# Automatic Resource Extraction

In a modern setup, Python packages are usually installed as directories, and all the files can be found on deterministic locations on the disk. This means that most of the tools expect package resources to be “real” files.

There are a few occasions however that packages are loaded in a different way (e.g., from a zip file), which is incompatible with the assumptions mentioned above. Moreover, a package developer may also include non-extension native libraries or other files that C extensions may expect to be able to access.

In these scenarios, the use of `importlib.resources` is recommended.

Old implementations (prior to the advent of `importlib.resources`) and long-living projects, however, may still rely on the library `pkg_resources` to access these files.

If you have to support such systems, or want to provide backward compatibility for `pkg_resources`, you may need to add an special configuration to `setuptools` when packaging a project. This can be done by listing as `eager_resources` (argument to `setup()` in `setup.py` or field in `setup.cfg`) all the files that need to be extracted together, whenever a C extension in the project is imported.

This is especially important if your project includes shared libraries other than `distutils`/`setuptools`-built C extensions, and those shared libraries use file extensions other than `.dll`, `.so`, or `.dylib`, which are the extensions that setuptools 0.6a8 and higher automatically detects as shared libraries and adds to the `native_libs.txt` file for you. Any shared libraries whose names do not end with one of those extensions should be listed as `eager_resources`, because they need to be present in the filesystem when he C extensions that link to them are used.

The `pkg_resources` runtime for compressed packages will automatically extract all C extensions and `eager_resources` at the same time, whenever any C extension or eager resource is requested via the `resource_filename()` API. (C extensions are imported using `resource_filename()` internally.) This ensures that C extensions will see all of the “real” files that they expect to see.

Note also that you can list directory resource names in `eager_resources` as well, in which case the directory’s contents (including subdirectories) will be extracted whenever any C extension or eager resource is requested.

Please note that if you’re not sure whether you need to use this argument, you don’t! It’s really intended to support projects with lots of non-Python dependencies and as a last resort for crufty projects that can’t otherwise handle being compressed. If your package is pure Python, Python plus data files, or Python plus C, you really don’t need this. You’ve got to be using either C or an external program that needs “real” files in your project before there’s any possibility of `eager_resources` being relevant to your project.