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.

:information_source: For a brief overview of the snap creation process, including how to install snapcraft and how it’s used, see Snapcraft overview. For a more comprehensive breakdown of the steps involved, take a look at Creating a snap.

Getting started

Snaps are defined in a single YAML file placed in the root folder of your project. The following example shows the entire snapcraft.yaml file for an existing project, OfflineIMAP. Don’t worry, we’ll break this down.

name: test-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.
base: core18
confinement: devmode


parts:
  test-offlineimap:
    plugin: python
    python-version: python2
    source: .
    stage-packages:
      - python-six

apps:
  test-offlineimap:
    command: bin/offlineimap

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: 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.

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 defines a special kind of snap that provides a run-time environment with a minimal set of libraries that are common to most applications. They’re transparent to users, but they need to be considered, and specified, when building a snap.

base: core18

core18 is the current standard base for snap building and is based on Ubuntu 18.04 LTS.

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.

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 offlineimap source code.

parts:
  test-offlineimap:
    plugin: python
    python-version: python2
    source: .
    stage-packages:
      - python-six

The 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.

The 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.

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.

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:
  test-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 name (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.

Building the snap

You can download the example repository with the following command:

$ git clone https://github.com/snapcraft-docs/offlineimap

After you’ve created the snapcraft.yaml, you can build the snap by simply executing the snapcraft command in the project directory:

$ snapcraft
Using 'snapcraft.yaml': Project assets will be searched for from the 'snap' directory.
Launching a VM.
[...]
Snapped test-offlineimap_v7.1.1+git6.7067be9-dirty_amd64.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 test-offlineimap_*.snap --devmode --dangerous

You can then try it out:

$ test-offlineimap -h

Removing the snap is simple too:

$ sudo snap remove test-offlineimap

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 push --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 a month ago. Help improve this document in the forum.