Ryan Harrison My blog, portfolio and technology related ramblings

Setting up a Python Virtual Environment

You should setup a Python virtual environment to ensure that your library dependencies are consistent and segregated from your global packages. This can help to prevent potential versioning conflicts and makes it easier to package your app for use by others (or you, but on a different machine).

Previously, you had to install dedicated packages to create and manage Python virtual environments - such as pipenv or virtualenv. Python does however now come with it’s own solution venv which accomplishes much of the same and can be run directly as a Python module without any installation steps.

Create a Virtual Environment

As the venv module comes preinstalled, you can create a new virtual environment by running:

python -m venv virtenv

This will create a new directory called virtenv in your current directory (you can call it whatever you want - general naming scheme is venv) which will include it’s own Python interpreter, pip installation and any packages you subsequently install once the environment is activated.

If you look inside the new directory, you will find it has it’s own Lib/site-packages structure (where any new packages will be installed), alongside it’s own Python / pip executables. The version of Python within your new virtual environment will be the same as the one you used to run the venv command above.

Activate the Environment

To ‘activate’ the virtual environment, you need to call the activate shell script which got created by the previous command. This sets a bunch of environment variables to point the python / pip commands to your newly created venv instead of the globally installed version - in effect creating a completely separate Python installation.

If on Windows - virtenv\Scripts\activate.bat

If on Linux/Mac - virtenv/Scripts/activate

You should notice that your shell prompt got changed to include the name of the venv at the start. If you now run the where / which commands to show the location of the executables, it should show those located in the virtenv directory.

where python ==> \virtenv\Scripts\python.exe

where pip ==> \virtenv\Scripts\pip.exe

Install Packages

Running the pip list command shows that we don’t currently have anything installed (even if you had installed something globally).

pip (19.1.1)
setuptools (28.8.0)

You can use the pip command now to install packages as you would normally e.g.

pip install requests

If we check the list of installed packages again, you can see that requests has been added. Note that this could be a different version to the one installed in the global site-packages.

certifi (2019.3.9)
chardet (3.0.4)
idna (2.8)
pip (19.1.1)
requests (2.22.0)
setuptools (28.8.0)
urllib3 (1.25.3)

If you check the virtenv\Lib\site-packages directory, you should find thatrequests has been installed there.

Generate requirements.txt

You can run the pip freeze command to generate a requirements file containing all the currently installed packages - helpful if you want to recreate the exact same environment on a different machine.

pip freeze > requirement.txt

The contents of which will be something like:

certifi==2019.3.9
chardet==3.0.4
idna==2.8
requests==2.22.0
urllib3==1.25.3

Installing Packages from requirements.txt

When on a new machine with another blank virtual environment, you can use the requirements.txt file generated by pip freeze to install all the packages required for your project at once:

pip install -r requirements.txt

Pip will run through each entry in the file and install the exact version number specified. This makes it easy to create consistent virtual environments - in which you know the exact version of every package installed without the hassle of installing each one manually.

Deactivate the Environment

To deactivate the virtual environment and return all the environment variables to their previous values (pointing instead to your global Python installation), simply run:

deactivate

The virtual environment name should be removed from your shell prompt to denote that no environment is currently active.