together and have complementary responsibilities. Pip downloads and installs application dependencies from the central PyPi repository.
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.
Python projects' dependencies for a web application should be specified in the requirements.txt with pegged dependencies like the following:
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
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.
There is another type of dependency specification for Python libraries known as setup.py. Setup.py is a standard for distributing and installing Python libraries. If you're building a Python library, such as requests or underwear 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 well written post for further clarification.
Jon Chu wrote a great introduction on virtualenv and pip basics.
"A non-magical introduction to virtualenv and pip breaks down what problems these tools solve and how to use them.
Tools of the modern Python hacker contains detailed explanations of virtualenv, Fabric, and pip.
Occasionally arguments about using Python's dependency manager versus one of Linux's depenency managers comes up. This provides one perspective on that debate.
