Concepts - installer

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# Concepts

This library has two main abstractions:

- `WheelSource`: Serves as source of information about a wheel file.

- `WheelDestination`: Handles all file writing and post-installation processing.

## WheelSource

These objects represent a wheel file, abstracting away how the actual file is stored or accessed.

This allows the core install logic to be used with in-memory wheel files, or unzipped-on-disk wheel, or with `zipfile.ZipFile` objects from an on-disk wheel, or something else entirely.

This protocol/abstraction is designed to be implementable without a direct dependency on this library. This allows for other libraries in the Python Packaging ecosystem to provide implementations of the protocol, allowing for more code reuse opportunities.

One of the benefits of this fully described interface is the possibility to decouple the implementation of additional validation on wheels (such as validating the RECORD entries in a wheel match the actual contents of the wheel, or enforcing signing requirements) based on what the specific usecase demands.

## WheelDestination

These objects are responsible for handling the writing-to-filesystem interactions, determining RECORD file entries and post-install actions (like generating .pyc files). While this is a lot of responsibility, this was explicitly provided to make it possible for custom `WheelDestination` implementations to be more powerful and flexible.

Most of these tasks can either be delegated to utilities provided in this library (eg: script generation), or to the Python standard libary (eg: generating `.pyc` files).