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Packaging Code for Release and Distribution

Overview

Teaching: 0 min
Exercises: 20 min
Questions
  • How do we prepare our code for sharing as a Python package?

  • How do we release our project for other people to install and reuse?

Objectives
  • Describe the steps necessary for sharing Python code as installable packages.

  • Use Poetry to prepare an installable package.

  • Explain the differences between runtime and development dependencies.

Why Package our Software?

We’ve now got our software ready to release - the last step is to package it up so that it can be distributed.

For very small pieces of software, for example a single source file, it may be appropriate to distribute to non-technical end-users as source code, but in most cases we want to bundle our application or library into a package. A package is typically a single file which contains within it our software and some metadata which allows it to be installed and used more simply - e.g. a list of dependencies. By distributing our code as a package, we reduce the complexity of fetching, installing and integrating it for the end-users.

In this session we’ll introduce one widely used method for building an installable package from our code. There are range of methods in common use, so it’s likely you’ll also encounter projects which take different approaches.

There’s some confusing terminology in this episode around the use of the term “package”. This term is used to refer to both:

Packaging our Software with Poetry

Installing Poetry

Because we’ve recommended GitBash if you’re using Windows, we’re going to install Poetry using a different method to the officially recommended one. If you’re on MacOS or Linux, are comfortable with installing software at the command line and want to use Poetry to manage multiple projects, you may instead prefer to follow the official Poetry installation instructions.

We can install Poetry much like any other Python distributable package, using pip:

$ source venv/bin/activate
$ pip3 install poetry

To test, we can ask where Poetry is installed:

$ which poetry
/home/alex/python-intermediate-inflammation/venv/bin/poetry

If you don’t get similar output, make sure you’ve got the correct virtual environment activated.

Poetry can also handle virtual environments for us, so in order to behave similarly to how we used them previously, let’s change the Poetry config to put them in the same directory as our project:

$ poetry config virtualenvs.in-project true

Setting up our Poetry Config

Poetry uses a pyproject.toml file to describe the build system and requirements of the distributable package. This file format was introduced to solve problems with bootstrapping packages (the processing we do to prepare to process something) using the older convention with setup.py files and to support a wider range of build tools. It is described in PEP 518 (Specifying Minimum Build System Requirements for Python Projects).

Make sure you are in the root directory of your software project and have activated your virtual environment, then we’re ready to begin.

To create a pyproject.toml file for our code, we can use poetry init. This will guide us through the most important settings - for each prompt, we either enter our data or accept the default.

Displayed below are the questions you should see with the recommended responses to each question so try to follow these, although use your own contact details!

NB: When you get to the questions about defining our dependencies, answer no, so we can do this separately later.

$ poetry init
This command will guide you through creating your pyproject.toml config.

Package name [example]:  inflammation
Version [0.1.0]: 1.0.0
Description []:  Analyse patient inflammation data
Author [None, n to skip]: James Graham <J.Graham@software.ac.uk>
License []:  MIT
Compatible Python versions [^3.8]: ^3.8

Would you like to define your main dependencies interactively? (yes/no) [yes] no
Would you like to define your development dependencies interactively? (yes/no) [yes] no
Generated file

[tool.poetry]
name = "inflammation"
version = "1.0.0"
description = "Analyse patient inflammation data"
authors = ["James Graham <J.Graham@software.ac.uk>"]
license = "MIT"

[tool.poetry.dependencies]
python = "^3.8"

[tool.poetry.dev-dependencies]

[build-system]
requires = ["poetry-core>=1.0.0"]
build-backend = "poetry.core.masonry.api"


Do you confirm generation? (yes/no) [yes] yes

We’ve called our package “inflammation” in the setup above, instead of “inflammation-analysis” like we did in our previous setup.py. This is because Poetry will automatically find our code if the name of the distributable package matches the name of our module package. If we wanted our distributable package to have a different name, for example “inflammation-analysis”, we could do this by explicitly listing the module packages to bundle - see the Poetry docs on packages for how to do this.

Project Dependencies

Previously, we looked at using a requirements.txt file to define the dependencies of our software. Here, Poetry takes inspiration from package managers in other languages, particularly NPM (Node Package Manager), often used for JavaScript development.

Tools like Poetry and NPM understand that there are two different types of dependency: runtime dependencies and development dependencies. Runtime dependencies are those dependencies that need to be installed for our code to run, like NumPy. Development dependencies are dependencies which are an essential part of your development process for a project, but are not required to run it. Common examples of developments dependencies are linters and test frameworks, like pylint or pytest.

