What is a source distribution?¶

Conceptually, a source distribution is an archive of the source code in raw form. Concretely, an sdist is a .tar.gz archive containing the source code plus an additional special file called PKG-INFO, which holds the project metadata. The presence of this file helps packaging tools to be more efficient by not needing to compute the metadata themselves. The PKG-INFO file follows the format specified in Core metadata specifications and is not intended to be written by hand [1].

You can thus inspect the contents of an sdist by unpacking it using standard tools to work with tar archives, such as tar -xvf on UNIX platforms (like Linux and macOS), or the command line interface of Python’s tarfile module on any platform.

Sdists serve several purposes in the packaging ecosystem. When pip, the standard Python package installer, cannot find a wheel to install, it will fall back on downloading a source distribution, compiling a wheel from it, and installing the wheel. Furthermore, sdists are often used as the package source by downstream packagers (such as Linux distributions, Conda, Homebrew and MacPorts on macOS, …), who, for various reasons, may prefer them over, e.g., pulling from a Git repository.

A source distribution is recognized by its file name, which has the form package_name-version.tar.gz, e.g., pip-23.3.1.tar.gz.

If you want technical details on the sdist format, read the sdist specification.

What is a wheel?¶

Conceptually, a wheel contains exactly the files that need to be copied when installing the package.

There is a big difference between sdists and wheels for packages with extension modules, written in compiled languages like C, C++ and Rust, which need to be compiled into platform-dependent machine code. With these packages, wheels do not contain source code (like C source files) but compiled, executable code (like .so files on Linux or DLLs on Windows).

Furthermore, while there is only one sdist per version of a project, there may be many wheels. Again, this is most relevant in the context of extension modules. The compiled code of an extension module is tied to an operating system and processor architecture, and often also to the version of the Python interpreter (unless the Python stable ABI is used).

For pure-Python packages, the difference between sdists and wheels is less marked. There is normally one single wheel, for all platforms and Python versions. Python is an interpreted language, which does not need ahead-of-time compilation, so wheels contain .py files just like sdists.

If you are wondering about .pyc bytecode files: they are not included in wheels, since they are cheap to generate, and including them would unnecessarily force a huge number of packages to distribute one wheel per Python version instead of one single wheel. Instead, installers like pip generate them while installing the package.

With that being said, there are still important differences between sdists and wheels, even for pure Python projects. Wheels are meant to contain exactly what is to be installed, and nothing more. In particular, wheels should never include tests and documentation, while sdists commonly do. Also, the wheel format is more complex than sdist. For example, it includes a special file – called RECORD – that lists all files in the wheel along with a hash of their content, as a safety check of the download’s integrity.

At a glance, you might wonder if wheels are really needed for “plain and basic” pure Python projects. Keep in mind that due to the flexibility of sdists, installers like pip cannot install from sdists directly – they need to first build a wheel, by invoking the build backend that the sdist specifies (the build backend may do all sorts of transformations while building the wheel, such as compiling C extensions). For this reason, even for a pure Python project, you should always upload both an sdist and a wheel to PyPI or other package indices. This makes installation much faster for your users, since a wheel is directly installable. By only including files that must be installed, wheels also make for smaller downloads.

On the technical level, a wheel is a ZIP archive (unlike sdists which are TAR archives). You can inspect its contents by unpacking it as a normal ZIP archive, e.g., using unzip on UNIX platforms like Linux and macOS, Expand-Archive in Powershell on Windows, or the command line interface of Python’s zipfile module. This can be very useful to check that the wheel includes all the files you need it to.

Inside a wheel, you will find the package’s files, plus an additional directory called package_name-version.dist-info. This directory contains various files, including a METADATA file which is the equivalent of PKG-INFO in sdists, as well as RECORD. This can be useful to ensure no files are missing from your wheels.

The file name of a wheel (ignoring some rarely used features) looks like this: package_name-version-python_tag-abi_tag-platform_tag.whl. This naming convention identifies which platforms and Python versions the wheel is compatible with. For example, the name pip-23.3.1-py3-none-any.whl means that:

• (py3) This wheel can be installed on any implementation of Python 3, whether CPython, the most widely used Python implementation, or an alternative implementation like PyPy;

• (none) It does not depend on the Python version;

• (any) It does not depend on the platform.

The pattern py3-none-any is common for pure Python projects. Packages with extension modules typically ship multiple wheels with more complex tags.

All technical details on the wheel format can be found in the wheel specification.

What about eggs?¶

“Egg” is an old package format that has been replaced with the wheel format. It should not be used anymore. Since August 2023, PyPI rejects egg uploads.

Here’s a breakdown of the important differences between wheel and egg.

• The egg format was introduced by Setuptools in 2004, whereas the wheel format was introduced by PEP 427 in 2012.

• Wheel has an official standard specification. Egg did not.

• Wheel is a distribution format, i.e a packaging format. [2] Egg was both a distribution format and a runtime installation format (if left zipped), and was designed to be importable.

• Wheel archives do not include .pyc files. Therefore, when the distribution only contains Python files (i.e. no compiled extensions), and is compatible with Python 2 and 3, it’s possible for a wheel to be “universal”, similar to an sdist.

• Wheel uses standard .dist-info directories. Egg used .egg-info.

• Wheel has a richer file naming convention. A single wheel archive can indicate its compatibility with a number of Python language versions and implementations, ABIs, and system architectures.

• Wheel is versioned. Every wheel file contains the version of the wheel specification and the implementation that packaged it.

• Wheel is internally organized by sysconfig path type, therefore making it easier to convert to other formats.