Advanced dependencies

Requesting USE flags on the Python interpreter

While the majority of Python standard library modules are available unconditionally, a few are controlled by USE flags. For example, the sqlite3 module requires sqlite flag to be enabled on the interpreter. If a package requires this module, it needs to enforce the matching flag via a USE dependency.

In order to create a USE dependency on the Python interpreter, set PYTHON_REQ_USE before inheriting the eclass. This will cause the eclass to generate appropriate dependency string in PYTHON_DEPS.

 # Copyright 1999-2020 Gentoo Authors
 # Distributed under the terms of the GNU General Public License v2

 EAPI=7

 PYTHON_COMPAT=( python3_6 )
 PYTHON_REQ_USE="sqlite"
 inherit python-r1 gnome2-utils meson xdg-utils

 DESCRIPTION="Modern music player for GNOME"
 HOMEPAGE="https://wiki.gnome.org/Apps/Lollypop"
 SRC_URI="https://adishatz.org/${PN}/${P}.tar.xz"
 KEYWORDS="~amd64"

 LICENSE="GPL-3"
 SLOT="0"
 REQUIRED_USE=${PYTHON_REQUIRED_USE}

 DEPEND="${PYTHON_DEPS}
     ..."

Full USE dependency syntax is permitted. For example, you can make the dependency conditional to a flag on the package:

 # Copyright 1999-2020 Gentoo Authors
 # Distributed under the terms of the GNU General Public License v2

 EAPI=6

 PYTHON_COMPAT=( python3_6 )
 PYTHON_REQ_USE="sqlite?"
 inherit distutils-r1

 DESCRIPTION="A lightweight password-manager with multiple database backends"
 HOMEPAGE="https://pwman3.github.io/pwman3/"
 SRC_URI="https://github.com/pwman3/pwman3/archive/v${PV}.tar.gz -> ${P}.tar.gz"

 LICENSE="GPL-3"
 SLOT="0"
 KEYWORDS="~amd64"
 IUSE="mongodb mysql postgres +sqlite"

Finally, there are cases when the problem cannot be fully solved using a single USE dependency. Additional Python interpreter dependencies with specific USE flags can be constructed using python_gen_impl_dep helper then. For example, the following ebuild requires Python with SQLite support when running tests:

 # Copyright 1999-2020 Gentoo Authors
 # Distributed under the terms of the GNU General Public License v2

 EAPI=7
 PYTHON_COMPAT=( python{2_7,3_{6,7,8}} pypy3 )

 inherit distutils-r1

 DESCRIPTION="Let your Python tests travel through time"
 HOMEPAGE="https://github.com/spulec/freezegun"
 SRC_URI="mirror://pypi/${PN:0:1}/${PN}/${P}.tar.gz"

 LICENSE="Apache-2.0"
 SLOT="0"
 KEYWORDS="~alpha ~amd64 ~arm ~arm64 ~hppa ~ia64 ~m68k ~mips ~ppc ~ppc64 ~s390 ~sh ~sparc ~x86 ~amd64-linux ~x86-linux ~ppc-macos ~x64-macos ~x86-macos"

 RDEPEND="
     >dev-python/python-dateutil-2.0[${PYTHON_USEDEP}]
     dev-python/six[${PYTHON_USEDEP}]
 "
 DEPEND="${RDEPEND}
     dev-python/setuptools[${PYTHON_USEDEP}]
     test? (
         $(python_gen_impl_dep sqlite)
         dev-python/mock[${PYTHON_USEDEP}]
         dev-python/nose[${PYTHON_USEDEP}]
     )
 "

 distutils_enable_tests pytest

Dependencies conditional to Python version

When packaging software for multiple Python versions, it is quite likely that you’ll find yourself needing some packages only with some of the versions, and not with others. This is the case with backports and other compatibility packages. It also happens if some of the optional dependencies do not support the full set of implementations your package supports.

A dependency that applies only to a subset of PYTHON_COMPAT can be created using python_gen_cond_dep function (the same as used in python-single-r1). It takes a dependency string template, followed by zero or more implementation arguments. The dependencies are output for every matching implementation.

The dependency template should contain literal (usually escaped through use of single quotes) ${PYTHON_USEDEP} that will be substituted with partial USE dependency by the eclass function (when using python-single-r1, ${PYTHON_SINGLE_USEDEP} is also permitted).

The implementation arguments can be:

  1. Literal implementation names. For example, if a particular feature is only available on a subset of Python implementations supported by the package:

    RDEPEND="
        cli? (
            $(python_gen_cond_dep '
                dev-python/black[${PYTHON_USEDEP}]
                dev-python/click[${PYTHON_USEDEP}]
            ' python3_{8..10})
        )
    "
    
  2. fnmatch(3)-style wildcard against implementation names. For example, CFFI is part of PyPy’s stdlib, so the explicit package needs to be only installed for CPython:

    RDEPEND="
        $(python_gen_cond_dep '
            dev-python/cffi[${PYTHON_USEDEP}]
        ' 'python*')
    "
    

    Remember that the patterns need to be escaped to prevent filename expansion from happening.

  3. Python standard library versions that are expanded into appropriate implementations by the eclass. For example, this makes it convenient to depend on backports:

    RDEPEND="
        $(python_gen_cond_dep '
            dev-python/backports-zoneinfo[${PYTHON_USEDEP}]
        ' 3.8)
    "
    

    The advantage of this form is that the dependencies automatically adjust as we switch PyPy3 to a newer stdlib version.

An important feature of python_gen_cond_dep is that it handles removal of old implementations gracefully. When one of the listed implementations is no longer supported, it silently ignores it. This makes it possible to remove old implementations without having to update all dependency strings immediately.

For example, in the following example the dependency became empty when Python 3.7 was removed:

RDEPEND="
    $(python_gen_cond_dep '
        dev-python/importlib_metadata[${PYTHON_USEDEP}]
    ' python3_7)"

Dependencies on CFFI and greenlet

The PyPy distribution includes special versions of the cffi and greenlet packages. For this reason, packages using CFFI and/or greenlet and supporting PyPy3 need to make the explicit dependencies conditional to CPython:

RDEPEND="
    $(python_gen_cond_dep '
        >=dev-python/cffi-1.1.0:=[${PYTHON_USEDEP}]
    ' 'python*')
"