Delft University of Technology
Jupyter notebook, the next generation of IPython, is an open-source web-application that allows the user to create and share documents containing live code, comments, formulas, images and more, all in one place. The current chapter analyses this project in depth. Our team was able to make six pull requests regarding technical debt and bug fixes, which were accepted. A good overview of the whole project is given by analysing stakeholders and the context view. Next, various viewpoints and perspectives on the architecture of the project are investigated. This chapter aims to help people who would like to join the Jupyter community and make contributions, by providing extensive analysis of the project.
Table of Contents
- Technical Debt
- Evolution Perspective
Jupyter Notebook is a cross-platform open source web application which allows running code, visualising its output (such as plots, images, tables etc.) and writing explanations in natural language, accompanied by equations and all this in one so-called Notebook file. It provides the opportunity to process large amount of data and supports Julia, Python, R (origin of the name Jupyter) and many other programming languages. It is intuitive, easy to install and use, which makes it suitable for scientists, programmers and learners.
It is adopted by teachers in universities such as UC Berkeley where they use JupyterHub (server, hosting Notebooks where users log in and immediately start coding) in which students do their assignments. Furthermore, Github supports the rendering of Notebook
.ipynb files. Entire books have been written with Notebook and published on Github. Multiple online courses on Machine Learning, Computer Science, Python programming, Data Analysis and others can be found in this Github repository.
This chapter aims to give users a quick understanding of how Jupyter Notebook is organised, developed and maintained. First, the Stakeholders and Context view are analysed to provide insights into the organisation of the project. Furthermore, detailed analysis of the architecture is presented in order to understand its structure. In addition, technical debt in terms of code, testing and documentation was also found throughout the analysis of the system and is presented in the chapter. This is followed by the evolution of the project explaining how Notebook emerged from IPython and discussing the JupyterLab project which is considered as the future of Notebook. Finally, the chapter concludes with our findings of the project.
This section discusses the parties that are involved in the development, maintenance, testing and building of the system. These stakeholders are then visualised using a context-view diagram.
A number of different types of stakeholders exist as defined by Rozanski & Woods  and this section goes through them w.r.t. Jupyter Notebook.
As Jupyter Notebook's team consists of fifteen core developers who oversee the conformance of the (programming & other) standards. They are responsible for handling pull requests and communicate them between each other to decide whether or not to merge. Furthermore, they do the planning of future releases and/or sub-systems of the Notebook.
Communicators are people who explain the system and the architecture to other stakeholders via documentation or training materials. A number of presentations are available online, prepared and presented by Fernando Perez and Matthias Bussonier. Jupyter Notebook communicators are also all the developers who have written the documentation. They are a small team and all of them update the documentation when necessary. The documentation is useful as well for developers and contributors to the system.
Two major types of Developer stakeholders were identified in Jupyter Notebook: core and contributors.
- Core: Core developers work actively to write, fix, test and improve the system. Their main job is to also go through the open pull requests on Github and ensure that the contribution proposals are well tested and do not interfere with the current system in any way other than to improve it. Core developers are @carreau, @takluyver, @jasongrout, @rgbkrk, @minrk and @blink1073.
- Contributors are important for open source projects as they often solve issues the core team has no time for or implement new features making the product even better. They contribute also by improving documentation, bug fixes and even by writing their own kernel supporting a new programming language. Examples of these contributors are @vidartf and @yuvipanda.
The Steering Council consists of Project Contributors writing substantial code of high quality and quantity for over one year. Working with the BDFL (Benevolent Dictator for Life) Fernando Perez the council ensures the long-term progress and existence of the project. The NumFOCUS Foundation serves as the projects' fiscal sponsor and acts as a parent legal entity. The NumFOCUS Subcommittee consists of four Council and one external member.
As per the book , support staff are the people who provide support to the users. Therefore Jupyter Notebook members taking care of issues in their Github repository could be considered support staff. These users are usually @minrk and @takluyver. In case they are not sure about something, they know who to get involved.
