venv ​

When developing Python applications, the system typically has only one version of Python 3 (3.x). All third-party packages installed via pip go into the Python 3 site-packages directory.

The Need for Isolation ​

If you're developing multiple applications, they will share the same Python environment. For example, if Application A needs Jinja 2.7 and Application B requires Jinja 2.6, conflicts arise.

To solve this, each application can have its own "isolated" Python runtime environment using venv.

Creating a Virtual Environment ​

Let’s assume you're starting a new project named project101 and need a dedicated Python environment:

1. Create a Directory: First, create a directory for your virtual environment. Here, we'll name it proj101env:

   bash

   $ mkdir proj101env
   $ cd proj101env/

2. Set Up the Virtual Environment: Create an isolated Python environment:

   bash

   proj101env$ python3 -m venv .

   After running this command, you will see several folders and a pyvenv.cfg file:

   bash

   proj101env$ ls
   bin  include  lib  pyvenv.cfg

   The command python3 -m venv <directory> sets up an independent Python environment. The bin directory contains executable files like python3 and pip3, which are symlinks to the system Python installation.

3. Activate the Virtual Environment: Change into the bin directory and activate the environment. On Linux/Mac, use:

   bash

   proj101env$ cd bin
   bin$ source activate

   On Windows, use:

   bash

   proj101env$ activate.bat

   Your command prompt will change to indicate that the proj101env environment is active:

   bash

   (proj101env) bin$

4. Install Packages: Now you can install packages without affecting the system Python environment:

   bash

   (proj101env) bin$ pip3 install jinja2

   You can verify the installation:

   bash

   (proj101env) bin$ python3
   >>> import jinja2
   >>> exit()

   Packages installed in the venv environment go into proj101env/lib/python3.x/site-packages, keeping the system environment untouched.

5. Deactivate the Environment: To exit the virtual environment, simply run:

   bash

   (proj101env) bin$ deactivate

   You'll return to the normal system environment, where pip and python point to the system installation.

Deleting a Virtual Environment ​

If you no longer need a particular venv, such as proj101env, you can easily delete it. Ensure it's not activated, then remove the directory:

$ rm -rf proj101env

Summary ​

venv provides isolated Python environments for applications, helping to avoid conflicts between different versions of packages required by various applications. This isolation allows for better management of dependencies across projects.