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$ lz TX_ţ d/ ? ===================================== The Internal Structure of Python Eggs ===================================== STOP! This is not the first document you should read! .. contents:: **Table of Contents** ---------------------- Eggs and their Formats ---------------------- A "Python egg" is a logical structure embodying the release of a specific version of a Python project, comprising its code, resources, and metadata. There are multiple formats that can be used to physically encode a Python egg, and others can be developed. However, a key principle of Python eggs is that they should be discoverable and importable. That is, it should be possible for a Python application to easily and efficiently find out what eggs are present on a system, and to ensure that the desired eggs' contents are importable. There are two basic formats currently implemented for Python eggs: 1. ``.egg`` format: a directory or zipfile *containing* the project's code and resources, along with an ``EGG-INFO`` subdirectory that contains the project's metadata 2. ``.egg-info`` format: a file or directory placed *adjacent* to the project's code and resources, that directly contains the project's metadata. Both formats can include arbitrary Python code and resources, including static data files, package and non-package directories, Python modules, C extension modules, and so on. But each format is optimized for different purposes. The ``.egg`` format is well-suited to distribution and the easy uninstallation or upgrades of code, since the project is essentially self-contained within a single directory or file, unmingled with any other projects' code or resources. It also makes it possible to have multiple versions of a project simultaneously installed, such that individual programs can select the versions they wish to use. The ``.egg-info`` format, on the other hand, was created to support backward-compatibility, performance, and ease of installation for system packaging tools that expect to install all projects' code and resources to a single directory (e.g. ``site-packages``). Placing the metadata in that same directory simplifies the installation process, since it isn't necessary to create ``.pth`` files or otherwise modify ``sys.path`` to include each installed egg. Its disadvantage, however, is that it provides no support for clean uninstallation or upgrades, and of course only a single version of a project can be installed to a given directory. Thus, support from a package management tool is required. (This is why setuptools' "install" command refers to this type of egg installation as "single-version, externally managed".) Also, they lack sufficient data to allow them to be copied from their installation source. easy_install can "ship" an application by copying ``.egg`` files or directories to a target location, but it cannot do this for ``.egg-info`` installs, because there is no way to tell what code and resources belong to a particular egg -- there may be several eggs "scrambled" together in a single installation location, and the ``.egg-info`` format does not currently include a way to list the files that were installed. (This may change in a future version.) Code and Resources ================== The layout of the code and resources is dictated by Python's normal import layout, relative to the egg's "base location". For the ``.egg`` format, the base location is the ``.egg`` itself. That is, adding the ``.egg`` filename or directory name to ``sys.path`` makes its contents importable. For the ``.egg-info`` format, however, the base location is the directory that *contains* the ``