Module 6
Modular Programming
6.3 Virtual Environment
Suppose you are working on two separate projects: Project_A and Project_B. Say, you are using a package package_one version 1.0 for Project_A. But, Project_B requires the same package package_one but its latest version, version 2.0.
All the packages installed using a third party installer such as pip are located in the site-packages folder.
>>> import site
>>> site.getsitepackages()
['/home/usr/anaconda3/lib/python3.6/site-packages']
So if you download a new version of a package, the latest version is downloaded and stored in the same folder. There is no way to distinguish between the two versions, and the latest version is used when the package is imported.
Virtual Environment
Virtual environment is a self-contained directory tree that contains a Python installation for a particular version of Python, plus a number of additional packages.
The simple idea behind virtual environment is to create an isolated environment for Python projects. Therefore, each project can have its own dependencies, regardless of what dependencies every other project has.
There is no limit to the number of virtual environments you can create.
Note: It is always a good idea to work in a virtual environment.
Installing
If you are using Python 3.6 or later, virtualenv tool comes built in. Otherwise you have to install it.
pip install virtualenv
Using virtual environment
Create a project directory.
mkdir my_project
cd my_project
Create a virtual environment
python3 -m venv virtual_environment_name
Or
python3 -m virtualenv virtual_environment_name
Here, virtual_environment_name can be any arbitrary string.
Example
python3 -m venv env
It creates a directory named env that contains a directory structre as follows.
├── bin
│ ├── activate
│ ├── activate.csh
│ ├── activate.fish
│ ├── easy_install
│ ├── easy_install-3.6
│ ├── pip
│ ├── pip3
│ ├── pip3.6
│ ├── python -> python3.6
│ ├── python3 -> python3.6
├── include
├── lib
│ └── python3.6
│ └── site-packages
└── pyvenv.cfg
Activating virtual envionment
On Windows run:
env\Scripts\activate.bat
On Unix or MacOS run:
source env/bin/activate
If the above action was successful, you should see the name of the virtual environment in your command line.
(env) $
Also try the following command:
which python
It shows the current Python file that is being used (It should be inside the virtual environment folder that we just created.)
/home/usr/my_project/env/bin/python
Install a third-party module
pip install requests
The new module will be downloaded to the env/lib/python3.6/site-packages folder.
Deactivating virtual environment
deactivate
Freezing dependencies and Source Control
You can create a file that lists all the dependencies used in the project. You can then share this file so that others can install the dependencies.
pip freeze > requirments.txt
Now we have a requirements.txt file with the following list.
certifi==2018.11.29
chardet==3.0.4
idna==2.8
requests==2.21.0
urllib3==1.24.1
To install from the requirements field, the following command can be used:
pip install -r requirements.txt
Using different versions of Python
You can also use a different version of Python other than your default one using virtual environment.
python3 -m virtualenv env --python=python2.7