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The Packaging Flow¶

The document aims to outline the flow involved in publishing/distributing a distribution package, usually to the Python Package Index (PyPI). It is written for package publishers, who are assumed to be the package author.

While the tutorial walks through the process of preparing a simple package for release, it does not fully enumerate what steps and files are required, and for what purpose.

Publishing a package requires a flow from the author’s source code to an end user’s Python environment. The steps to achieve this are:

Have a source tree containing the package. This is typically a checkout from a version control system (VCS).
Prepare a configuration file describing the package metadata (name, version and so forth) and how to create the build artifacts. For most packages, this will be a pyproject.toml file, maintained manually in the source tree.
Create build artifacts to be sent to the package distribution service (usually PyPI); these will normally be a source distribution (“sdist”) and one or more built distributions (“wheels”). These are made by a build tool using the configuration file from the previous step. Often there is just one generic wheel for a pure Python package.
Upload the build artifacts to the package distribution service.

At that point, the package is present on the package distribution service. To use the package, end users must:

Download one of the package’s build artifacts from the package distribution service.
Install it in their Python environment, usually in its site-packages directory. This step may involve a build/compile step which, if needed, must be described by the package metadata.

These last 2 steps are typically performed by pip when an end user runs pip install.

The steps above are described in more detail below.

The source tree¶

The source tree contains the package source code, usually a checkout from a VCS. The particular version of the code used to create the build artifacts will typically be a checkout based on a tag associated with the version.

The configuration file¶

The configuration file depends on the tool used to create the build artifacts. The standard practice is to use a pyproject.toml file in the TOML format.

At a minimum, the pyproject.toml file needs a [build-system] table specifying your build tool. There are many build tools available, including but not limited to flit, hatch, pdm, poetry, Setuptools, trampolim, and whey. Each tool’s documentation will show what to put in the [build-system] table.

For example, here is a table for using hatch:

[build-system]
requires = ["hatchling"]
build-backend = "hatchling.build"

With such a table in the pyproject.toml file, a “frontend” tool like build can run your chosen build tool’s “backend” to create the build artifacts. Your build tool may also provide its own frontend. An install tool like pip also acts as a frontend when it runs your build tool’s backend to install from a source distribution.

The particular build tool you choose dictates what additional information is required in the pyproject.toml file. For example, you might specify:

a [project] table containing project Core Metadata (name, version, author and so forth),
a [tool] table containing tool-specific configuration options.

Refer to the pyproject.toml guide for a complete guide to pyproject.toml configuration.

Build artifacts¶

The source distribution (sdist)¶

A source distribution contains enough to install the package from source in an end user’s Python environment. As such, it needs the package source, and may also include tests and documentation. These are useful for end users wanting to develop your sources, and for end user systems where some local compilation step is required (such as a C extension).

The build package knows how to invoke your build tool to create one of these:

python3 -m build --sdist source-tree-directory

Or, your build tool may provide its own interface for creating an sdist.

The built distributions (wheels)¶

A built distribution contains only the files needed for an end user’s Python environment. No compilation steps are required during the install, and the wheel file can simply be unpacked into the site-packages directory. This makes the install faster and more convenient for end users.

A pure Python package typically needs only one “generic” wheel. A package with compiled binary extensions needs a wheel for each supported combination of Python interpreter, operating system, and CPU architecture that it supports. If a suitable wheel file is not available, tools like pip will fall back to installing the source distribution.

The build package knows how to invoke your build tool to create one of these:

python3 -m build --wheel source-tree-directory

Or, your build tool may provide its own interface for creating a wheel.

Note

The default behaviour of build is to make both an sdist and a wheel from the source in the current directory; the above examples are deliberately specific.

Upload to the package distribution service¶

The twine tool can upload build artifacts to PyPI for distribution, using a command like:

twine upload dist/package-name-version.tar.gz dist/package-name-version-py3-none-any.whl

Or, your build tool may provide its own interface for uploading.

Download and install¶

Now that the package is published, end users can download and install the package into their Python environment. Typically this is done with pip, using a command like:

python3 -m pip install package-name

End users may also use other tools like Pipenv, poetry, or pdm.

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