When we add a dependency using Poetry, Poetry will add it to the list of dependencies in the pyproject.toml file, add a reference to it in a new poetry.lock file, and automatically install the package into our virtual environment. If we don’t yet have a virtual environment activated, Poetry will create it for us - using the name .venv, so it appears hidden unless we do ls -a. Because we’ve already activated a virtual environment, Poetry will use ours instead. The pyproject.toml file has two separate lists, allowing us to distinguish between runtime and development dependencies.

$ poetry add matplotlib numpy
$ poetry add --dev pylint
$ poetry install

These two sets of dependencies will be used in different circumstances. When we build our package and upload it to a package repository, Poetry will only include references to our runtime dependencies. This is because someone installing our software through a tool like pip is only using it, but probably doesn’t intend to contribute to the development of our software and does not require development dependencies.

In contrast, if someone downloads our code from GitHub, together with our pyproject.toml, and installs the project that way, they will get both our runtime and development dependencies. If someone is downloading our source code, that suggests that they intend to contribute to the development, so they’ll need all of our development tools.

Have a look at the pyproject.toml file again to see what’s changed.

Packaging Our Code

The final preparation we need to do is to make sure that our code is organised in the recommended structure. This is the Python module structure - a directory containing an __init__.py and our Python source code files. Make sure that the name of this Python package (inflammation - unless you’ve renamed it) matches the name of your distributable package in pyproject.toml unless you’ve chosen to explicitly list the module packages.

By convention distributable package names use hyphens, whereas module package names use underscores. While we could choose to use underscores in a distributable package name, we cannot use hyphens in a module package name, as Python will interpret them as a minus sign in our code when we try to import them.

Once we’ve got our pyproject.toml configuration done and our project is in the right structure, we can go ahead and build a distributable version of our software:

$ poetry build

This should produce two files for us in the dist directory. The one we care most about is the .whl or wheel file. This is the file that pip uses to distribute and install Python packages, so this is the file we’d need to share with other people who want to install our software.

Now if we gave this wheel file to someone else, they could install it using pip - you don’t need to run this command yourself, you’ve already installed it using poetry install above.

$ pip3 install dist/inflammation*.whl

The star in the line above is a wildcard, that means Bash should use any filenames that match that pattern, with any number of characters in place for the star. We could also rely on Bash’s autocomplete functionality and type dist/inflammation, then hit the Tab key if we’ve only got one version built.

After we’ve been working on our code for a while and want to publish an update, we just need to update the version number in the pyproject.toml file (using SemVer perhaps), then use Poetry to build and publish the new version. If we don’t increment the version number, people might end up using this version, even though they thought they were using the previous one. Any re-publishing of the package, no matter how small the changes, needs to come with a new version number. The advantage of SemVer is that the change in the version number indicates the degree of change in the code and thus the degree of risk of breakage when we update.

$ poetry build

In addition to the commands we’ve already seen, Poetry contains a few more that can be useful for our development process. For the full list see the Poetry CLI documentation.

The final step is to publish our package to a package repository. A package repository could be either public or private - while you may at times be working on public projects, it’s likely the majority of your work will be published internally using a private repository such as JFrog Artifactory. Every repository may be configured slightly differently, so we’ll leave that to you to investigate.

What if We Need More Control?

Sometimes we need more control over the process of building our distributable package than Poetry allows. In these cases, we have to use the method that we’ve seen already in this course - a setup.py file. Because this is a Python file, we can use the full power of Python to describe how to setup our project.

One of the common cases where this is particularly useful is if our project has components in different languages. For example, to speed up some of the core parts we might write some of our functions in C, then call these from our Python code. Using a setup.py gives us the flexibility to handle building these components in different ways and bring them together at the end.

Alternative Python Packaging Methods?

In the unit testing episode, you may recall we created a setup.py file to represent our inflammation code as a package, so it could be ‘found’ by pytest. Using a setup.py is part of one older convention to packaging Python, and in this episode we looked at another.

There many ways to distribute Python code in packages, with some degree of flux in terms of which methods are most popular. For a more comprehensive overview of Python packaging you can see the Python docs on packaging, which contains a helpful guide to the overall packaging process, or ‘flow’, using the Twine tool to upload created packages to PyPI for distribution as an alternative.

Optional Exercise: Enhancing our Package Metadata

The Python Packaging User Guide provides documentation on how to package a project using a manual approach to building a pyproject.toml file, and using Twine to upload the distribution packages to PyPI.

Referring to the section on metadata in the documentation, enhance your pyproject.toml with some additional metadata fields to improve the information your package.

Key Points

  • Poetry allows us to produce an installable package and upload it to a package repository.

  • Making our software installable with Pip makes it easier for others to start using it.

  • For complete control over building a package, we can use a setup.py file.