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zip_safe.rst

Understanding the ``zip_safe`` flag =================================== The ``zip_safe`` flag is a ``setuptools`` configuration mainly associated with the ``egg`` distribution format (which got replaced in the ecosystem by the newer ``wheel`` format) and the ``easy_install`` command (deprecated in ``setuptools`` v58.3.0). It is very unlikely that the values of ``zip_safe`` will affect modern deployments that use :pypi:`pip` for installing packages. Moreover, new users of ``setuptools`` should not attempt to create egg files using the deprecated ``build_egg`` command. Therefore, this flag is considered **obsolete**. This document, however, describes what was the historical motivation behind this flag, and how it was used. Historical Motivation --------------------- 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. In the past, ``setuptools`` would install a project distributed as a zipfile or a directory (via the ``easy_install`` command or ``python setup.py install``), the default choice being determined by the project's ``zip_safe`` flag. How the ``zip_safe`` flag was used? ----------------------------------- To set this flag, a developer would pass a boolean value for the ``zip_safe`` argument to the ``setup()`` function, or omit it. When omitted, the ``bdist_egg`` command would analyze the project's contents to see if it could detect any conditions preventing the project from working in a zipfile. This was extremely conservative: ``bdist_egg`` would consider the project unsafe if it contained any C extensions or datafiles whatsoever. This does *not* mean that the project couldn't or wouldn't work as a zipfile! It just means that the ``bdist_egg`` authors were not yet comfortable asserting that the project *would* work. If the project did not contain any C or data files, and did not attempt to perform ``__file__`` or ``__path__`` introspection or source code manipulation, then there was an extremely solid chance the project will work when installed as a zipfile. (And if the project used ``pkg_resources`` for all its data file access, then C extensions and other data files shouldn't be a problem at all. See the :ref:`Accessing Data Files at Runtime` section for more information.) The developer could manually set ``zip_safe`` to ``True`` to perform tests, or to override the default behaviour (after checking all the warnings and understanding the implications), this would allow ``setuptools`` to install the project as a zip file. Alternatively, by setting ``zip_safe`` to ``False``, developers could force ``setuptools`` to always install the project as a directory. Modern ways of loading packages from zip files ---------------------------------------------- Currently, popular Python package installers (such as :pypi:`pip`) and package indexes (such as PyPI_) consider that distribution packages are always installed as a directory. It is however still possible to load packages from zip files added to :obj:`sys.path`, thanks to the :mod:`zipimport` module and the :mod:`importlib` machinery provided by Python standard library. When working with modules loaded from a zip file, it is important to keep in mind that values of ``__file__`` and ``__path__`` might not work as expected. Please check the documentation for :mod:`importlib.resources`, if file locations are important for your use case. .. _PyPI: https://pypi.org

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