Table of Contents
At any given time, the binary package python3
will represent the current default Debian Python 3 version; the
binary package python will represent the
current default Debian Python 2 version. As far as is reasonable,
Python 3 and Python 2 should be treated as separate runtime
systems with minimal interdependencies.
In some cases, Python policy explicitly references Python helper
tools. For Debian Stretch, the dh-python
package provides the only such tools; earlier helpers have been
removed from Debian.
It is a design goal to fully specify required interfaces and functions in policy for Python 3 and to avoid enshrining specific implementation details in policy. Except as noted, policy for Python 2 is the same as Python 3 with the exception of the different major version number as needed to distinguish them.
The default Debian Python version, for each of Python 3 and Python 2, should always be the latest stable upstream version that can be fully integrated in Debian.
There may be newer supported or unsupported versions included in Debian if they are not fully integrated for a particular release.
Apart from the default version, legacy versions of Python or beta releases of future upstream versions may be included as well in Debian, 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, a Python version is synonymous with all micro versions within that minor version. e.g. Python 3.5.0 and 3.5.1 are micro versions of the same Python version 3.5, but Python 3.4 and 3.5 are indeed different versions.
For any version, the main binary package must be called
python.
X.Y
The set of currently supported Python 3 versions can be found
in /usr/share/python3/debian_defaults; the supported
interface to this information is
through /usr/bin/py3versions.
The set of currently supported Python 2 versions can be found in
/usr/share/python/debian_defaults; the supported
interface to this information is /usr/bin/pyversions.
These files are in Python configparser format. They
define (in the DEFAULT section) the following options:
default-version: The name of the interpreter for
the current default Debian Python.
supported-versions: The set of interpreter names
currently supported and for which modules should be built and
byte-compiled. This includes default-version.
old-versions: The set of interpreter names which
might still be on the system but for which modules should not
be built.
unsupported-versions: The set of interpreter
names which should not be supported at all, that is modules
should not be built or byte-compiled for these. This includes
(is a superset of) old-versions.
Newer versions might also appear in unsupported-versions
before being moved to supported-versions.
For every Python version provided in Debian, the binary
package python shall
provide a complete distribution for deployment of Python
scripts and applications. The package must ensure that the binary
X.Y/usr/bin/python is provided.
X.Y
Installation of python
shall provide the modules of the upstream Python distribution with
some exceptions.
X.Y
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
python.
X.Y-dev
Documentation will be provided separately as well.
At any time, the python3 binary package must
ensure that /usr/bin/python3 is provided, as a
symlink to the current python3.
executable. The package must depend on
the Ypython3. package that installs
the executable.
Y
The version of the python3 package must be
greater than or equal to 3.Y and lower than
3.Y+1.
At any time, the python binary package must
ensure that /usr/bin/python2 is provided, as a
symlink to the current python2.
executable. The package must depend on
the Ypython2. package that installs
the executable.
Y
The version of the python package must be
greater than or equal to 2.Y and lower than
2.Y+1.
The python binary package must also ensure
that /usr/bin/python is provided, as a symlink to the
current python2. executable. See
PEP 394 for details.
Y
For every Python version provided in Debian, the binary package
python might
exist and should not be depended upon by other packages except the
Python runtime packages themselves.
X.Y-minimal
The different Python major versions require different interpreters (see Section 2.2, “Main packages”).
Python scripts that require the default Python 3 version should
specify python3 as the interpreter name.
Python scripts that require the default Python 2 version should
specify python2 as the interpreter name.
Python scripts may specify python as the
interpreter name only if they do not require any particular
version of Python. (Note: this means any python2 version)
Python scripts that only work with a specific Python minor
version must explicitly use the versioned interpreter name
(python).
X.Y
Python scripts should specify the Debian Python interpreter, to ensure that the Debian Python installation is used and all dependencies on additional Python modules are met.
The preferred specification for the Python 3 interpreter is
/usr/bin/python3 (or
/usr/bin/python3. if it requires Python
3.YY).
The preferred specification for the Python 2 interpreter is
/usr/bin/python2 (or
/usr/bin/python2. if it requires Python
2.YY).
Scripts requiring the default Python 2 version may instead
specify the interpreter /usr/bin/python.
Maintainers should not override the Debian Python interpreter
using /usr/bin/env . This is not
advisable as it bypasses Debian's dependency checking and makes
the package vulnerable to incomplete local installations of
Python.
name
By default, Python modules are searched in the directories listed
in the PYTHONPATH environment variable and in
the sys.path Python variable. For all supported Debian
releases, sys.path does not include
a /usr/lib/python entry.
XY.zip
Directories with private Python modules must be absent from the
sys.path.
Public Python 3 modules must be installed in the system Python 3
modules directory, /usr/lib/python3/dist-packages.
Public Python 2 modules must be installed in the system Python 2
modules directory
/usr/lib/python2., where
2.Y/dist-packagesY is the Python 2 version.
A special directory is dedicated to public Python modules
installed by the local administrator,
/usr/lib/python3/dist-packages for all Python 3 versions,
/usr/local/lib/python2. for
Python 2.
Y/dist-packages
For local installation of Python modules by the system
administrator, special directories are reserved. The
directory /usr/local/lib/python3/site-packages is in
the Python 3 runtime module search path. The
directory /usr/local/lib/python2.
is in the Python 2.Y/site-packagesY runtime module search path.
Additional information on appending site-specific paths to the
module search path is available in the official documentation of
the site module.
Python modules which work with multiple supported Python 2
versions must install to version-specific locations, for instance
/usr/lib/python2.6/dist-packages/foo.py and
/usr/lib/python2.7/dist-packages/foo.py. These should
point to a common file.
Architecture-independent public Python 3 modules must be installed
to /usr/lib/python3/dist-packages.
Architecture-independent public Python 2 modules should be
installed to /usr/lib/python2.7/dist-packages. The
historical location for this was /usr/share/pyshared.
Since Python 2.7 is the last Python 2 version and the only
supported version in Wheezy and later releases, a version-specific
location is sufficient.
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/python3/runtime.d/*.rtinstall,
/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 python) and the
third and fourth argument are the old and new version of this
packaged runtime if this runtime was already installed but
unsupported.
X.Y
/usr/share/python3/runtime.d/*.rtremove,
/usr/share/python/runtime.d/*.rtremove: These are
called when a runtime is removed or stops being supported. The
first argument is rtremove, and the second argument
is the affected runtime (for
example python).
X.Y
/usr/share/python3/runtime.d/*.rtupdate,
/usr/share/python/runtime.d/*.rtupdate: 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 python), and the
third argument is the new default runtime (for example
X.Ypython).
X.Z
Python documentation is split out in separate binary packages
python.
X.Y-doc
The binary package python3-doc will always
provide the documentation for the default Debian Python 3 version.
The binary package python-doc will always
provide the documentation for the default Debian Python 2 version.
TODO: Policy for documentation of third party packages.