An introduction to the h codebase

If you’re new to the team, or to the Hypothesis project, you probably want to get up to speed as quickly as possible so you can make meaningful improvements to h. This document is intended to serve as a brief “orientation guide” to help you find your way around the codebase.

This document is a living guide, and is at risk of becoming outdated as we continually improve the software. If you spot things that are out of date, please submit a pull request to update this document.

This guide was last updated on 11 Apr 2017.

A lightning guide to Pyramid

The h codebase is principally a Pyramid web application. Pyramid is more of a library of utilities than a “framework” in the sense of Django or Rails. As such, the structure (or lack thereof) in our application is provided by our own conventions, and not the framework itself.

Important things to know about Pyramid that may differ from other web application frameworks you’ve used:

  • Application setup is handled explicitly by a distinct configuration step at boot. You’ll note includeme functions in some modules – these are part of that configuration system.
  • The request object is passed into views explicitly rather than through a threadlocal (AKA “global variable”), and is often passed around explicitly to provide request context to other parts of the application. This has a number of advantages but can get a bit messy if not managed appropriately.

You can read more about the distinguishing features of Pyramid in the excellent Pyramid documentation.

Application components

The important parts of the h application can be broken down into:

SQLAlchemy models representing the data objects that live in our database. These live in h.models.
Views (and templates)

Views are code that is called in response to a particular request. Templates can be used to render the output of a particular view, typically as HTML.

With a few exceptions, views live in h.views, and templates live in the h/templates/ directory.


Putting business logic in views can quickly lead to views that are difficult to test. Putting business logic in models can lead to model objects with a large number of responsibilities.

As such, we put most business logic into so-called “services.” These are objects with behaviour and (optionally) state, which can be retrieved from the request object.

Services live in


Tasks are bits of code that run in background workers and which can be easily triggered from within the context of a request.

We use Celery for background tasks, and task definitions can be found in h.tasks.

There are a number of other modules and packages in the h repository. Some of these (e.g. h.auth, h.settings) do one-off setup for a booting application. Others may be business logic that dates from before we introduced the services pattern, and thus might be more appropriately moved into a service in the future.