Comprehensive Beginner's Virtualenv Tutorial

Comprehensive beginner's virtualenv tutorial?

This is very good: http://simononsoftware.com/virtualenv-tutorial-part-2/

And this is a slightly more practical one: https://web.archive.org/web/20160404222648/https://iamzed.com/2009/05/07/a-primer-on-virtualenv/

Creating virtualenv for an existing project

You can just create an virtual enviroment with virtualenv venv and start it with venv/bin/activate.

You will need to reinstall all dependencies using pip, but the rest should just work fine.

What are dependencies and why should I care about them?

Virtualenvs are isolated python environments that can be created within your machine/server, and they are useful as each of them holds specific/relevant libraries for each python project/programs of various nature that you might have (be it web applications, machine learning applications, data processing microservices, IoT, etc).

For example, let's say your machine/server is hosting 2 or more python projects/programs. Each of them might require different versions of Django, MySQL-connector, etc (which can be installed via pip). Hence, you need separate python environments for each of these projects/programs to prevent clashes.

Creating virtualenvs are simple, you can install them via pip.
See: https://virtualenv.pypa.io/en/latest/

Thereafter, you can go about creating different virtualenv for each python project to isolate the python environments and libraries/packages needed (installed via pip again for each environment) for each project.

Python 3.5 install pyvenv

Kesh's answer led me in the right direction.

The problem was that I didn't actually have pip installed in my venv.

It turns out, when I built python3.5 from source, I did not have the libssl-dev package. It looks like one of the dependencies of ensurepip was the python ssl package that didn't get installed because I didn't have libssl-dev.

To fix the problem, I rebuilt python 3.5 for source with the libssl-dev package installed. The rebuilt python now included the ssl package, which allowed ensurepip to install pip in my virtual environment.

importance of virtual environment setup for django with python

1. A virtual environment is a way for you to have multiple versions of
   python on your machine without them clashing with each other, each
   version can be considered as a development environment and you can
   have different versions of python libraries and modules all isolated
   from one another

2. Yes it's very important. For example without a virtualenv, if you're
   working on an open source project that uses django 1.5 but locally on
   your machine, you installed django 1.9 for other personal projects.
   It's almost impossible for you to contribute because you'll get a lot of
   errors due to the difference in django versions. If you decide to
   downgrade to django 1.5 then you can't work on your personal projects
   anymore because they depend on django 1.9.

   A virtualenv handles all this for you by enabling you to create seperate
   virtual (development) environments that aren't tied to each other and can
   be activated and deactivated easily when you're done. You can also have
   different versions of python

3. You're not forced to but you should, it's as easy as:

   virtualenv newenv

   cd newenv

   source bin/activate # The current shell uses the virtual environment

   Moreover it's very important for testing, lets say you want to port
   a django web app from 1.5 to 1.9, you can easily do that by creating
   different virtualenv's and installing different versions of django.
   it's impossible to do this without uninstalling one version (except
   you want to mess with sys.path which isn't a good idea)

Is there a way to version my python distribution?

virtualenv + requirements.txt are your friend.

You can create several virtual python installs for your projects, everything containing exactly those library versions you need (Tip: pip freeze spits out a requirements.txt with the exact library versions).

Find a good reference to virtualenv here: http://simononsoftware.com/virtualenv-tutorial/ (it's from this question Comprehensive beginner's virtualenv tutorial?).

Alternatively, if you just want to distribute your code together with libraries, PyInstaller is worth a try. You can package everything together in a static executable - you don't even have to install the software afterwards.

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