Testers are people who test the system to ensure it is suitable to use. Since the Notebook team consists of only fifteen core developers, each of them has to write tests whenever they implement a new feature or improve the current code. All of them can thus be considered testers as well. However, while going through the project a few members were identified who wrote new or updated the old tests frequently: @jdfreder (no longer active), @minrk (core dev), @ssanderson (contributor) and others.
The two biggest categories of users are the following:
- Scientists: Jupyter is mainly used by data scientists working with R and Python on daily basis, however, it is not only aimed at them but is generally used by other types of users as well.
- Learners are stakeholders who, for example, use Notebook for study & teaching purposes.
Sponsors are an additional stakeholder class to the main ones, related to the project. It consists of companies and universities that support the project financially. Some of them are Google, Microsoft, Rackspace, Leona M. and Harry B Helmsley Charitable Trust, Gordon and Betty Moore Foundation etc. They do not take decisions about the projects' progress, however, they may influence these decisions.
In this section, the context viewpoint of Jupyter Notebook is described. The context view describes and visualises the relationships and interactions between Jupyter Notebook with the environment.
According to the Jupyter foundation website, the Jupyter Notebook is
an open-source web application that allows you to create and share documents that contain live code, equations, visualisations and explanatory text.
The Notebooks are typically used for data cleaning and transformation, numerical simulation and machine learning tasks; the target audience is the broad tech audience, spanning from high school students to the most advanced researchers of the world.
Jupyter Notebook is the main application of the broader Jupyter project, which focuses on interactive and exploratory computing that is reproducible and multi-language.
Figure 1 - Context Model of Jupyter Notebook.
In Figure 1, the context model of Jupyter Notebook is displayed. A short description of the most important entities in the diagram follows.
Thanks to the interoperability of Python, the project can run on most platforms, including Windows, Mac OS X and other Unix-like systems. There are no clear indications of officially supported browsers, but the documentation recommends the usage of Chrome, Firefox or Safari.
The system has multiple external dependencies, both in the front-end and in the back-end. In the former case, the key ones are Bootstrap, Backbone.js, CodeMirror, Marked and MathJax. In the latter, they are Jinja2 and Tornado. The dependencies are managed using package managers, in particular, Bower is used for the front-end and NPM is used for the building tools.
Sponsoring occurs in two ways: directly from companies like Google, Microsoft and Rackspace or by employing members of the Steering Council. The latter is the case for companies like Bloomberg, Netflix and for the Berkeley University of California. The development is carried on by the members of the Steering Council, including the BFDL Fernando Pérez, and by the Github community.
As far as the documentation goes, it is maintained using the Sphinx documentation generator and published on the Read The Docs hosting platform.
The main competitors are the Beaker Notebook and Apache Zeppelin, both of which have shorter history and have not yet reached the maturity level of Jupyter.
Jupyter Notebook is licensed using the 3-clause BSD license, while the copyright of the code belongs to the respective authors
The development view describes the architecture of a project from the viewpoint of the developers. According to Rozanski and Woods , the development view is responsible for addressing different aspects of the system development process such as code structure and dependencies, build and configuration management of deliverables, system-wide design constraints, and system-wide standards to ensure technical integrity. In the following sections, the development view of Jupyter Notebook is presented based on the three models that Rozanski and Woods  define in their book: Module Structure Model, Common Design Model and Codeline Model. In addition to this, a high-level view of Jupyter Notebook is included in order to ensure a thorough understanding of the project in different granularity.
Figure 2 - High-level view of Jupyter Notebook. 
The high-level view is visualized in Figure 2. First, the user interacts with the browser, consequently, a request is sent to the Notebook server. This can be either HTTP or WebSocket request. If user code has to be executed, the notebook server sends it to the kernel in ZeroMQ messages. The kernel returns the results of the execution. At last, the notebook server returns an HTML page to the user.
