Using Python Virtual Environments

Refer: https://packaging.python.org/guides/installing-using-pip-and-virtual-environments/

Install on Ubuntu 18.04

sudo apt-get install python3-venv

Create a virtual environment

python3 -m venv env

Activating a virtual environment

source env/bin/activate

Confirm you are pointing to your virtual environment

which python

You should seem something like

.../env/bin/python

Leaving the virtual environment

deactivate

Installing packages, when inside virtual environment

python3 -m pip install requests

Installing specific versions

python3 -m pip install requests==2.18.4

Upgrading packages

pip can upgrade packages in-place using the --upgrade flag. For example, to install the latest version of requests and all of its dependencies:

python3 -m pip install --upgrade requests

Using requirements files

Instead of installing packages individually, pip allows you to declare all dependencies in a Requirements File. For example you could create a requirements.txt file containing:

requests==2.18.4
google-auth==1.1.0

Tell pip to install all the packages in this file using the -r flag:

python3 -m pip install -r requirements.txt

Freezing dependencies

Pip can export a list of all installed packages and their versions using the freeze command:

python3 -m pip freeze

Which will output a list of package specifiers such as:

cachetools==2.0.1
certifi==2017.7.27.1
chardet==3.0.4
google-auth==1.1.1
idna==2.6
pyasn1==0.3.6
pyasn1-modules==0.1.4
requests==2.18.4
rsa==3.4.2
six==1.11.0
urllib3==1.22

This is useful for creating Requirements Files that can re-create the exact versions of all packages installed in an environment.