Python Package Set-up
OverviewTeaching: 40 min
Exercises: 5 minQuestions
What is the layout of a Python package?
How can I quickly create the structure of a Python package?
What license should I choose for my project?Objectives
Explain Python package structure.
Use the CMS CookieCutter to build a Python package.
For this workshop, we are going to create a Python package that performs analysis and creates visualizations for molecules. We will start from a Jupyter notebook that has some functions and analysis. (You should have downloaded the Jupyter notebook during setup.)
The idea is that we would like to take this Jupyter notebook and convert the functions we have created into a Python package. That way, if anyone (a lab-mate, for example) would like to use our functions, they can do so by installing the package and importing it into their own scripts.
To start, we will first use a tool called CookieCutter, which will set up a Python package structure and several tools we will use during the workshop.
Examples of Python package structure
Having a similar way to lay out Python packages allows people to more easily understand and contribute to your code.
Creating a Python package using CookieCutter
To create a skeletal structure for our project, we will use the MolSSI Computational Molecular Science (CMS) CookieCutter. The CMS CookieCutter is a special cookiecutter created specifically by MolSSI to use the tools and services we recommend in developing a Python project.
CookieCutter will not only create our directory layout, but will also set up many tools we will use including testing, continuous integration, documentation, and version control using git. We will discuss what all of these are later in the workshop.
You should have the general CookieCutter installed, according to the directions given in the setup portion of this workshop. If you do not, please navigate to setup and follow the instructions given there.
$ cookiecutter gh:molssi/cookiecutter-cms
This command runs the cookiecutter software (
cookiecutter in the command)
and tells cookiecutter to look at GitHub (
gh) in the repository under
This repository contains a template that cookiecutter uses to create your project,
once you have provided some starting information.
You will see an interactive prompt which asks questions about your project.
Here, the prompt appears first, followed by the default value in square brackets.
The first question will be on your project name.
You have very cleverly decided to give it the name
(it’s like molecule, but with
cool instead, because of your cool visualizations - get it?)
Answer the questions according to the following.
If nothing is given after the colon (
:), hit enter to use the default value.
project_name [ProjectName]: molecool repo_name [molecool]: first_module_name [molecool]: functions author_name [Your name (or your organization/company/team)]: *YOUR_NAME_HERE* author_email [Your email (or your organization/company/team)]: *YOUR_EMAIL_ADDRESS_HERE* description [A short description of the project (less than one line).]: A Python package for analyzing and visualizing xyz files. Select open_source_license: 1 - MIT 2 - BSD-3-Clause 3 - LGPLv3 4 - Not Open Source Choose from 1, 2, 3, 4 (1, 2, 3, 4) : 2 Select dependency_source: 1 - Prefer conda-forge over the default anaconda channel with pip fallback 2 - Prefer default anaconda channel with pip fallback 3 - Dependencies from pip only (no conda) Choose from 1, 2, 3 (1, 2, 3) : Select include_ReadTheDocs: 1 - y 2 - n Choose from 1, 2 (1, 2) :
About these decisions
The first two questions are for the project and repository name.
The project name is the name of the project, while the repo name is the name of the folder that cookiecutter will create. Usually, you will leave these two to be the same thing.
repo_name is the name which you will use to import the package you eventually create, and because of that has some rules.
repo_name must be a valid Python module name and cannot contain spaces.
The next choice is about the first module name.
Modules are the
.py files which contain python code.
The default for this is the
repo_name, but we will change this to avoid confusion (the module
molecool.py in a folder named
molecool in a folder named
For now, we’ll just name our first module
functions, and this is where we will put all of our starting functions.
Another thing that CookieCutter checks for is your email address.
Be sure to provide a valid email address to
(It must have an
@ symbol followed by a domain name, or
cookiecutter will fail.)
Note that your email address is not recorded or kept by the CookieCutter software, itself.
cookiecutter inserts your email address into generated files so that people using your software will have contact information for you.
Choosing which license to use is often confusing for new developers. The MIT license (option 1) is a very common license, and the default on GitHub. It allows for anyone to use, modify, or redistribute your work with no restrictions (and also no warranty).
Here, we have chosen the
BSD-3-Clause license is an open-source, permissive license (meaning that few requirements are placed on developers of derivative works), similar to the MIT license.
