Examples not working?

If possible, we recommend to be explicit about which version to use by specifying either python2 and python3 in your scripts.

Work with Python

Python 2 and Python 3 are both available via python2 and python3. The command python is an alias for python2. We recommend to be explicit about which version you want to use, also when using the default Python 2, i.e. use python2 when you know your script requires Python 2 and python3 when you know it requires Python 3.

The below examples uses Python 3, but it works analogously in Python 2, i.e. just replace python3 with python2.

Installing Python packages

The standard way to install Python packages is by using the pip package management system. You often find installation instructions online such as:

$ pip install HTSeq

It will not work. If you attempt to run this as-is on the cluster, you get lots of errors complaining about lack of write permissions etc., which is because it tries to install the package in the system-wide Python package folder (to which only sysadms have write permission). You might also see instructions saying you should use sudo ... - that will also not work for the same reason.

There are two ways for non-privileged users to install Python packages using the ‘pip’ module:

Install globally to your home directory (typically ~/.local/lib/python3.6/site-packages/) using python3 -m pip install --user ...
Install locally to a project-specific folder (e.g. ~/my_project/) using python3 -m pip install ... in a self-contained Python virtual environment

Both are done from the terminal. Which one you choose depends on your needs; for some projects you may want to use the virtual environment approach whereas for your everyday work you might want to work toward your global Python package stack. Installing globally is the easiest, because you don’t have to remember to activate a virtual environment and if you need the Python package in different places, you only have to install it once. However, if you are concerned about reproducibility, or being able to coming back to an old project of yours, you most likely want to use a virtual environment for that project so that its Python packages are not updated when you update or install Python packages globally. This is also true if you collaborate with others in a shared project folder.

1. Installing globally (aka “user-site”)

First of all, if an online installation instructions says pip install ..., replace that with python3 -m pip install .... Second, to install globally to your home directory, remember to always specify the --user option. For example,

[alice@dev2 ~]$ python3 -m pip install --user HTSeq
Collecting HTSeq
  Using cached https://files.pythonhosted.org/packages/7e/2e/60045d925c3b8da3f6a7869ce9ae77fd6360ea8749c3ab7f62198b3deed7/HTSeq-0.13.5.tar.gz
Requirement already satisfied: numpy in /usr/lib64/python3.6/site-packages (from HTSeq)
Collecting pysam (from HTSeq)
  Using cached https://files.pythonhosted.org/packages/87/a1/73e80a7a873f3fb0e52d368a4343eb9882b737c932b95020d82251f1087e/pysam-0.16.0.1-cp36-cp36m-manylinux1_x86_64.whl
Installing collected packages: pysam, HTSeq
  Running setup.py install for HTSeq: started
    Running setup.py install for HTSeq: finished with status 'done'
Successfully installed HTSeq-0.13.5 pysam-0.16.0.1

To see all Python packages that you have installed globally, use python3 -m pip list --user. To also see packages installed site wide on the cluster, use python3 -m pip list. Packages installed with python3 -m pip list --user are typically installed to your ~/.local/lib/python3.6/site-packages/ folder. If CLI executables are installed with one of those packages, they are often installed to ~/.local/bin/.

2. Installing to a virtual environment (aka “virtualenv”)

Virtual environment are not used just on computer clusters - many Python users and developers choose to use virtual environment on their local computers whenever they work in Python.

An alternative to install globally to your home directory, is to install to a local folder using a, so called, Python virtual environment. A virtual environment is a self-contained folder that contains the Python executable and any Python packages you install. When you activate a virtual environment, environment variables like PATH is updated such that you will use the Python executable and the packages in the virtual environment and not the globally installed ones.

Below is an example on how to set up a virtual environment and install the HTSeq package and all of its dependencies into it.

