Migration guides

Migrating from old PYTHON_USEDEP syntax in python-single-r1

Prior to February 2020, python-single-r1 used to provide a single PYTHON_USEDEP variable alike the two other eclasses. However, getting it to work correctly both on single-impl and multi-impl packages required a gross hack.

The current eclass API requires using python_gen_cond_dep function to generate multi-impl deps instead, with PYTHON_USEDEP serving as a placeholder. Single-impl deps can be expressed with PYTHON_SINGLE_USEDEP that can be used either as placeholder, or directly as a variable.

During a transitional period, PYTHON_USEDEP was banned entirely and PYTHON_MULTI_USEDEP was used instead. As of EAPI 8, the opposite is true — PYTHON_USEDEP is to be used, and PYTHON_MULTI_USEDEP was removed.

The recommended rule of thumb for migrating old ebuilds is to:

  1. Replace all instances of ${PYTHON_USEDEP} for simple-impl deps with ${PYTHON_SINGLE_USEDEP}. If you don’t know the type of given dep, dependency check (repoman, pkgcheck) will tell you if you chose wrong.

  2. Wrap the dependencies using ${PYTHON_USEDEP} in a single python_gen_cond_dep block (reordering may be desirable).

  3. Run pkgcheck scan or repoman full. If you get syntax errors, you probably missed python_gen_cond_dep or did not escape the $ in placeholder properly. If you get unmatched dependency, you probably got single-impl vs. multi-impl wrong.

This way, you can generally convert ebuilds using trial-and-error method.

Migrating from EAPI 7 to EAPI 8

EAPI 8 has banned everything that’s been deprecated in EAPI 7, as well as some other obsolete stuff. The following table lists all banned things along with their suggested replacements.

Deprecated thing


Private eclass API


python_setup / getters



Obsolete API

distutils_install_for_testing --via-home

no argument (--via-root) or --via-venv





mydistutilsargs rename



Post-Python 2 cleanup

python_gen* -2 / python2 / pypy

remove entirely

python_gen* -3

make unconditional


always assume true

The changes can be split roughly into four groups: ban of now-private eclass API, ban of obsolete API functions, mydistutilsargs rename and bans related to post-Python 2 cleanup.

The private eclass API part involves python_export and python_wrapper_setup. Both were deprecated in March 2020, and they were never covered in this guide. The former was historically used to get information about the Python interpreter (either the current ${EPYTHON} or an arbitrary choice), the latter to create the wrapper directory containing python and other executables.

When the functions were used to establish a Python build environment, the replacement for both is a single python_setup call. When python_export was used to grab additional details about the Python interpreter, the various python_get* functions should be used instead.

src_configure() {
    # ...

    # OLD:

    # NEW:

The second group involves sundry API that were deprecated earlier. These are:

  1. distutils_install_for_testing --via-home layout that stopped working correctly at some point. The default --via-root should work most of the time, and -via-venv replace the remaining cases for the removed layout.

  2. python_gen_usedep function that was historically used to generate partial USE dependencies, and was generally combined with REQUIRED_USE to force specific (usually old) Python interpreters for specific features. This was really ugly. Nowadays, you should really use python_gen_cond_dep instead.

  3. PYTHON_MULTI_USEDEP placeholder that was temporarily used in python-single-r1 ebuilds. PYTHON_USEDEP is equivalent now.

The third group is a sole rename of mydistutilsargs variable. Since you usually need to pass the same arguments in all phase functions, this variable was not really used in local scope. It has been renamed to uppercase DISTUTILS_ARGS to follow the common pattern for global scope variables.

Finally, the fourth group involves banning some of the features that were specifically used in order to support distinguish between Python 2 and Python 3. This is meant to force cleaning up old cruft from ebuilds. It comes in three parts:

  1. Banning arguments to python_gen* that reference Python 2 (e.g. -2, python2*, python2_7, pypy). Since Python 2 is no longer supported in the relevant code paths, the relevant calls should just be removed.

  2. Banning the -3 short-hand to python_gen*. Since all supported interpreters are compatible with Python 3 now, the relevant code should be made unconditional. Note that python3* is still useful, as it distinguishes CPython from PyPy3.

  3. Banning the python_is_python3 function. Since the removal of Python 2 support, it always evaluated to true.

All the aforementioned replacements are available in all EAPIs.

Migrating to PEP 517 builds

As of January 2022, the distutils-r1 can use PEP 517 build backends instead of calling setuptools directly. The new mode is particularly useful for:

  • packages using flit and poetry, as a better replacement for the deprecated dev-python/pyproject2setuppy hack

  • packages using other PEP 517 build systems (such as pdm) that are not supported in legacy mode at all

  • packages using setuptools without setup.py

  • packages using plain distutils, as the mode handles the switch from deprecated stdlib distutils to the version vendored in setuptools safely

The PEP 517 mode provides the test phase with venv-style installed package tree (alike distutils_install_for_testing --via-venv) that should make testing more streamlined.

Unfortunately, the new mode can cause issues with customized distutils and setuptools build systems. It is important to verify the installed file list after the migration. Packages that require custom configure phases or passing arguments are not supported at the moment.

For simple packages, the migration consists of:

  1. Adding DISTUTILS_USE_PEP517 above the inherit line. The value indicates the build system used, e.g. flit, poetry, setuptools (used also for distutils).

  2. Removing DISTUTILS_USE_SETUPTOOLS. If the previous value was rdepend (and indeed a runtime dependency is required), then dev-python/setuptools needs to be explicitly added to RDEPEND.

  3. Removing distutils_install_for_testing and/or --install option to distutils_enable_tests. This should no longer be necessary and tests should work out of the box.