However, it adds a copyright notice with your name and requires redistributors of the code to keep the notice.
It also prohibits others from using the name of the project or its contributors to promote derived products without written consent.
You can see more detailed information on each license at choosealicense.com or by clicking the links below:
- MIT License
- Not Open Source - In this case, the cookiecutter will not generate a license. You can add a custom license, or choose to not add a license. If there is no license in a repository, you should assume that the project is not open source, and you cannot modify or redistribute the software.
For most of your projects, it is likely that the license you choose won’t matter a great deal. However, remember that if you ever want to change a license, you may have to get permission of all contributors. So, if you ever start a project that becomes popular or has contributors, be sure to decide your license early!
Types of Open-Source Licenses
Open-source licenses can either be ‘permissive’ or ‘copy-left’. Copy-left licenses require that derivative works also be open source. Out of the choices given above, MIT and BSD-3-Clause are permissive, while LGPLv3 is a copy left license.
This determines some things in set-up for what will be used to install dependencies for testing. This mostly has consequence for the section on Continuous Integration. We have chosen to install dependencies from anaconda with pip fallback. Don’t worry too much about this choice for now.
Support for ReadTheDocs
This option is to choose whether you would like files associated with the documentation hosting service ReadTheDocs. Choose “yes” for this workshop.
Reviewing directory contents
Now we can examine the project layout the CookieCutter has set up for us.
Navigate to the newly created
You should see the following directory structure.
. ├── CODE_OF_CONDUCT.md <- Code of Conduct for developers and users ├── LICENSE <- License file ├── MANIFEST.in <- Packaging information for pip ├── README.md <- Description of project which GitHub will render ├── molecool <- Basic Python Package import file │ ├── __init__.py <- Basic Python Package import file │ ├── functions.py <- Starting package module │ ├── data <- Sample additional data (non-code) which can be packaged. Just an example, delete in production │ │ ├── README.md │ │ └── look_and_say.dat │ └── tests <- Unit test directory with sample tests │ ├── __init__.py │ └── test_molecool.py ├── devtools <- Deployment, packaging, and CI helpers directory │ ├── README.md │ ├── conda-envs <- Conda environments for testing │ │ └── test_env.yaml │ ├── legacy-miniconda-setup <- Legacy Travis CI Helper, will likely be removed in later version │ │ └── before_install.sh │ └── scripts │ └── create_conda_env.py <- OS agnostic Helper script to make conda environments based on simple flags ├── docs <- Documentation template folder with many settings already filled in │ ├── Makefile │ ├── README.md <- Instructions on how to build the docs │ ├── _static │ │ └── README.md │ ├── _templates │ │ └── README.md │ ├── api.rst │ ├── conf.py │ ├── getting_started.rst │ ├── index.rst │ ├── make.bat │ └── requirements.yaml <- Documenation building specific requirements. Usually a smaller set than the main program ├── pyproject.toml <- Generic Python build system configuration (PEP-517). ├── readthedocs.yml ├── setup.cfg <- Near-master config file to make house INI-like settings for Coverage, Flake8, YAPF, etc. ├── setup.py <- Your package's setup file for installing with additional options that can be set ├── .codecov.yml <- Codecov config to help reduce its verbosity to more reasonable levels ├── .github <- GitHub hooks for user contribution, pull request guides and GitHub Actions CI │ ├── CONTRIBUTING.md │ ├── PULL_REQUEST_TEMPLATE.md │ └── workflows │ └── CI.yaml ├── .gitignore <- Stock helper file telling git what file name patterns to ignore when adding files └── .lgtm.yml
To visualize your project like above you will use
If you do not have
tree, you can get it using
sudo apt-get install tree on Linux,
brew install tree on Mac.
tree will not show you the helpful labels after
<- (those were added by us).
CookieCutter has created a lot of files!
They can be thought of as three sections.
In the top level of our project we have a folder for tools related to development (
docs) and to the package itself (
We will first be working in the
molecool folder to build our package, and adding more things later.
... ├── molecool │ ├── __init__.py <- Basic Python Package import file │ ├── functions.py <- Starting package module │ ├── data <- Sample additional data (non-code) which can be packaged │ │ ├── README.md │ │ └── look_and_say.dat │ ├── tests <- Unit test directory with sample tests │ │ ├── __init__.py │ │ └── test_functions.py
This the only folder we actually have to work with to build our package.