2.1 Install required tools (once per account)

In order to use virtual environments, we need the virtualenv tool. Following the above instructions, you can install it to your global stack as:

[alice@dev2 ~]$ python3 -m pip install --user virtualenv
Collecting virtualenv
  Downloading https://files.pythonhosted.org/packages/23/5e/12fd05915bd86af400c075fbe2bf57058c02dafe377623ab0a73961428ce/virtualenv-20.4.0-py2.py3-none-any.whl (5.7MB)
Collecting importlib-metadata>=0.12; python_version < "3.8" (from virtualenv)
  Using cached https://files.pythonhosted.org/packages/f3/ed/da40116a204abb5c4dd1d929346d33e0d29cedb2cedd18ea98f0385dcd92/importlib_metadata-3.4.0-py3-none-any.whl
Collecting filelock<4,>=3.0.0 (from virtualenv)
  Using cached https://files.pythonhosted.org/packages/93/83/71a2ee6158bb9f39a90c0dea1637f81d5eef866e188e1971a1b1ab01a35a/filelock-3.0.12-py3-none-any.whl
Collecting six<2,>=1.9.0 (from virtualenv)
  Using cached https://files.pythonhosted.org/packages/ee/ff/48bde5c0f013094d729fe4b0316ba2a24774b3ff1c52d924a8a4cb04078a/six-1.15.0-py2.py3-none-any.whl
Collecting importlib-resources>=1.0; python_version < "3.7" (from virtualenv)
  Downloading https://files.pythonhosted.org/packages/82/70/7bf5f275a738629a7252c30c8461502d3658a75363db9f4f88ddbeb9eeac/importlib_resources-5.1.0-py3-none-any.whl
Collecting distlib<1,>=0.3.1 (from virtualenv)
  Using cached https://files.pythonhosted.org/packages/f5/0a/490fa011d699bb5a5f3a0cf57de82237f52a6db9d40f33c53b2736c9a1f9/distlib-0.3.1-py2.py3-none-any.whl
Collecting appdirs<2,>=1.4.3 (from virtualenv)
  Using cached https://files.pythonhosted.org/packages/3b/00/2344469e2084fb287c2e0b57b72910309874c3245463acd6cf5e3db69324/appdirs-1.4.4-py2.py3-none-any.whl
Collecting typing-extensions>=3.6.4; python_version < "3.8" (from importlib-metadata>=0.12; python_version < "3.8"->virtualenv)
  Using cached https://files.pythonhosted.org/packages/60/7a/e881b5abb54db0e6e671ab088d079c57ce54e8a01a3ca443f561ccadb37e/typing_extensions-3.7.4.3-py3-none-any.whl
Collecting zipp>=0.5 (from importlib-metadata>=0.12; python_version < "3.8"->virtualenv)
  Using cached https://files.pythonhosted.org/packages/41/ad/6a4f1a124b325618a7fb758b885b68ff7b058eec47d9220a12ab38d90b1f/zipp-3.4.0-py3-none-any.whl
Installing collected packages: typing-extensions, zipp, importlib-metadata, filelock, six, importlib-resources, distlib, appdirs, virtualenv
Successfully installed appdirs-1.4.4 distlib-0.3.1 filelock-3.0.12 importlib-metadata-3.4.0 importlib-resources-5.1.0 six-1.15.0 typing-extensions-3.7.4.3 virtualenv-20.4.0 zipp-3.4.0
[alice@dev2 ~]$ which virtualenv
~/.local/bin/virtualenv
[alice@dev2 ~]$ virtualenv --version
virtualenv 20.4.0 from ~/.local/lib/python3.6/site-packages/virtualenv/__init__.py

2.2 Create a virtual environment (once per project)

Start by creating a folder specific to the project you are currently working on. Each project folder will have its own unique set of installed packages. For a project that requires Python 3, do the following (once):

[alice@dev2 ~]$ virtualenv -p python3 my_project
created virtual environment CPython3.6.8.final.0-64 in 15874ms
  creator CPython3Posix(dest=~/my_project, clear=False, no_vcs_ignore=False, global=False)
  seeder FromAppData(download=False, pip=bundle, setuptools=bundle, wheel=bundle, via=copy, app_data_dir=~/.local/share/virtualenv)
    added seed packages: pip==20.3.3, setuptools==51.1.2, setuptools==51.3.3, wheel==0.36.2
  activators BashActivator,CShellActivator,FishActivator,PowerShellActivator,PythonActivator,XonshActivator

Always remember to specify option -p python3 or -p python2 when you call virtualenv. It makes sure that the intended version of Python is used in the virtual environment when python is called.