When the user saves the document, it is sent from the browser to the notebook server. The server saves it on disk as a JSON file with a
.ipynb extension. This notebook file contains the code, output and markdown notes. 
Module Structure Model
The module structure model defines the organisation of the system's code clustering related source code files into modules and determining the dependencies between them . In this section first the modules of the project are briefly described and then the dependencies between them are visualised in a diagram. It should be also noted that this section focuses only on the internal modules of the project and not the external dependencies.
Here follows a short description of the modules that make up the project:
- notebookapp: it is the entry point of the web application, including the Tornado-based server; it is responsible for the configuration of the server, including the mapping of requests to the handlers classes.
- auth: it manages the authentication to the notebook and other security-related features.
- base: it contains the base Tornado handlers that are extended by the other ones throughout the structure of the directory hierarchy; moreover, the handlers for the ZeroMQ sockets, which are responsible for the communication with the kernel, are included.
- bundler: it is used in order to bundle the notebook documents, packaging them in tarballs or zip archives.
- edit, kernelspecs, nbconvert, view, files, terminal, tree, notebook: they contain the handlers for rendering the text editor interface, handling the views that are displayed to the user, converting notebook files to other formats, serving files via the content manager, rendering the terminal interface, displaying the tree view and controlling the live notebook view respectively.
- frontend: this module contains the code related to the user interface; in particular,
templatesconsists of the Jinja2 HTML templates rendered by Tornado.
- services: it contains the handlers for the backend services, including kernels, kernel specifications and contents web services; all the handlers registered here start with the
/api/prefix and communicate with the frontend using JSON whereas the other handlers render the HTML views.
The module structure and dependencies are shown in Figure 3.
Figure 3 - Module dependencies diagram.
Furthermore, it should be noted that the figure above shows high-level dependencies between the most significant modules of the system. In this graph modules that share a similar role in the system are grouped together for the convenience of the reader. The same purpose serve the colours of each of the modules. The diagram starts from the top with the
notebookapp module which constitutes the entry point of the web application. It is worth mentioning that there are no circular dependencies which makes the system easier to understand and maintain.
Source code structure
The overall structure of the directory hierarchy of the Jupyter Notebook is organised as follows. The root folder consists of several files that are used to configure the system. More specifically, the root folder contains YAML files that are responsible for configuring the continuous integration of the project such as
appveyor.yml. Other important configuration files that are included in the root folder are
bower.json manifest file keeps track of the right versions of the front-end packages that are needed for the system. The
setupbase.py files are essential for configuring the installation of packages and their dependencies.
Furthermore, the root folder is divided into five separate folders: the
docs/ folder where all the documentation files that are built using Sphinx are located, the
scripts/ which contains scripts that are used as an interface to the components of the notebook for the command line, the
tools/ folder containing tools used in the build process and the
notebook/ which contains all of the source code.
notebook/ folder is divided into sub-folders by grouping together source files with similar purpose and functionality. First off, it is divided into sub-folders that contain back-end functionality which are structured in a similar way. The related initializers can be found in
__init__.py for the given sub-folder and next to that every sub-folder contains a module called
handlers.py in which the Tornado handlers are included and registered. In addition, a
tests/ folder is included which consists of files that test the Tornado handlers functionality.
static folder using as the name for the sub-folders the one of the back-end components they interact with. The Jinja2 HTML templates are included in the
templates/ folder. Concerning the tests that are responsible for testing the front-end functionality, they are contained in the
notebook/tests/ folder. Finally, the
notebook/ folder contains the entry point of the web application which is the
notebookapp.py module and some common utilities that are used throughout the whole source code. A graphical overview can be found in Figure 4.
Figure 4 - Source code structure of Jupyter Notebook.
Continuous Integration and Testing Approach
Continuous Integration (CI) refers to the development practice in which developers integrate the code to a shared repository frequently. Each integration is verified by an automated build and test system.
