A WHL file is a package saved in the Wheel format, which is the standard built-package format used for Python distributions. It contains all the files for a Python install and metadata, which includes the version of the wheel implementation and specification used to package it. WHL files are compressed using Zip compression.

The Wheel format was introduced in PEP 427, which is a Python Enhancement Proposal authored by Daniel Holth and accepted in 2012. It was developed as a quicker and more reliable method of installing Python software than re-building from source code every time. WHL files only have to be moved to the correct location on the target system to be installed, whereas a source distribution requires a build step before installation. There are three types of WHL files: Universal Wheel - It only contains Python files, no compiled extensions, and natively supports Python 2 and 3. Pure Python Wheel - It only contains Python files and no compiled extensions, but does not natively support Python 2 and 3. Platform Wheel - It contains Python files and compiled extensions but does not natively support Python 2 and 3. This type of wheel is specific to a platform, such as Windows or macOS, because it contains compiled extensions.