4. Packaged Modules¶
The goal of these policies is to reduce the work necessary for Python
transitions.
Python modules are internally very dependent on a specific Python
version.
However, we want to automate recompiling modules when possible, either
during the upgrade itself (re-compiling bytecode files *.pyc
and *.pyo
) or shortly thereafter with automated rebuilds (to
handle C extensions).
These policies encourage automated dependency generation and loose
version bounds whenever possible.
4.1. Types of Python Modules¶
There are two kinds of Python modules, “pure” Python modules, and extension modules. Pure Python modules are Python source code that generally works across many versions of Python. Extensions are C code compiled and linked against a specific version of the Python runtime, and so can only be used by one version of Python.
Debian Python does not link extensions to libpython
(as is done in
some operating systems).
Symbols are resolved by /usr/bin/pythonX.Y
which is not
linked to libpython
.
Python packages are a way of structuring Python’s module namespace by using “dotted module names”. See Python’s glossary for details on how packages are defined in Python terms (a package in the Python sense is unrelated to a Debian package). Python packages must be packaged into the same directory (as done by upstream). Splitting components of a package across directories changes the import order and may confuse documentation tools and IDEs.
There are two ways to distribute Python modules.
Public modules are installed in a public directory as listed in
Module Path.
They are accessible to any program.
Private modules are installed in a private directory such as
/usr/share/package-name
or /usr/lib/package-name
.
They are generally only accessible to a specific program or suite of
programs included in the same package.
4.2. Wheels¶
PEP 427 defines a built-package format called “wheels”, which is a
Zip format archive containing Python code and a *.dist-info
metadata directory, in a single file named with the .whl
suffix.
As Zip files, wheels containing pure Python can be put on sys.path
and modules in the wheel can be imported directly by Python’s import
statement.
(Importing extension modules from wheels is not yet supported as of
Python 3.4.)
Except as described below, packages must not build or provide wheels.
They are redundant to the established way of providing Python libraries
to Debian users, take no advantage of distro-based tools, and are less
convenient to use.
E.g. they must be explicitly added to sys.path
, cannot be easily
grepped, and stack traces through Zip files are more difficult to debug.
A very limited set of wheel packages are available in the archive, but
these support the narrow purpose of enabling the pip
,
virtualenv
, and pyvenv
tools in a Debian policy compliant way.
These packages build their own dependent wheels through the use of the
dirtbike
“rewheeling” tool, which takes installed Debian packages
and turns them back into wheels.
Only universal wheels (i.e. pure-Python, Python 3 and 2 compatible
packages) are supported, with the exception of wheels of packages that
no longer support Python 2.
Wheels built for these packages are not required to be universal.
Since only the programs that require wheels need build them, only they
may provide -whl
packages, e.g. python3-pip-whl
.
When these binary packages are installed, *.whl
files must be
placed in the /usr/share/python-wheels
directory.
The location inside a virtual environment will be rooted in the virtual
environment, instead of /usr
.
4.3. Module Package Names¶
Public Python modules must be packaged separately by major Python version, to preserve run time separation between Python 2 and Python 3.
Public Python 3 modules used by other packages must have their binary
package name prefixed with python3-
.
It is recommended to use this prefix for all packages with public
modules as they may be used by other packages in the future.
The binary package for module foo
should preferably be named
python3-foo
, if the module name allows.
This is not required if the binary package installs multiple modules, in
which case the maintainer shall choose the name of the module which best
represents the package.
For the purposes of package naming, the name that is used for a module
is the name that can be used with import
, which is not necessarily
the same as the name used in setuptools PKG-INFO
and .egg-info
files and directories.
For example, the module described in pyxdg-*.egg-info
is used
via import xdg
, so its package name is python3-xdg
and not
python3-pyxdg
.
Some modules have names that contain underscores or capital letters,
which are not allowed in Debian package names.
The recommendation is to replace underscores with hyphen/minus and
capital letters with lower-case.
For example, the modules that can be used with import distro_info
and import Xlib
are packaged as python3-distro-info
and
python3-xlib
respectively.
For subpackages such as foo.bar
, the recommendation is to name
the binary package python3-foo.bar
.
