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<title>Debian Python Policy - Packaged Modules</title>
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<link href="ch-python3.html" rel="chapter" title="1 On the move to Python 3">
<link href="ch-python.html" rel="chapter" title="2 Python Packaging">
<link href="ch-module_packages.html" rel="chapter" title="3 Packaged Modules">
<link href="ch-programs.html" rel="chapter" title="4 Python Programs">
<link href="ch-embed.html" rel="chapter" title="5 Programs Embedding Python">
<link href="ch-other.html" rel="chapter" title="6 Interaction with Locally Installed Python Versions">
<link href="ap-build_dependencies.html" rel="appendix" title="A Build Dependencies">
<link href="ap-packaging_tools.html" rel="appendix" title="B Packaging Tools">
<link href="ap-upgrade.html" rel="appendix" title="C Upgrade Procedure">
<link href="ch-python.html#s-versions" rel="section" title="2.1 Versions">
<link href="ch-python.html#s-base" rel="section" title="2.2 Main packages">
<link href="ch-python.html#s-minimal" rel="section" title="2.3 Minimal packages">
<link href="ch-python.html#s-interpreter" rel="section" title="2.4 Python Interpreter">
<link href="ch-python.html#s-paths" rel="section" title="2.5 Module Path">
<link href="ch-python.html#s-runtimes_hooks" rel="section" title="2.6 Hooks for updates to installed runtimes">
<link href="ch-python.html#s-docs" rel="section" title="2.7 Documentation">
<link href="ch-module_packages.html#s3.1" rel="section" title="3.1 Types of Python Modules">
<link href="ch-module_packages.html#s-wheels" rel="section" title="3.2 Wheels">
<link href="ch-module_packages.html#s-package_names" rel="section" title="3.3 Module Package Names">
<link href="ch-module_packages.html#s-specifying_versions" rel="section" title="3.4 Specifying Supported Versions">
<link href="ch-module_packages.html#s-dependencies" rel="section" title="3.5 Dependencies">
<link href="ch-module_packages.html#s-provides" rel="section" title="3.6 Provides">
<link href="ch-module_packages.html#s-byte_compilation" rel="section" title="3.7 Modules Byte-Compilation">
<link href="ch-programs.html#s-interpreter-directive" rel="section" title="4.1 Interpreter directive (“Shebang”)">
<link href="ch-programs.html#s-version_indep_progs" rel="section" title="4.2 Programs using the default Python">
<link href="ch-programs.html#s-version_dep_progs" rel="section" title="4.3 Programs Using a Particular Python Version">
<link href="ch-embed.html#s-build_embedded" rel="section" title="5.1 Building Embedded Programs">
<link href="ch-embed.html#s-embedded_deps" rel="section" title="5.2 Embedded Python Dependencies">
<link href="ap-packaging_tools.html#s-distutils" rel="section" title="B.1 distutils">
<link href="ap-packaging_tools.html#s-setuptools" rel="section" title="B.2 setuptools">
<link href="ap-packaging_tools.html#s-dh-python" rel="section" title="B.3 dh-python">
<link href="ap-packaging_tools.html#s-pybuild" rel="section" title="B.4 pybuild">
<link href="ap-packaging_tools.html#s-cdbs" rel="section" title="B.5 CDBS">
<link href="ap-packaging_tools.html#s-pysupport" rel="section" title="B.6 python-support (removed)">
<link href="ap-packaging_tools.html#s-pycentral" rel="section" title="B.7 python-central (removed)">
<link href="ch-python.html#s-interpreter_name" rel="subsection" title="2.4.1 Interpreter Name">
<link href="ch-python.html#s-interpreter_loc" rel="subsection" title="2.4.2 Interpreter Location">
<link href="ch-programs.html#s-current_version_progs" rel="subsection" title="4.2.1 Programs Shipping Private Modules">
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<p><a name="ch-module_packages"></a></p>
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[ <a href="ch-python.html">previous</a> ]
[ <a href="index.html#contents">Contents</a> ]
[ <a href="ch-python3.html">1</a> ]
[ <a href="ch-python.html">2</a> ]
[ 3 ]
[ <a href="ch-programs.html">4</a> ]
[ <a href="ch-embed.html">5</a> ]
[ <a href="ch-other.html">6</a> ]
[ <a href="ap-build_dependencies.html">A</a> ]
[ <a href="ap-packaging_tools.html">B</a> ]
[ <a href="ap-upgrade.html">C</a> ]
[ <a href="ch-programs.html">next</a> ]
</p>
<hr>
<h1>
Debian Python Policy
<br>Chapter 3 - Packaged Modules
</h1>
<hr>
<p>
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
<code>*.pyc</code> and <code>*.pyo</code>) or shortly thereafter with automated
rebuilds (to handle C extensions). These policies encourage automated
dependency generation and loose version bounds whenever possible.
</p>
<hr>
<h2 id="s3.1">3.1 Types of Python Modules</h2>
<p>
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.
