How to bootstrap Python VirtualEnvs with a specific Python version

This shows how to create a "bootstrap" VirtualEnv, which will allow the creation of (mostly) self-contained VirtualEnvs that are used for projects.

1. Install OS dependencies like libsqlite3-dev
• Otherwise you won't be able to import sqlite3
• See Django problem: sqlite error when running"manage.py migrate" for what sort of error this results in
2. Build Python from source:
1. Download from https://www.python.org/download/releases
2. Extract tarball
3. cd into directory
4. Run "./configure --prefix=/opt/python3"
5. Run "make"
6. Run "sudo make install"
3. Manually grab the latest virtualenv module to avoid dependency problems with the existing Python installation:
1. Download virtualenv-X.Y.Z.tar.gz from https://pypi.python.org/pypi/virtualenv#downloads
2. Extract into /opt/virtualenv-X.Y.Z
4. Create and configure the "bootstrap" VirtualEnv:
1. E.g. run "/opt/virtualenv-15.0.3/virtualenv.py -p /opt/python3/bin/python3 lib/py3venv"
2. Run "lib/py3venv/bin/pip install virtualenv"

Now, whenever you need to create a self-contained VirtualEnv for a project, run this command, substituting the placeholder for the desired output directory:

lib/py3venv/bin/virtualenv env_dir

The VirtualEnv directory that is created will contain its own Python executable, but will rely on the global packages in /opt/python3.

See The "Virtual Environments and Packages" section of the Python tutorial for more info.

Django problem: sqlite error when running"manage.py migrate"

Just ran into a problem with creating a Django virtualenv.  I built Python from source, but I didn't have the libsqlite3-dev library installed.  (I'm using Debian, but the same problem applies to Ubuntu.) 

So when I tried to run "demo/manage.py migrate" in my Django virtualenv, I got this error:

ImportError: No module named 'pysqlite2'

Once I installed libsqlite3-dev rebuilt Python with "make clean install", it worked!

The fix is documented in the Stack Overflow post "No module named _sqlite3".