Automatic Resource Extraction#

If you are using tools that expect your resources to be “real” files, or your project includes non-extension native libraries or other files that your C extensions expect to be able to access, you may need to list those files in the eager_resources argument to setup(), so that the files will 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-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.

Defining Additional Metadata#

Some extensible applications and frameworks may need to define their own kinds of metadata to include in eggs, which they can then access using the pkg_resources metadata APIs. Ordinarily, this is done by having plugin developers include additional files in their ProjectName.egg-info directory. However, since it can be tedious to create such files by hand, you may want to create a distutils extension that will create the necessary files from arguments to setup(), in much the same way that setuptools does for many of the setup() arguments it adds. See the section below on Creating distutils Extensions for more details, especially the subsection on Adding new EGG-INFO Files.

Setting the zip_safe flag#

For some use cases (such as bundling as part of a larger application), Python packages may be run directly from a zip file. Not all packages, however, are capable of running in compressed form, because they may expect to be able to access either source code or data files as normal operating system files. So, setuptools can install your project as a zipfile or a directory, and its default choice is determined by the project’s zip_safe flag.

You can pass a True or False value for the zip_safe argument to the setup() function, or you can omit it. If you omit it, the bdist_egg command will analyze your project’s contents to see if it can detect any conditions that would prevent it from working in a zipfile. It will output notices to the console about any such conditions that it finds.

Currently, this analysis is extremely conservative: it will consider the project unsafe if it contains any C extensions or datafiles whatsoever. This does not mean that the project can’t or won’t work as a zipfile! It just means that the bdist_egg authors aren’t yet comfortable asserting that the project will work. If the project contains no C or data files, and does no __file__ or __path__ introspection or source code manipulation, then there is an extremely solid chance the project will work when installed as a zipfile. (And if the project uses pkg_resources for all its data file access, then C extensions and other data files shouldn’t be a problem at all. See the Accessing Data Files at Runtime section above for more information.)

However, if bdist_egg can’t be sure that your package will work, but you’ve checked over all the warnings it issued, and you are either satisfied it will work (or if you want to try it for yourself), then you should set zip_safe to True in your setup() call. If it turns out that it doesn’t work, you can always change it to False, which will force setuptools to install your project as a directory rather than as a zipfile.

In the future, as we gain more experience with different packages and become more satisfied with the robustness of the pkg_resources runtime, the “zip safety” analysis may become less conservative. However, we strongly recommend that you determine for yourself whether your project functions correctly when installed as a zipfile, correct any problems if you can, and then make an explicit declaration of True or False for the zip_safe flag, so that it will not be necessary for bdist_egg to try to guess whether your project can work as a zipfile.

Controlling files in the distribution#

For the most common use cases, setuptools will automatically find out which files are necessary for distributing the package. This includes all pure Python modules in the py_modules or packages configuration, and the C sources (but not C headers) listed as part of extensions when creating a Source Distribution (or “sdist”).

However, when building more complex packages (e.g. packages that include non-Python files, or that need to use custom C headers), you might find that not all files present in your project folder are included in package distribution archive.

In these situations you can use a setuptools plugin, such as setuptools-scm or setuptools-svn to automatically include all files tracked by your Revision Control System into the sdist.

Alternatively, if you need finer control, you can add a file at the root of your project. This file contains instructions that tell setuptools which files exactly should be part of the sdist (or not). A comprehensive guide to syntax is available at the PyPA’s Packaging User Guide.

Once the correct files are present in the sdist, they can then be used by binary extensions during the build process, or included in the final wheel 1 if you configure setuptools with include_package_data=True.


Please note that, when using include_package_data=True, only files inside the package directory are included in the final wheel, by default.

So for example, if you create a Python project that uses setuptools-scm and have a tests directory outside of the package folder, the tests directory will be present in the sdist but not in the wheel 2.

See Data Files Support for more information.


You can think about the build process as two stages: first the sdist will be created and then the wheel will be produced from that sdist.


This happens because the sdist can contain files that are useful during development or the build process itself, but not in runtime (e.g. tests, docs, examples, etc…). The wheel, on the other hand, is a file format that has been optimized and is ready to be unpacked into a running installation of Python or Virtual Environment. Therefore it only contains items that are required during runtime.