The Python backend is tested using the
nose testing framework, a package built on top of
unittest to simplify test-driven development.
This testing framework has been in maintenance mode for several years now and its maintainers are suggesting to move to the newer framework
nose2, but the Jupyter Notebook maintainers don't seem to have any plan to move forward.
Continuous Integration Services
The project uses the integration offered by Github to automate building and testing of the pull requests as they are opened or updated. In particular, three CI services are used: Travis CI, AppVeyor and Codecov.
Travis CI is used by the project to automate build and testing on Linux. The configuration, specified using the
.travis.yml file, is such that:
- Testing is done using both Python 2.7 and 3.5.1
- Both the frontend and the backend are tested
- When testing is done, the Python coverage reports generated by Nosetests are uploaded to Codecov.
Since Travis CI does not yet support the Windows OS, AppVeyor is used for the testing on this platform. The configuration is such that only the backend is tested, using both Python 2.7 and 3.5, and code coverage is not checked.
Codecov is a hosted CI service that computes a coverage score of the unit tests.
The Jupyter Notebook project uses Codecov to analyse the coverage of the Python backend tests at each pull request. The coverage is not checked for the frontend because the testing framework does not support the generation of coverage reports. The service is configured by means of the
codecov.yml file  so that the automatic checking of the pull requests fails if a decrease in the coverage score greater than 10% is observed.
Rozanski and Woods  explain that the functional view "defines the architectural elements that deliver the function of the system being described" (p. 294).
This view documents the key runtime functional elements, their responsibilities, interfaces and primary interactions.
In the notebook project, different key functional elements are present that interact with internal and external elements during run-time.
Interfaces like in
Java are not present in
But these functional elements do behave like interfaces, as they have well-defined functions that can be called by other elements, to perform different actions.
The functional view of our project is visualised on figure 5. The key elements and the interaction between them is described next.
Figure 5: UML diagram of the Functional View.
The main entry point of the web application is an instance of the class
It also launches an instance of the
NotebookWebApplication class. The server delegates all the incoming requests to the
This class is a sub-class of
Tornado.Web.Application, which is used to map the different types of incoming requests to the correct handlers.
The handlers use external elements to leverage existing functionalities from the other Jupyter projects.
Jupyter_core is used for basic functionalities like handling configuration files and filesystem locations.
Jupyter_client provides APIs for working with kernels. Kernels are used to execute user code.
nbformat provides APIs for working with notebook files, such as loading, reading and saving.
nbconvert to export notebook files into various static formats such as PDF, HTML and LaTeX.
NotebookApp can only be used by a single user with no login. On top of the notebook, there are other external elements which provide additional functionalities.
tmpnb can launch a temporary Jupyter notebook server for multi-user use, without login.
jupyterhub can create a multi-user hub with login which manages multiple instances of the single-user
nbgrader provides a toolbar extension which allows a teacher to make and grade assignments on the notebook.
The technical debt concept is related to the extra development work that is introduced when solutions that are easy to implement in the short run are used instead of applying the optimal ones that will keep the project maintainable in the long run; it is not only related to the code, it can also be associated with the (lack of) documentation, or with the low testing coverage.
This section focuses on the following three types of technical debt: code debt, testing debt and documentation debt.
Coping with code debt as soon as possible is important because the size of the code, as it grows, will make the maintenance difficulty ever increasing. The identification of code debt has been done using both automated analysis tools and via manual inspection.
In relation to the code readability, the Jupyter project documentation states in the coding style section that the code should follow the PEP8 guidelines.
To investigate the adherence of the code to the guidelines the
flake8 tool was used to lint the codebase, discovering numerous inconsistencies, including mis-indentation, usage of tabs, unused imports, multiple imports on the same line and lambda expressions assigned to variables, for a total of 2075 errors/warnings.
