Python

Python has rich tools for packaging, distributing and sandboxing applications. Snapcraft builds on top of these familiar tools such as pip, setup.py and requirements.txt to create snaps for people to install on Linux.

What problems do snaps solve for Python applications?

Linux install instructions for Python applications often get complicated. System dependencies, which differ from distribution to distribution, must be separately installed. To prevent modules from different Python applications clashing with each other, developer tools like virtualenv or venv must be used. With snapcraft it’s one command to produce a bundle that works anywhere.

Here are some snap advantages that will benefit many Python projects:

  • Bundle all the runtime requirements, including the exact versions of system libraries and the Python interpreter.
  • Simplify installation instructions, regardless of distribution, to snap install mypythonapp.
  • Directly control the delivery of automatic application updates.
  • Extremely simple creation of daemons.

Getting started

Let’s take a look at offlineimap and youtube-dl by way of examples. Both are command line applications. offlineimap uses Python 2 and only has Python module requirements. youtube-dl uses Python 3 and has system package requirements, in this case ffmpeg.

offlineimap

Snaps are defined in a single yaml file placed in the root of your project. The offlineimap example shows the entire snapcraft.yaml for an existing project. 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.

grade: devel
confinement: devmode

apps:
  offlineimap:
    command: bin/offlineimap

parts:
  offlineimap:
    plugin: python
    python-version: python2
    source: .

Metadata

The snapcraft.yaml 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. The summary: can not exceed 79 characters. You can use a pipe with the description: to declare a multi-line description.

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

To get started we won’t confine this application. Unconfined applications, specified with devmode, can only be released to the hidden “edge” channel where you and other developers can install them.

confinement: devmode

Parts

Parts define how to build your app. Parts can be anything: programs, libraries, or other assets needed to create and run your application. In this case we have one: the offlineimap source code. In other cases these can point to local directories, remote git repositories, or tarballs.

The Python plugin will also bundle Python in the snap, so you can be sure that the version of Python you test against is included with your app. Dependencies from install_requires in your setup.py will also be bundled. Dependencies from a requirements.txt file can also be bundled using the requirements: option.

parts:
  offlineimap:
    plugin: python
    python-version: python2
    source: .

Apps

Apps are the commands and services exposed to end users. 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.

If you don’t want your command prefixed you can request an alias for it on the Snapcraft forum. These command aliases are set up automatically when your snap is installed from the Snap Store.

apps:
  offlineimap:
    command: bin/offlineimap

If your application is intended to run as a service, add the line daemon: simple after the command keyword. This will automatically keep the service running on install, update and reboot.

Building the snap

You’ll first need to install snap support, and then install the snapcraft tool:

sudo snap install --beta --classic snapcraft

If you have just installed snap support, start a new shell so your PATH is updated to include /snap/bin. You can then build this example yourself:

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

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 offlineimap_*.snap --devmode --dangerous

You can then try it out:

offlineimap

Removing the snap is simple too:

sudo snap remove offlineimap

Jump ahead to Share with your friends or continue to read another example.

youtube-dl

The youtube-dl example shows a snapcraft.yaml using a tarball of a Python application and ffmpeg bundled in the snap to satisfy the runtime requirements. Here is the entire snapcraft.yaml for youtube-dl. We’ll break this down.

name: youtube-dl
version: 2017.06.18
summary: YouTube Downloader.
description: |
  youtube-dl is a small command-line program to download videos from
  YouTube.com and a few more sites.

grade: devel
confinement: devmode

parts:
  youtube-dl:
    source: https://github.com/rg3/youtube-dl/archive/$SNAPCRAFT_PROJECT_VERSION.tar.gz
    plugin: python
    python-version: python3
    after: [ffmpeg]

apps:
  youtube-dl:
    command: bin/youtube-dl

Parts

The $SNAPCRAFT_PROJECT_VERSION variable is derived from the version: stanza and used here to reference the matching release tarball. Because the python plugin is used, snapcraft will bundle a copy of Python in the snap using the version specified in the python-version: stanza, in this case Python 3.

youtube-dl makes use of ffmpeg to transcode or otherwise convert the audio and video file it downloads. In this example, youtube-dl is told to build after the ffmpeg part. Because the ffmpeg part specifies no plugin, it will be fetched from the parts repository. This is a collection of community-contributed definitions which can be used by anyone when building a snap, saving you from needing to specify the source and build rules for each system dependency. You can use snapcraft search to find more parts to use and snapcraft define <part-name> to verify how the part is defined.

parts:
  youtube-dl:
    source: https://github.com/rg3/youtube-dl/archive/$SNAPCRAFT_PROJECT_VERSION.tar.gz
    plugin: python
    python-version: python3
    after: [ffmpeg]

Building the snap

You can build this example yourself by running the following:

git clone https://github.com/snapcraft-docs/youtube-dl
cd youtube-dl
snapcraft

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 youtube-dl_*.snap --devmode --dangerous

Run the command:

youtube-dl “https://www.youtube.com/watch?v=k-laAxucmEQ”

Removing the snap is simple too:

sudo snap remove youtube-dl

Share with your friends

To share your snaps you need to publish them in the Snap Store. First, create an account on dashboard.snapcraft.io. Here you can customize 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.

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 mypthonsnap_*.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.

Further customisations

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

You can view them locally by running:

snapcraft help python

Extending and overriding behaviour

You can extend the behaviour of any part in your snapcraft.yaml with shell commands. These can be run after pulling the source code but before building by using the prepare keyword. The build process can be overridden entirely using the build keyword and shell commands. The install keyword is used to run shell commands after building your code, useful for making post build modifications such as relocating build assets.

Using the youtube-dl example above, we can run the test suite at the end of the build. If this fails, the snap creation will be terminated:

parts:
  youtube-dl:
    source: https://github.com/rg3/youtube-dl/archive/$SNAPCRAFT_PROJECT_VERSION.tar.gz
    plugin: python
    python-version: python3
    stage-packages: [ffmpeg, python-nose]
    install: |
      nosetests