Running Python Apps in a Virtual Environment

You can run Python apps in a virtual environment that contains a separate, stand-alone copy of Python and the Python add-ons that the application requires.

A virtual environment lets you configure and test apps in a stable environment that isolates the app from other Python stuff on your computer.
This solves several problems:

you want to install and run someone else’s app on your computer, without changing the Python packages on your computer
you want to verify that your app can run on someone else’s computer (may not have the same Python packages as yours)
you want to have multiple different Python “environments” or Python versions

Using a virtual environment solves most of these problems.

Creating a Virtual Environment

There are several commands for creating a virtual environment:

python -m venv
virtualenv (Python package)
pipenv (another Python package)

In this write up we will use the builtin venv module. It has less functionality than the virtualenv or pipenv packages, but is good enough.

Create a virtual env directory named env inside a project directory. You can use any name instead of “env” (“env” and “venv” are typical names):
```
cd /some/directory/django-polls
python3 -m venv  env 
```
This creates an env subdirectory with structure like this:
```
env
env/bin
env/include
env/lib
env/share
```
The env/bin directory contains python, python3, pip, and pip3 commands. But they aren’t on your shell’s “path” until you “activate” the virtual env.
Start the virtual environment by “sourcing” the activate script. In a Bash or Zshell (Linux, MacOS, or Windows with git-bash installed), type:
```
 source env/bin/activate
```
or:
```
 . env/bin/activate
```
On Microsoft Windows:
```
 env\Scripts\activate
```
When the virtual env is activated it prepends (env) to your shell prompt so you know it is active.
```
 (env) cmd>
```
(First time only) Install requirements inside the virtual environment:
```
 (env) cmd>  pip install -r requirements.txt
```
Requirements are installed only in the virtual env directory (env).

Run the application or do whatever you want:

 (env) cmd>  python3 manage.py runserver

Exit the virtualenv using deactivate.
You can also exit by closing the shell window.
```
(env) cmd>  deactivate
```

After initial setup, to run the app just “activate” the virtualenv and run the app.

Exclude virtual environment directories from Git!

Add the virtualenv directory (env) to your .gitignore file and don’t commit virtualenv directories into git.

Pipenv

Pipenv is a newer package that fixes some virtualenv problems of dependency mismatch, improves the management interface, and has a consistent interface on all OS (including Windows). “Under the hood” pipenv uses virtualenv for the virtual environment.

References

Installing packages using pip and virtualenv has complete instructions for using virtualenv on Windows, MacOS, and Linux/Unix.
Virtualenvwrapper docs describe what is does and how to use it.
Pipenv documents the official documentation and Github Site.
Pipenv: A guide to the new packaging tool on RealPython explains the dependency conflicts that can arise with virtualenv and requirements.txt.