Deprecation warnings are another symptom of code debt, as they mean that the developers are not keeping the code up to date with the evolution of the ecosystem. An example of this kind of debt was discovered while running the automated backend test suite. In particular, the
decodestring() is a deprecated alias, use decodebytes() deprecation warning was shown.
As the Python documentation states, there is a new API to work with base 64 encoding (base64.b64decode) which is supposed to be used. A pull request (#2280) was submitted to the project repository to fix this issue.
Another way of identifying code debt is by manually searching for "TODO" and "FIXME" comments in the codebase. As of 13/03/2017 Jupyter Notebook had 21 "TODO" and 10 "FIXME" comments in the code. A TODO comment was found in the page.js file, referring to the removal of hard-coded selectors from the code. Hard-coded strings are generally considered a bad coding practice that seriously hinders the flexibility of the code. A pull request (#2279) that tackles this code debt refactoring the parameters to be dynamic was submitted. A FIXME comment was also found in the file handlers.py, mentioning that a certain functionality should have been implemented in the frontend rather than in the backend. An examination of the frontend code revealed that the functionality was already present there and thus pull request #2281 was submitted to remove the redundancy.
For what concerns the backend testing, the coverage is reported by
nose and analysed by CodeCov. The actual coverage on the master branch is 77.24%. Test coverage usefulness as a metric has been debated, and the general consensus is that it is mainly a tool to discover untested parts of the codebase, rather than being an assurance that the codebase is well tested. The source files with the lowest reported coverage were analysed, and in the process, it was discovered that the tests for a certain file were not run because of a missing requirement in Travis CI; pull request #2283 was opened to fix the issue resulting in a 1.09% increase of the overall coverage.
It also seems like not all the developers are up to date with the frontend testing. In addition to this, Travis CI was configured to use the
travis_retry for the frontend tests, in order to repeat them 3 times in the case that they fail. This function is supposed to be used to deal with network timeouts during the requirements installation and is therefore misused in this context.
Developers need to write documentation of their source code, which may be of help for other developers to understand their code. More importantly, when they would not work anymore for a project, people coming after and new contributors should also be able to understand the system and its setup. In addition, documentation for the end user of the product is needed in order to understand how to download, install and use the product.
Jupyter Notebook documentation is lacking in many parts of the project. Specific examples are listed below:
- An example of bad documentation is observed in issue #603 where a new user tries to use the shortcuts within Notebook but runs into trouble because the functionality is not clearly explained. This is supposed to be fixed in release 5.0 as @pkgw and @ellisonbg advised.
- Another documentation debt is identified in the issue #1754 where user @dsblank wanted to contribute to the project and tried to install the development environment on Ubuntu but got a lot of errors. Eventually, he managed to fix the issue by looking at the setup of the
.travis.ymlfile. The user notes that this is not explained in the documentation.
Finally, the biggest part of documentation debt was found from the first sight, by just looking at the title, which says (source: old IPython wiki) (IPython Development Guide) and the whole documentation is outdated. There is, in fact, a note that states:
This is copied verbatim from the old IPython wiki and is currently under development. Much of the information in this part of the development guide is out of date.
This is a clear indication of debt that needs to be paid and may cause confusion about where the correct documentation is if such exists and where is it located. Improvements of documentation are always welcome by the core developers and they try to encourage contributors to help them with this.
In this section, the evolution of the project is presented, along with the most important facts and changes part of its history.
The Past: IPython
The predecessor of Jupyter Notebook is IPython. It was born in 2001 as an afternoon hack by the hands of Fernando Perez , in the attempt to improve the Python REPL. It was a 250 lines Python script trying to mimic the Wolfram Mathematica prompt system to enable scientific computing with Python.
Over the years the need for a notebook-like front-end grew, and after multiple unsuccessful attempts, in 2011 the IPython Notebook was born. The key in building this fully working web-based notebook system was the coupling of the Tornado webserver for asynchronous WebSocket-based communication and ZeroMQ for the communication with the kernels.
