At any given time, the binary package python will represent the
current default Debian Python version. The binary package python3
will represent the current Debian Python 3 version. As far as is reasonable,
python and python3 should be treated as separate runtime systems with minimal
interdependencies. In some cases, Python policy explicitly references Python
helper programs such as python-support and python-central. None of these
references apply to Python 3. It is a design goal to fully specify required
interfaces and functions in policy for Python 3 and to avoid enshrining
specific implmentation details in policy. Except as noted, policy for Python 3
is the same as Python with the addition of the version number as needed to
distinguish them.
The default Debian Python version should always be the latest stable upstream
release that can be fully integrated in the distribution. There may be newer
supported or unsupported versions included in the distribution if they are not
fully integrated for a particular release.
Apart from the default version, legacy versions of Python or beta versions of
future releases may be included as well in the distribution, as long as they
are needed by other packages, or as long as it seems reasonable to provide
them. (Note: For the scope of this document, Python versions are synonymous to
feature releases, i.e. Python 2.5 and 2.5.1 are sub-minor versions of the same
Python version 2.5, but Python 2.4 and 2.5 are indeed different versions.)
For any version, the main binary package must be called
pythonX.Y.
The set of currently supported python versions can be found in
/usr/share/python/debian_defaults. This file is in Python
ConfigParser format and defines four variables in its DEFAULT section:
default-version which is the current default Python runtime, supported-versions
which is the set of runtimes currently supported and for which modules should
be built and byte-compiled, old-versions which is the list of runtimes which
might still be on the system but for which should not be built anymore, and
unsupported-versions which is the list of runtimes which should not be
supported at all, that is modules should not be built or byte-compiled for
these. The supported interface to this file is
/usr/bin/pyversions. The Python 3 interface is through
/usr/bin/py3versions.
unsupported-versions is a superset of (includes) old-versions and the
default-version is always in supported-versions.
Newer versions might also appear in unsupported-versions before being moved to
supported-versions.
1.2 Main packages
For every Python version provided in the distribution, the binary package
pythonX.Y shall provide a complete
distribution for deployment of Python scripts and applications. The
package must ensure that the binary
/usr/bin/pythonX.Y is provided.
Installation of pythonX.Y shall provide the
modules of the upstream Python distribution with some exceptions.
Excluded are modules that cannot be included for licensing reasons (for example
the profile module), for dependency tracking purposes (for example the
GPL-licensed gdbm module) or that should not be included for packaging reasons
(for example the tk module which depends on Xorg). Some tools and files for
the development of Python modules are split off in a separate binary
package pythonX.Y-dev. Documentation will
be provided separately as well.
At any time, the python binary package must ensure that
/usr/bin/python is provided as a symlink to the current
pythonX.Y executable. The
python binary package must also depend on the appropriate
pythonX.Y to ensure this runtime is
installed.
The version of the python binary package must be greater than or
equal to X.Y and smaller than
X.Y+1.
1.3 Minimal packages
For every Python version provided in the distribution, the binary package
pythonX.Y-minimal might exist and should not
be depended upon by other packages except the Python runtime packages
themselves.
1.4 Python Interpreter
1.4.1 Interpreter Name
Python scripts depending on the default Python version (see Main packages, Section 1.2) or not depending on a specific
Python version should use python (without a version) as the
interpreter name.
Python scripts that only work with a specific Python version must explicitly
use the versioned interpreter name
(pythonX.Y).
1.4.2 Interpreter Location
The preferred specification for the Python interpreter is
/usr/bin/python or
/usr/bin/pythonX.Y. This ensures that a
Debian installation of python is used and all dependencies on additional python
modules are met.
If a maintainer would like to provide the user with the possibility to override
the Debian Python interpreter, he may want to use /usr/bin/env
python or /usr/bin/env pythonX.Y.
However this is not advisable as it bypasses Debian's dependency checking and
makes the package vulnerable to incomplete local installations of python.
1.5 Module Path
By default, Python modules are searched in the directories listed in the
PYTHONPATH environment variable and in the sys.path Python variable. Since
python2.4 version 2.4.5-3, python2.5 version 2.5.2-7, and python2.6 version
2.6.2-1 sys.path does not include a /usr/lib/pythonXY.zip
entry anymore. Directories with private Python modules must be absent from the
sys.path. Public Python modules not handled by python-central or
python-support must be installed in the system Python modules directory,
/usr/lib/pythonX.Y/dist-packages for python2.6 and later,
and /usr/lib/pythonX.Y/site-packages for python2.5 and
earlier. As an exception to the above, modules managed by python-support are
installed in another directory which is added to the sys.path using the .pth
mechanism. The .pth mechanism is documented in the Python documentation of the
site module. A special directory is dedicated to public Python
modules installed by the local administrator,
/usr/local/lib/pythonX.Y/dist-packages for python2.6 and
later, and /usr/local/lib/pythonX.Y/site-packages for
python2.5 and earlier. For a local installation by the administrator of
python2.6 and later, a special directory is reserved to Python modules which
should only be available to this Python,
/usr/local/lib/pythonX.Y/site-packages. Unfortunately,
for python2.5 and earlier this directory is also visible to the system Python.
Additional information on appending site-specific paths to the module search
path is available in the official documentation of the site module.
When binary packages ship identical source code for multiple Python versions,
for instance /usr/lib/python2.6/dist-packages/foo.py and
/usr/lib/python2.5/site-packages/foo.py, these should point to a common file.
A common location to share, across Python versions, arch-independent files
which would otherwise go to the directory of system public modules is
/usr/share/pyshared.
1.6 Hooks for updates to installed runtimes
The python binary package has special hooks to allow other
packages to act upon updates to the installed runtimes. This mechanism is
required to handle changes of the default Python runtime in some packages and
to enable the Python packaging helpers. There are three supported hook types
which come in the form of scripts which are invoked from the maintainer scripts
of the Python runtime packages when specific installations, removals, or
upgrades occur.
/usr/share/python/runtime.d/*.rtinstall: these are called when a runtime is
installed or becomes supported. The first argument is "rtinstall",
the second argument is the affected runtime (for example
pythonX.Y) and the third and fourth argument are the old
and new version of this packaged runtime if this runtime was already installed
but unsupported.
/usr/share/python/runtime.d/*.rtremove: these are called when a runtime is
installed or stops being supported. The first argument is
"rtremove", and the second argument is the affected runtime (for
example pythonX.Y).
/usr/share/python/runtime.d/*.rtupate: these are called when the default
runtime changes. The first argument is either "pre-rtupdate", called
before changing the default runtime, or "rtupdate", called when
changing the default runtime, or "post-rtupdate", called immediately
afterwards. The second argument is the old default runtime (for example
pythonX.Y), and the third argument is the new default
runtime (for example pythonX.Z).
1.7 Documentation
Python documentation is split out in separate binary packages
pythonX.Y-doc. The binary package
python-doc will always provide the documentation for the default
Debian Python version.
TODO: Policy for documentation of third party packages.