Python apps

Python has several options that can assist when packaging and distributing applications. There’s Setuptools, and the Python Package Index (pip), and Virtualenv allows developers to isolate an application and its dependencies from the rest of the system.

But 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.

Why are snaps good for Python projects?

• Snaps are easy to discover and install Millions of users can browse and install snaps graphically in the Snap Store or from the command-line.
• Snaps install and run the same across Linux They bundle the exact version of Python required, along with all of your app’s dependencies, be they Python modules or system libraries.
• Snaps automatically update to the latest version Four times a day, users’ systems will check for new versions and upgrade in the background.
• Upgrades are not disruptive Because upgrades are not in-place, users can keep your app open as it’s upgraded in the background.
• Upgrades are safe If your app fails to upgrade, users automatically roll back to the previous revision.

Build a snap in 20 minutes

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.

Getting started

Snaps are defined in a single YAML file placed in the root folder of your project. The following example shows an entire snapcraft.yaml file based on the snap of an existing project, yt-dlp. There are minor differences, such as the version definition and confinement level, but these can be easily changed after the snap is working. Don’t worry, we’ll break this down.

name: yt-dlp
summary: A fork of youtube-dl with additional features and patches
description: |
      Download and play videos on your local system. Runs from the command 
      line and with all the features and patches of youtube-dlc in addition
      to the latest youtube-dl.
version: git
grade: stable
confinement: devmode
base: core20
architectures:
  - build-on: [arm64, armhf, amd64]

apps:
  yt-dlp:
    command: bin/yt-dlp
    plugs: [home, network, network-bind, removable-media]

parts:
  yt-dlp:
    plugin: python
    source: https://github.com/yt-dlp/yt-dlp.git

Metadata

The 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: yt-dlp
summary: A fork of youtube-dl with additional features and patches
description: |
      Download and play videos on your local system. Runs from the command 
      line and with all the features and patches of youtube-dlc in addition
      to the latest youtube-dl.

The name must be unique in the Snap Store. Valid snap names consist of lower-case alphanumeric characters and hyphens. They cannot be all numbers and they also cannot start or end with a hyphen.

By specifying git for the version, the current git tag or commit will be used as the version string. Versions carry no semantic meaning in snaps.

The summary can not exceed 79 characters. You can use a chevron ‘>’ in the description key to declare a multi-line description.

Base

The base keyword declares which base snap to use with your project. A base snap is a special kind of snap that provides a run-time environment alongside a minimal set of libraries that are common to most applications:

base: core20

As used above, core20 is the current standard base for snap building and is based on Ubuntu 20.04 LTS. See Base snaps for more details.

Security model

The next section describes the level of confinement applied to your app.

confinement: devmode

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 journalctl -xe.

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.

The above example will also work if you change devmode to strict, as you would before a release.

Parts

Parts define what sources are needed to assemble your app. Parts can be anything: programs, libraries, or other needed assets, but for now, we’re only going to use one part: the yt-dlp source code.

parts:
  yt-dlp:
    plugin: python
    source: https://github.com/yt-dlp/yt-dlp.git

The Python plugin can be used by either Python 2 or Python 3 based parts using a setup.py script for building the project, or using a package published to PyPI, and optionally any of the following:

• a requirements.txt file used to import Python modules
• packages installed directly from pip

Our example project uses neither of the above but they can be added to your own project with the python-requirement and python-packages keywords, as described in our Python plugin documentation.

The 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.

If you need additional packages, the stage-packages keyword simply lists any package dependencies needed to run your app. A corresponding build-packages keyword can also be used to specify packages only needed during the build phase.

For more details on Python-specific metadata, see The Python plugin.

Apps

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.

The 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 usr/sbin, usr/bin, sbin, and bin sub directories of your snap.

apps:
  yt-dlp:
    command: bin/yt-dlp
    plugs: [home, network, network-bind, removable-media]

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 name (yt-dlp.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.

Building the snap

You can download the example repository with the following command:

$ git clone https://github.com/degville/snap-yt-dlp.git

After you’ve created the snapcraft.yaml (which already exists in the above repository), you can build the snap by simply executing the snapcraft command in the project directory:

$ snapcraft
Launching a container.
Waiting for container to be ready
[...]
Staging yt-dlp
+ snapcraftctl stage
Priming yt-dlp
+ snapcraftctl prime
Determining the version from the project repo (version: git).
The version has been set to '0+git.9e6dc74-dirty'
Snapping |
Snapped yt-dlp_0+git.9e6dc74-dirty_multi.snap

The resulting snap can be installed locally. This requires the --dangerous flag because the snap is not signed by the Snap Store. The --devmode flag acknowledges that you are installing an unconfined application:

$ sudo snap install yt-dlp_0+git.*_multi.snap --devmode --dangerous

You can then try it out:

$ yt-dlp -h

Removing the snap is simple too:

$ sudo snap remove yt-dlp

You can also clean up the build environment, although this will slow down the next initial build:

$ snapcraft clean

By default, when you make a change to snapcraft.yaml, snapcraft only builds the parts that have changed. Cleaning a build, however, forces your snap to be rebuilt in a clean environment and will take longer.

Publishing your snap

To share your snaps you need to publish them in the Snap Store. First, create an account on the dashboard. Here you can customise how your snaps are presented, review your uploads and control publishing.

You’ll need to choose a unique “developer namespace” as part of the account creation process. This name will be visible by users and associated with your published snaps.

Make sure the snapcraft command is authenticated using the email address attached to your Snap Store account:

$ snapcraft login

Reserve a name for your snap

You can publish your own version of a snap, provided you do so under a name you have rights to. You can register a name on dashboard.snapcraft.io, or by running the following command:

$ snapcraft register mypythonsnap

Be sure to update the name: in your snapcraft.yaml to match this registered name, then run snapcraft again.

Upload your snap

Use snapcraft to push the snap to the Snap Store.

$ snapcraft upload --release=edge mypythonsnap_*.snap

If you’re happy with the result, you can commit the snapcraft.yaml to your GitHub repo and turn on automatic builds so any further commits automatically get released to edge, without requiring you to manually build locally.

Congratulations! You’ve just built and published your first Python snap. For a more in-depth overview of the snap building process, see Creating a snap.

Last updated 8 months ago.