Python has Setuptools, and the Python Package Index, to assist in packaging and distributing applications to other developers. And Virtualenv allows developers to isolate an application and its dependencies from the rest of the system. However, pip and virtualenv are not user-oriented tools. Nor do they offer a solution for notifying users of available updates.
Snaps address these gaps, while building upon the work you’ve already done to teach Python how to package your app.
Ready to get started? By the end of this guide you’ll understand how to make a snap of your Python app that can be published in the Snap Store, showcasing it to millions of Linux users.
Snaps are defined in a single YAML file placed in the root of your project. The offlineimap example shows the entire snapcraft.yaml file for an existing project. Don’t worry, we’ll break this down.
name: offlineimap version: git summary: OfflineIMAP description: > OfflineIMAP is software that downloads your email mailbox(es) as local Maildirs. OfflineIMAP will synchronize both sides via IMAP. confinement: devmode parts: offlineimap: plugin: python python-version: python2 source: . apps: offlineimap: command: bin/offlineimap
snapcraft.yaml file starts with a small amount of human-readable metadata, which usually can be lifted from the GitHub description or project README.md. This data is used in the presentation of your app in the Snap Store.
name: offlineimap version: git summary: OfflineIMAP description: > OfflineIMAP is software that downloads your email mailbox(es) as local Maildirs. OfflineIMAP will synchronize both sides via IMAP.
name must be unique in the Snap Store. Valid snap names consist of lower-case alphanumeric characters and hyphens. They cannot be all numbers. They also cannot start or end with a hyphen.
git for the version, the current git tag or commit will be used as the version string. Versions carry no semantic meaning in snaps.
summary can not exceed 79 characters. You can use a chevron ‘>’ in the
description key to declare a multi-line description.
The next section describes the level of confinement applied to your app.
Snaps are containerised to ensure more predictable application behaviour and greater security. Unlike other container systems, the shape of this confinement can be changed through a set of interfaces. These are declarations that tell the system to give permission for a specific task, such as accessing a webcam or binding to a network port.
It’s best to start a snap with the confinement in warning mode, rather than strictly applied. This is indicated through the
devmode keyword. When a snap is in devmode, runtime confinement violations will be allowed but reported. These can be reviewed by running
Because devmode is only intended for development, snaps must be set to strict confinement before they can be published as “stable” in the Snap Store. Once an app is working well in devmode, you can review confinement violations, add appropriate interfaces, and switch to strict confinement.
Parts define what sources are needed to assemble your app. Parts can be anything: programs, libraries, or other needed assets. We’ll deal with libraries and other assets later, so for now we just have one part: the offlineimap source code.
parts: offlineimap: plugin: python python-version: python2 source: .
python plugin builds upon the work you’ve already done to describe your Python dependencies in setup.py and requirements.txt. It will automatically include these in your snap.
python-version keyword instructs the python plugin to include a copy of Python 2 in your snap. If unspecified, this option defaults to Python 3.
source keyword points to the root of your Python project and can be a local directory or remote Git repository. Note that your Python project should be using setuptools and You should be able to run
python setup.py bdist_wheel without errors. If either of these are not true, please consult the setuptools documentation.
Apps are the commands you want to expose to users and any background services your application provides. Each key under
apps is the command name that should be made available on users’ systems.
command specifies the path to the binary to be run. This is resolved relative to the root of your snap contents and automatically searches in the
bin subdirectories of your snap.
apps: offlineimap: command: bin/offlineimap
If your command name matches the snap
name, users will be able run the command directly. If the names differ, then apps are prefixed with the snap
offlineimap.command-name, for example). This is to avoid conflicting with apps defined by other installed snaps.
You can request an alias on the Snapcraft forum if your command name and snap name do not match but you don’t want your command prefixed. These aliases are set up automatically when your snap is installed from the Snap Store.
Here are all the Python plugin-specific keywords:
- requirements: (string) Path to a requirements.txt file - constraints: (string) Path to a constraints file - process-dependency-links: (bool; default: false) Enable the processing of dependency links in pip, which allow one project to provide places to look for another project - python-packages: (list) A list of dependencies to get from PyPI - python-version: (string; default: python3) The python version to use. Valid options are: python2 and python3
That’s it. You now have a snapcraft.yaml file which describes how the core of your app is assembled, presented, and run.
Continue on to learn how you can bundle your app’s dependencies, such as system libraries, into this snap so your app is portable across Linux distributions.