2.3 Activate virtual environment (each time you use project)

Now, each time you want to work on your project, go to its folder and active the virtual environment:

[alice@dev2 ~]$ cd my_project 
[alice@dev2 my_project]$ . bin/activate   ## IMPORTANT! Note period in front
(my_project) [alice@dev2 my_project]$ 

Note how (my_project) is prepended to the shell prompt when the virtual environment my_project is activate. This tells you that you run in a customized Python environment. Specifically, python3 now points to a local, frozen version:

(my_project) [alice@dev2 my_project]$ which python3
~/my_project/bin/python3

Similarly, python points to:

(my_project) [alice@dev2 my_project]$ which python
~/my_project/bin/python
(my_project) [alice@dev2 my_project]$ python --version
Python 3.6.8

Note how this local python command points to the local python3 command. What is interesting, and important to notice, is that if we set up a Python 3 virtual environment, then the local python command will point to the local python3 command. In other words, when we use virtual environments, the python command will be using either Python 2 or Python3 at our choice.

To see what Python packages are installed in the virtual environment, use:

(my_project) [alice@dev2 my_project]$ python3 -m pip list
Package    Version
---------- --------
HTSeq      0.13.5
mpi4py     1.3.1
numpy      1.19.5
pip        20.3.3
pysam      0.16.0.1
setuptools 51.3.3
wheel      0.36.2
(my_project) [alice@dev2 my_project]$ 

2.4 Installing packages (once per package)

With a virtual environment enabled, you can install Python packages to the project folder using python3 -m pip install ... without specifying --user. For instance,

(my_project) [alice@dev2 ~]$ python3 -m pip install HTSeq
Requirement already satisfied: HTSeq in ./my_project/lib64/python3.6/site-packages (0.13.5)
Requirement already satisfied: pysam in ./my_project/lib64/python3.6/site-packages (from HTSeq) (0.16.0.1)
Requirement already satisfied: numpy in ./my_project/lib64/python3.6/site-packages (from HTSeq) (1.19.5)

To see which packages are now installed in the virtual environment (the “project folder”) and what their versions are, do:

(my_project) [alice@dev2 my_project]$ python3 -m pip list
Package    Version
---------- --------
HTSeq      0.13.5
mpi4py     1.3.1
numpy      1.19.5
pip        20.3.3
pysam      0.16.0.1
setuptools 51.3.3
wheel      0.36.2
(my_project) [alice@dev2 my_project]$ 

2.5 Returning to a project

Whenever you open a new terminal, make sure to activate the virtual environment (“project folder”), otherwise it will not find the packages you’ve installed. Pay attention to the shell prompt:

[alice@dev2 ~]$ cd my_project 
[alice@dev2 my_project]$ . bin/activate   ## ACTIVATE
(my_project) [alice@dev2 my_project]$ pip3 show HTSeq
Name: HTSeq
Version: 0.13.5
Summary: A framework to process and analyze data from high-throughput sequencing (HTS) assays
Home-page: https://github.com/htseq
Author: Simon Anders, Fabio Zanini
Author-email: fabio.zanini@unsw.edu.au
License: GPL3
Location: ~/my_project/lib64/python3.6/site-packages
Requires: pysam, numpy
Required-by: 
(my_project) [alice@dev2 my_project]$ 

When submitting a job to the scheduler, make sure the job script loads all required modules and activates the virtual environment.

To deactivate a Python virtual environment, either open a fresh terminal (e.g. log out and back in), or use:

(my_project) [alice@dev2 ~]$ deactivate
[alice@dev2 ~]$ 

Note how prefix (my_project) was dropped from the shell prompt and python3 now points to the system-wide installation;

[alice@dev2 ~]$ which python3
/usr/bin/python3

Appendix

Upgrading pip

You will at times get warnings that you are running an old version of ‘pip’:

You are using pip version 8.1.2, however version 20.3.3 is available.
You should consider upgrading via the 'pip install --upgrade pip' command.

Don’t use the suggested command call in that message. Instead, use:

[alice@dev2 ~]$ python3 -m pip install --user --upgrade pip
Collecting pip
  Using cached https://files.pythonhosted.org/packages/54/eb/4a3642e971f404d69d4f6fa3885559d67562801b99d7592487f1ecc4e017/pip-20.3.3-py2.py3-none-any.whl
Installing collected packages: pip
Successfully installed pip-20.3.3

To check the installed version of the ‘pip’ module, use:

[alice@dev2 ~]$ python3 -m pip --version
pip 20.3.3 from ~/.local/lib/python3.6/site-packages/pip (python 3.6)