The other folders relate to “best practices”,
which do not technically have to be used in order for your package to be working (but you should do them,
and we will talk about them later).
You could build this directory structure by hand, but we have just used
cookiecutter to set it up for us.
This directory will contain all of our Python code for our project,
as well as sample data (in the
data folder), and tests (in the
Packages and modules
What ‘packages’ or ‘modules’ are in Python may be confusing. In general, ‘module’ refers to a single
.pyfile containing Python definitions and statements. It may be imported for use in another module or script. The module name is determined by the file name. A function defined in a module is used (once the module is imported) using the syntax
module_name.function_name(). ‘Package’ refers to a collection of Python modules. The package may also have an
To read more about Python packages vs. modules, check out Python’s documentation.
Navigate inside our package directory. From the directory where you ran CookieCutter,
$ cd molecool
__init__.py file is a special file recognized by the Python interpreter which makes a directory into a package.
This file can be blank in some cases, however, we will use it to define how the user interacts with the functions in our package.
"""A Python package for analyzing and visualizing xyz files.""" # Add imports here from .functions import * from ._version import __version__
The very first section of this file contains a string opened and closed with three quotations. This is a docstring, and has a short description of the file.
The section we will be concerned with is under
# Add imports here.
This is how we define the way functions from modules are used.
In particular, the line
from .functions import *
goes to the
functions.py file, and brings everything that is defined there into the file.
When we use our function defined in
that means we will be able to just say
molecool.canvas() instead of giving the full path
If that’s confusing, don’t worry too much for now.
We will be returning to
__init__.py in a few minutes.
For now, just note that it exists and makes our directory into a package.
Our first module
Once inside the
molecool folder (
molecool/molecool), examine the files that are there.
View the module (
functions.py) in a text editor.
We see a few things about this file.
The top begins with a description of this module surrounded by three quotations (
Right now, that is the file name, followed by our short description,
then the sentence “Handles the primary functions”.
We will change this to be more descriptive later.
CookieCutter has also created a placeholder function called
At the start of the
canvas function, we have a
docstring (more about this in [documentation]),
which describes the function.
We will be moving all of the functions we defined in the Jupyter notebook into python modules (
.py files) like these.
Before proceeding, make sure your
setuptoolspackages are up-to-date.
conda update pip setuptools
pip install --upgrade pip setuptools
Installing from local source.
You may be accustomed to
pip automatically retrieving packages from the internet.
To develop this package, we will want to use what is called “development mode”, or an “editable install”,
so that we can try out our functions and package as we develop it.
We access development mode using the
-e option to
Reviewing the generated config files
Return to the top directory (
Two of the files CookieCutter generated are
These are the configuration files for our packaging and testing tools.
pyproject.toml tells setuptools about your package (such as the name and version) as well as which code files to include.
We’ll be using this file in the next section.
Installing your package
A development install will allow you to import your package and use it from anywhere on your computer.
You will then be able to import your package into scripts in the same way you import
A development installation inserts a link to your project into your Python
site-packages folder so that updates are immediately available the next time
you launch Python, without having to reinstall your package.
To find the location of your site-packages folder, you can check your Python path.
Open Python (type
python into your terminal window), and type
>>> import sys >>> sys.path
This will give a list of locations python looks for packages when you do an import.
One of the locations should end with
The site packages folder is where all of your installed packages for a particular environment are located.
To do a development mode install, type
$ pip install -e .
-e indicates that we are installing this project in editable mode
. indicates to install from the local directory (you could also specify a path here).
Now, if you examine the contents of your site packages folder,
you should see a link to
The folder has also been added to your path (check
Now, we can use our package from any directory, similar to how we can use other installed packages like
Open Python, and type
>>> import molecool >>> molecool.canvas()
This should print a quote.
'The code is but a canvas to our imagination.\n\t- Adapted from Henry David Thoreau'
This should work from anywhere on your computer.
What happens if we use
conda deactivateand attempt to execute the code above? What if we switch directories?
If you are in the project directory, the code will still work. However, it will not work in any other location.
There is a common way to structure Python packages
You can use the CMS CookieCutter to quickly create the layout for a Python package