</p>
<p>
Debian Python does not link extensions to <code>libpython</code> (as is done in
some operating systems). Symbols are resolved by
<code>/usr/bin/python<var>X</var>.<var>Y</var></code> which is not linked to
<code>libpython</code>.
</p>
<p>
Python packages are a way of structuring Python’s module namespace by using
“dotted module names”. See <code><a
href="https://docs.python.org/3/glossary.html#term-package">Python's
glossary</a></code> 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.
</p>
<p>
There are two ways to distribute Python modules. Public modules are installed
in a public directory as listed in <a href="ch-python.html#s-paths">Module
Path, Section 2.5</a>. They are accessible to any program. Private modules
are installed in a private directory such as
<code>/usr/share/<var>package-name</var></code> or
<code>/usr/lib/<var>package-name</var></code>. They are generally only
accessible to a specific program or suite of programs included in the same
package.
</p>
<hr>
<h2 id="s-wheels">3.2 Wheels</h2>
<p>
<code><a href="https://www.python.org/dev/peps/pep-0427/">PEP 427</a></code>
defines a built-package format called "wheels", which is a Zip format
archive containing Python code and a <code>*.dist-info</code> metadata
directory, in a single file named with the <code>.whl</code> 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 <samp>import</samp> statement.
(Importing extension modules from wheels is not yet supported as of Python
3.4.)
</p>
<p>
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 <samp>sys.path</samp>, cannot be easily
grepped, and stack traces through Zip files are more difficult to debug.
</p>
<p>
A very limited set of wheel packages are available in the archive, but these
support the narrow purpose of enabling the <code>pip</code>,
<code>virtualenv</code>, and <code>pyvenv</code> tools in a Debian policy
compliant way. These packages build their own dependent wheels through the use
of the <code>dirtbike</code> "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. Since only the
programs that require wheels need build them, only they may provide
<code>-whl</code> packages, e.g. <code>python3-pip-whl</code>.
</p>
<p>
When these binary packages are installed, <code>*.whl</code> files must be
placed in the <code>/usr/share/python-wheels</code> directory. The location
inside a virtual environment will be rooted in the virtual environment, instead
of <code>/usr</code>.
</p>
<hr>
<h2 id="s-package_names">3.3 Module Package Names</h2>
<p>
Public Python modules must be packages separately by major Python version, to
preserve run time separation between Python 2 and Python 3.
</p>
<p>
Public Python 3 modules used by other packages must have their binary package
name prefixed with <code>python3-</code>. Public Python 2 modules used by
other packages must have their binary package name prefixed with
<code>python-</code>. It is recommended to use this prefix for all packages
with public modules as they may be used by other packages in the future.
</p>
<p>
The binary package for module <var>foo</var> should preferably be named
<code>python3-<var>foo</var></code> (for Python 3) or
<code>python-<var>foo</var></code> (for Python 2), 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.
</p>
<p>
For subpackages such as <var>foo.bar</var>, the recommendation is to name the
binary package <code>python3-<var>foo.bar</var></code> (for Python 3) or
<code>python-<var>foo.bar</var></code> (for Python 2).
</p>
<p>
Such a package should support the current Debian Python version, and more if
possible (there are several tools to help implement this, see <a
href="ap-packaging_tools.html">Packaging Tools, Appendix B</a>). For example,
if Python 3.3, 3.4, and 3.5 are supported, the Python statement
</p>
<pre>
import foo
</pre>
<p>
should import the module when the program interpreter is any of
<code>/usr/bin/python3.3</code>, <code>/usr/bin/python3.4</code>, and
<code>/usr/bin/python3.5</code>. This requirement also applies to extension
modules; binaries for all the supported Python versions should be included in a
single package.
</p>
<p>
Packages intended for use with Django (<code>python3-django</code>/
<code>python-django</code>) are installed in the same namespace as other python
packages for a variety of reasons. Many such packages are named django_$name
upstream. These are then packaged as <code>python3-django-$name</code> and
<code>python-django-$name</code>. 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.
</p>
<hr>
<h2 id="s-specifying_versions">3.4 Specifying Supported Versions</h2>
<p>
The <code>debian/control</code> source paragraph may contain optional fields to
specify the versions of Python the package supports.
</p>
<p>
The optional <samp>X-Python3-Version</samp> field specifies the versions of
Python 3 supported. When not specified, it defaults to all currently supported
Python 3 versions.
</p>
<p>
Similarly, the optional fields <samp>X-Python-Version</samp> or
<samp>XS-Python-Version</samp> were used to specify the versions of Python 2
supported by the source package. They are obsolete and can be removed now that
only Python 2.7 is supported.