Many features were added in the following years following the users' needs, and the codebase developed organically incorporating different components that were increasingly becoming distinct projects. Moreover, at this point many different languages were supported by the project, so the name Interactive Python was starting to feel odd.
The Present: Jupyter
For these reasons, in 2014 at the SciPy conference, the Jupyter project was announced, incorporating all the language-agnostic features of IPython, such as the notebook.
Figure 6 - Evolution of LOC of the IPython project. 
In 2015 the first phase of The Big Split™ was completed, splitting all the components into subprojects. As can be seen from Figure 6, a vast amount of code was removed from the IPython project and moved to separate repositories corresponding to the different subprojects, such as traitlets, nbformat and the notebook itself. IPython continues to live to this day as a kernel for Jupyter and as the interactive shell environment.
The Jupyter project gained more popularity, not only among data scientist but also among software engineers. At this point, Jupyter Notebook was not only about the notebook experience because it also shipped with a web-based terminal, text editor, and file browser. All these components were not blended together and the user community started expressing the need for a more integrated experience.
The Future: JupyterLab
At the SciPy 2016 conference, the JupyterLab project was announced. It was described as the natural evolution of the Jupyter Notebook interface.
The codebase of the Notebook was showing its age and it was becoming more and more difficult to extend. The cost of maintaining the old codebase and implementing new features on top of it was ever increasing.
The developers incorporated in this new project all the lessons that they learned from the usage patterns of the Notebook, to build a robust and clean foundation for a flexible interactive computing experience and an improved user interface.
Figure 7 - JupyterLab User Interface.
In figure 7 the user interface of the JupyterLab project, based on the PhosporJS framework, is shown. The division in tabs and panes allows using different components (file browser, terminal, notebook), that can be used only separately in the classic Notebook, at the same time.
As of April 2017, the project is still in the early preview phase, and it is not suitable for general usage yet. A series of phases are planned to enable a smooth transition from the classic Notebook to the new environment, as eventually JupyterLab will become the default UI and the classic notebook will only be available as a separate download.
Jupyter notebook is currently the most popular and widely used tool for students and data scientists to create and share documents containing live code, explanations and visualisations. Its popularity can be validated by the fact that Github natively supports the rendering of notebook files.
While going through the code, our team found five issues that could give problems in the long-run i.e. non-compliance to code style, hard-coded variables, duplicate code, deprecated methods and low test coverage. Pull requests were made to pay off these technical-debt-related issues and four of them got accepted. Two additional pull requests, related to bug fixes, were made and both were merged to master. The developers provided extensive feedback and suggestions to our pull-requests which was very helpful.
Even though the documentation of the project was lacking for the most part and the analysis of the project's architecture proved to be more challenging than we thought, our team managed to understand the inner workings of the system. We learned that real-world software does not always correspond to the ideal architectures that we discuss in theory, but it can nonetheless be successful. We also learned that The code is the truth, but it is not the whole truth (Grady Booch), and that the Jupyter ecosystem, including its community, plays a major part in the success of the project. We are excited to see what the future has in store for the Jupyter project.
If you are considering joining the Jupyter notebook community and make contributions, do not hesitate. The Jupyter community will welcome you with open arms. This chapter together with the contribution guidelines and documentation will guide you towards your contributions.
- Nick Rozanski and Eoin Woods. Software Systems Architecture: Working with Stakeholders using Viewpoints and Perspectives. Addison-Wesley, 2012.
- Jupyter documentation (http://jupyter.readthedocs.io/en/latest/architecture/how_jupyter_ipython_work.html)
- The IPython notebook: a historical retrospective (http://blog.fperez.org/2012/01/ipython-notebook-historical.html)
- IPython LOC Analysis (https://www.openhub.net/p/ipython/analyses/latest/languages_summary)