Python Virtual Environments

This article describes how you can use "virtual environments" to install Python packages that are not provided in the common software environments on JASMIN. You might wish to do this if you want to use different packages/versions from those installed on the system, or if you have requested for a package to be installed system-wide but wish to start using it before this request can be acted upon.

To decide whether you should use a Python virtual environment or a Conda environment for this purpose, see: overview of software environments.

What is a "virtual environment"?

A "virtual environment" is a self-contained directory tree that contains a Python installation for a particular version of Python (such as 2.7, 3.7, 3.8), plus a number of additional packages. It provides a very useful method for managing multiple environments on a single platform that can be used by different applications.

Creating a virtual environment

As a pre-requisite, when using any modern Python (i.e. Python2.7 onwards), you should activate a Jaspy environment before following the instructions below.

Python allows you to create a directory containing a private virtual environment, into which you can install your packages of choice. This is done differently for python2 and python3, as follows:

# Python 3 onwards:
python -m venv /path/to/my_virtual_env

# Python 2.7
virtualenv /path/to/my_virtual_env

The path can be an absolute or relative path, but it should not already exist. Note: /path/to/my_virtual_env here (and also in the commands shown below) should be replaced by the actual path where you choose to create your virtual environment.

Using the system "site-packages" with your virtual environment

Note that if you create a virtual environment using the above syntax, the packages initially installed in it will only be those in the standard Python library. This means, for example, that the numpy package (not in the standard library, but installed as part of Jaspy) will be unavailable unless you install it yourself. If you would prefer as a starting point to have all the add-on packages which have already been installed in Jaspy, then use instead:

python -m venv --system-site-packages /path/to/my_virtual_env

This will work for most packages in Jaspy. We have seen situations where one or two packages from Jaspy do not work in private virtual environments, and if you are affected by this then please see the "package-specific fixes" section below.

Activating a virtual environment

Before the virtual environment can be used, it needs to be " activated". This is done by running the activate script using the source command:

source /path/to/my_virtual_env/bin/activate

(If you prefer, you can use . instead of source.)

After you run the activate script, some environment variables will be set so that the python (or python2.7, python3) command will point to the one in the virtual environment, allowing installation and use of packages in that environment.

You can see that python points to the python executable in the virtual environment, with:

$ which python
/home/users/my_username/my_virtual_env/bin/python

Note that you have to source the activate script in every shell (login session) in which you intend to use the virtual environment. If there is a particular virtual environment which you want to use consistently, you might consider putting the command to source the activate script in your $HOME/.bashrc file.

If you wish to deactivate the currently active virtual environment in a particular shell, just type deactivate. The environment variable changes will be undone, and you will again be using the system default set of packages. This is also reflected in the shell prompt.

Installing packages into a virtual environment

Once you have activated a virtual environment, the pip utility will be available. This allows package installation into the environment using the command:

pip install your_package

pip is quite flexible what you can use for your_package. It can include:

a package name in the Python Package Index (PyPI)
a URL pointing to a package repository
the local path of a .tar.gz or .zip file containing the package source
the local path of a directory containing the extracted package source
the download URL of a .tar.gz or .zip file

If the package requires other packages that are not already installed into the virtual environment, then pip will use the package's requirements file to install them automatically from PyPI.

To upgrade an existing package, use:

pip install --upgrade your_package

If your Python package cannot be installed with pip for any reason, it can also be installed directly from the setup.py file after activating the virtual environment.

python setup.py install

To install a specific version of a package, this can be specified with:

pip install your_package==1.2.3

Inspecting the virtual environment

To list the packages installed into the virtual environment, with their version numbers, type:

pip freeze

Using the virtual environment

Interactive use

After you have activated the virtual environment in your shell, any packages that you have installed into it can be imported into an interactive python session.

$ python # automatically uses python in your virtualenv

>>> import my_package

Scripts

If a script is run using the python command on the command-line in a similar way to when starting an interactive Python session, this will use any virtual environment that has been activated in the calling shell.

$ python my_script.py

If an executable script is run using the #! mechanism, and the first line of the script has the hard-coded path to the executable in the virtual environment, then it is not necessary to activate the virtual environment in the calling shell.

$  head -n 1 myscript.py  # show the first line
#!/path/to/my_virtual_env/bin/python3.7

$  chmod u+x myscript.py  # ensure that it is executable

$  ./myscript.py  # run it

As an alternative to hard-coding the path of the virtual environment, it is possible to use the /usr/bin/env approach to ensure that the script is run using whichever python executable is found via $PATH. The script will then run using any virtual environment that has been activated in the calling shell. This makes the script more portable, although at the expense of having to source the activate script.

$  head -n 1 myscript.py
#!/usr/bin/env python3.7

$  chmod u+x myscript.py

$  ./myscript.py

Package-specific fixes

When using the --system-site-packages option in combination with Jaspy, it has been found that some packages provided by Jaspy (and which work correctly in the Jaspy environment itself) require fixes in order to use them in virtual environments that are created on top of Jaspy. In particular:

if you use shapely, we suggest to reinstall this into your virtual environment using pip install --ignore-installed shapely after activating the environment
if you use geopandas, you will need to reinstall shapely as above, and also when running python you will need to set an environment variable to enable it to find the spatialindex library. After loading Jaspy and activating the virtual environment, you could use either one of the following:
- export LD_LIBRARY_PATH=$CONDA_PREFIX/lib:$LD_LIBRARY_PATH
- export LD_PRELOAD=$CONDA_PREFIX/lib/libspatialindex.so:$CONDA_PREFIX/lib/libspatialindex_c.so
Note that these environment variables could potentially also affect the behaviour of other Linux commands, although unlikely, so you might prefer to set them only for the python session (using a command of the form env variable_name=value python) rather than using export.
if you use cartopy (also used by iris), you may need to create a symbolic link into your virtual environment to allow the correct loading of libgeos_c.so during import cartopy or import iris. To do this:
```
ln -s $CONDA_PREFIX/lib/libgeos_c.so /path/to/my_virtual_env/lib/
	
```