</p>
<p>
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:
</p>
<pre>
X-Python3-Version: >= X.Y
X-Python3-Version: >= A.B, << X.Y
XS-Python-Version: A.B, X.Y
XS-Python-Version: all
</pre>
<p>
The keyword <samp>all</samp> means that the package supports any Python 2
version available but might be deprecated in the future since using version
numbers is clearer than <samp>all</samp> and encodes more information. The
keyword <samp>all</samp> is limited to Python 2 versions and must be ignored
for Python 3 versions.
</p>
<p>
A comma-separated list of multiple individual versions (e.g. <samp>3.3, 3.4,
3.5</samp>) in <samp>XS-Python-Version</samp> will continue to be supported,
but is not recommended. The use of multiple individual versions in
<samp>X-Python-Version</samp> or <samp>X-Python3-Version</samp> is not
supported for Wheezy and later releases.
</p>
<p>
The keyword <samp>current</samp> has been deprecated and used to mean that the
package would only have to support a single Python 2 version (even across
default version changes). It must be ignored for Python 3 versions.
</p>
<p>
The use of <samp>XB-Python-Version</samp> in the binary package paragraphs of
<code>debian/control</code> 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.
</p>
<hr>
<h2 id="s-dependencies">3.5 Dependencies</h2>
<p>
Any package that installs modules for the default Python version (or many
versions including the default) as described in <a
href="#s-package_names">Module Package Names, Section 3.3</a>, 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.
</p>
<p>
For Python 3, the correct dependencies are <samp>Depends:
python3 (>= 3.<var>Y</var>)</samp> and any corresponding
<code>python3-<var>foo</var></code> packages.
</p>
<p>
For Python 2, the correct dependencies are <samp>Depends:
python (>= 2.<var>Y</var>)</samp> and any corresponding
<code>python-<var>foo</var></code> packages.
</p>
<p>
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. <samp>Depends:
python3 (<< <var>X</var>.<var>Y</var>)</samp>.
</p>
<hr>
<h2 id="s-provides">3.6 Provides</h2>
<p>
Binary packages that declare Provides dependencies of the form
<code>python<var>X</var>.<var>Y</var>-<var>foo</var></code> were never
supported for Python 3 and are no longer useful for Python 2. They should be
removed in the normal course of package updates. Future provision of values
for the substituation variable <samp>python:Provides</samp> is not guaranteed.
</p>
<hr>
<h2 id="s-byte_compilation">3.7 Modules Byte-Compilation</h2>
<p>
If a binary package provides any binary-independent modules
(<code><var>foo</var>.py</code> files), the corresponding byte-compiled modules
(<code><var>foo</var>.pyc</code> files) and optimized modules
(<code><var>foo</var>.pyo</code> 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 <code><var>foo</var>.pyc</code> and <code><var>foo</var>.pyo</code> are
removed.
</p>
<p>
A binary package should only byte-compile the files which belong to the
package.
</p>
<p>
The file <code>/etc/python/debian_config</code> allows configuration how
modules should be byte-compiled. The post-install scripts should respect these
settings.
</p>
<p>
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.
</p>
<p>
Public Python extensions should be bin-NMUed.
</p>
<p>
Private Python extensions should be subject to binary NMUs every time the
default interpreter changes, unless the extension is updated through a
<code>*.rtupdate</code> script.
</p>
<hr>
<p>
[ <a href="ch-python.html">previous</a> ]
[ <a href="index.html#contents">Contents</a> ]
[ <a href="ch-python3.html">1</a> ]
[ <a href="ch-python.html">2</a> ]
[ 3 ]
[ <a href="ch-programs.html">4</a> ]
[ <a href="ch-embed.html">5</a> ]
[ <a href="ch-other.html">6</a> ]
[ <a href="ap-build_dependencies.html">A</a> ]
[ <a href="ap-packaging_tools.html">B</a> ]
[ <a href="ap-upgrade.html">C</a> ]
[ <a href="ch-programs.html">next</a> ]
</p>
<hr>
<p>
Debian Python Policy
</p>
<address>
version 0.10.1.1<br>
<br>
Neil Schemenauer <code><a href="mailto:nas@debian.org">mailto:nas@debian.org</a></code><br>
Matthias Klose <code><a href="mailto:doko@debian.org">mailto:doko@debian.org</a></code><br>
Gregor Hoffleit <code><a href="mailto:flight@debian.org">mailto:flight@debian.org</a></code><br>
Josselin Mouette <code><a href="mailto:joss@debian.org">mailto:joss@debian.org</a></code><br>
Joe Wreschnig <code><a href="mailto:piman@debian.org">mailto:piman@debian.org</a></code><br>
Loïc Minier <code><a href="mailto:lool@debian.org">mailto:lool@debian.org</a></code><br>
Scott Kitterman <code><a href="mailto:scott@kitterman.com">mailto:scott@kitterman.com</a></code><br>
Barry Warsaw <code><a href="mailto:barry@debian.org">mailto:barry@debian.org</a></code><br>
Ben Finney <code><a href="mailto:ben+debian@benfinney.id.au">mailto:ben+debian@benfinney.id.au</a></code><br>
<br>
</address>
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