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.