Such a package should support the current Debian Python version, and more if possible (there are several tools to help implement this, see Packaging Tools). For example, if Python 3.3, 3.4, and 3.5 are supported, the Python statement
import foo
should import the module when the program interpreter is any of
/usr/bin/python3.3
, /usr/bin/python3.4
, and
/usr/bin/python3.5
.
This requirement also applies to extension modules; binaries for all the
supported Python versions should be included in a single package.
Packages intended for use with Django (python3-django
) are installed
in the same namespace as other python packages for a variety of reasons.
Many such packages are named django_name
upstream.
These are then packaged as python3-django-name
.
This makes it clear that they are intended for use with Django and not
general purpose Python modules.
Debian maintainers are encouraged to work with their upstreams to
support consistent use of this approach.
If the documentation for a module foo
provided in
python3-foo
is large enough that a separate binary package for
documentation is desired, then the documentation package should
preferably be named python-foo-doc
(and in particular, not
python3-foo-doc
).
4.4. Specifying Supported Versions¶
The debian/control
source paragraph may contain optional fields
to specify the versions of Python the package supports.
The optional X-Python3-Version
field specifies the versions of
Python 3 supported.
When not specified, it defaults to all currently supported Python 3
versions.
Similarly, the optional fields X-Python-Version
or
XS-Python-Version
were used to specify the versions of Python 2
supported by the source package.
They are obsolete and must be removed.
These fields are used by some packaging scripts to automatically generate appropriate Depends and Provides lines. The format of the field may be one of the following:
X-Python3-Version: >= X.Y
X-Python3-Version: >= A.B, << X.Y
XS-Python-Version: A.B, X.Y
The keyword all
is no longer to be used since using version numbers
is clearer than all
and encodes more information.
The keyword all
must be ignored for Python 3 versions.
A comma-separated list of multiple individual versions (e.g. 3.3, 3.4,
3.5
) in XS-Python-Version
will continue to be supported, but
is not recommended.
The use of multiple individual versions in X-Python-Version
or
X-Python3-Version
is not supported for Wheezy and later releases.
The keyword current
has been deprecated and must not be used.
It must be ignored for Python 3 versions.
The use of XB-Python-Version
in the binary package paragraphs of
debian/control
file has been deprecated and should be removed in
the normal course of package updates.
It never achieved sufficient deployment to support its intended purpose
of managing Python transitions.
This purpose can be adequately accomplished by examining package
dependencies.
4.5. Dependencies¶
Any package that installs modules for the default Python version (or many versions including the default) as described in Module Package Names, must declare a dependency on the default Python runtime package. If it requires other modules to work, the package must declare dependencies on the corresponding packaged modules. The package must not declare dependency on any version-specific Python runtime or module package.
For Python 3, the correct dependencies are Depends:
python3 (>= 3.Y)
and any corresponding python3-foo
packages.
If any Python 2 packages remain, the correct dependencies are
Depends: python2 (>= 2.Y)
and any corresponding
python2-foo
packages.
Any package that installs Python modules or Python 3 binary extensions
must also declare a maximum version it supports as currently built.
This is accomplished by declaring a maximum version constraint strictly
less than one higher than the current maximum version, i.e.
Depends: python3 (<< X.Y)
.
4.6. Provides¶
Binary packages that declare Provides dependencies of the form
pythonX.Y-foo
were never supported for Python 3.
They should be removed in the normal course of package updates.
Future provision of values for the substitution variable
python:Provides
is not guaranteed.
4.7. Modules Byte-Compilation¶
If a binary package provides any binary-independent modules
(foo.py
files), the corresponding byte-compiled modules
(foo.pyc
files) and optimized modules (foo.pyo
files) must not ship in the package.
Instead, they should be generated in the package’s post-install script,
and removed in the package’s pre-remove script.
The package’s prerm
has to make sure that both foo.pyc
and foo.pyo
are removed.
A binary package should only byte-compile the files which belong to the package.
The file /etc/python/debian_config
allows configuration how
modules should be byte-compiled.
The post-install scripts should respect these settings.
Pure Python modules in private installation directories that are byte-compiled with the default Python version must be forcefully byte-compiled again when the default Python version changes.
Public Python extensions should be bin-NMUed.
Private Python extensions should be subject to binary NMUs every time
the default interpreter changes, unless the extension is updated through
a *.rtupdate
script.