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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>Chapter 3. Packaged Modules</title><meta name="generator" content="DocBook XSL Stylesheets V1.79.1" /><link rel="home" href="index.html" title="Debian Python Policy" /><link rel="up" href="index.html" title="Debian Python Policy" /><link rel="prev" href="python.html" title="Chapter 2. Python Packaging" /><link rel="next" href="programs.html" title="Chapter 4. Python Programs" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Chapter 3. Packaged Modules</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="python.html">Prev</a> </td><th width="60%" align="center"> </th><td width="20%" align="right"> <a accesskey="n" href="programs.html">Next</a></td></tr></table><hr /></div><div class="chapter"><div class="titlepage"><div><div><h1 class="title"><a id="module_packages"></a>Chapter 3. Packaged Modules</h1></div></div></div><div class="toc"><p><strong>Table of Contents</strong></p><dl class="toc"><dt><span class="section"><a href="module_packages.html#idm319">3.1. Types of Python Modules</a></span></dt><dt><span class="section"><a href="module_packages.html#wheels">3.2. Wheels</a></span></dt><dt><span class="section"><a href="module_packages.html#package_names">3.3. Module Package Names</a></span></dt><dt><span class="section"><a href="module_packages.html#specifying_versions">3.4. Specifying Supported Versions</a></span></dt><dt><span class="section"><a href="module_packages.html#dependencies">3.5. Dependencies</a></span></dt><dt><span class="section"><a href="module_packages.html#provides">3.6. Provides</a></span></dt><dt><span class="section"><a href="module_packages.html#byte_compilation">3.7. Modules Byte-Compilation</a></span></dt></dl></div><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 class="filename">*.pyc</code>
	and <code class="filename">*.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><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="idm319"></a>3.1. Types of Python Modules</h2></div></div></div><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 class="filename">libpython</code>
	  (as is done in some operating systems). Symbols are resolved by
	  <code class="filename">/usr/bin/python<em class="replaceable"><code>X</code></em>.<em class="replaceable"><code>Y</code></em></code> which is not
	  linked to <code class="filename">libpython</code>.
	</p><p>
	  Python packages are a way of structuring Python’s module namespace
	  by using “dotted module names”. See
	  <a class="ulink" href="https://docs.python.org/3/glossary.html#term-package" target="_blank">Python's glossary</a> 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 class="xref" href="python.html#paths" title="2.5. Module Path">Section 2.5, “Module Path”</a>.
	  They are accessible to any program. Private modules are installed
	  in a private directory such
	  as <code class="filename">/usr/share/<em class="replaceable"><code>package-name</code></em></code>
	  or <code class="filename">/usr/lib/<em class="replaceable"><code>package-name</code></em></code>. They are
	  generally only accessible to a specific program or suite of
	  programs included in the same package.
	</p></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="wheels"></a>3.2. Wheels</h2></div></div></div><p>
	  <a class="ulink" href="https://www.python.org/dev/peps/pep-0427/" target="_blank">PEP 427</a>
	  defines a built-package format called "wheels", which is a Zip
	  format archive containing Python code and
	  a <code class="filename">*.dist-info</code> metadata directory, in a single file
	  named with the <code class="filename">.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 <code class="literal">import</code>
	  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 <code class="literal">sys.path</code>, 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 class="filename">pip</code>, <code class="filename">virtualenv</code>,
	  and <code class="filename">pyvenv</code> tools in a Debian policy compliant way.
	  These packages build their own dependent wheels through the use of
	  the <code class="filename">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 class="filename">-whl</code> packages,
	  e.g. <code class="literal">python3-pip-whl</code>.
	</p><p>
	  When these binary packages are installed, <code class="filename">*.whl</code> files
	  must be placed in the <code class="filename">/usr/share/python-wheels</code>
	  directory. The location inside a virtual environment will be
	  rooted in the virtual environment, instead of <code class="filename">/usr</code>.
	</p></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="package_names"></a>3.3. Module Package Names</h2></div></div></div><p>
	  Public Python modules must be packaged 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 class="literal">python3-</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 <em class="replaceable"><code>foo</code></em> should preferably be
	  named <code class="literal">python3-<em class="replaceable"><code>foo</code></em></code>, 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 <em class="replaceable"><code>foo.bar</code></em>, the recommendation is
	  to name the binary
	  package <code class="literal">python3-<em class="replaceable"><code>foo.bar</code></em></code>.
	</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 class="xref" href="packaging_tools.html" title="Appendix B. Packaging Tools">Appendix B, <em>Packaging Tools</em></a>). For example, if Python 3.3,
	  3.4, and 3.5 are supported, the Python statement
	  </p><pre class="programlisting">
import foo
	  </pre><p>
	  should import the module when the program interpreter is any
	  of <code class="filename">/usr/bin/python3.3</code>, <code class="filename">/usr/bin/python3.4</code>,
	  and <code class="filename">/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 class="literal">python3-django</code>
          are installed in the same namespace as other python packages
	  for a variety of reasons.  Many such packages are
          named django_$name upstream.  These are then packaged as
          <code class="literal">python3-django-$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></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="specifying_versions"></a>3.4. Specifying Supported Versions</h2></div></div></div><p>
	  The <code class="filename">debian/control</code> source paragraph may contain
	  optional fields to specify the versions of Python the package
	  supports.
	</p><p>
	  The optional <code class="literal">X-Python3-Version</code> 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 <code class="literal">X-Python-Version</code>
	  or <code class="literal">XS-Python-Version</code> were used to specify the versions of
	  Python 2 supported by the source package. They are obsolete and
	  must be removed.
	</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 class="programlisting">
X-Python3-Version: &gt;= X.Y
X-Python3-Version: &gt;= A.B, &lt;&lt; X.Y
XS-Python-Version: A.B, X.Y
	  </pre><p>
	</p><p>
	  The keyword <code class="literal">all</code> is no longer to be used
	  since using version numbers is clearer than <code class="literal">all</code> and
	  encodes more information. The keyword <code class="literal">all</code>
	  must be ignored for Python 3 versions.
	</p><p>
	  A comma-separated list of multiple individual versions
	  (e.g. <code class="literal">3.3, 3.4, 3.5</code>) in <code class="literal">XS-Python-Version</code> will
	  continue to be supported, but is not recommended. The use of
	  multiple individual versions in <code class="literal">X-Python-Version</code>
	  or <code class="literal">X-Python3-Version</code> is not supported for Wheezy and
	  later releases.
	</p><p>
	  The keyword <code class="literal">current</code> has been deprecated and must
	  not be used. It must be ignored for Python 3 versions.
	</p><p>
	  The use of <code class="literal">XB-Python-Version</code> in the binary package
	  paragraphs of <code class="filename">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></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="dependencies"></a>3.5. Dependencies</h2></div></div></div><p>
	  Any package that installs modules for the default Python version
	  (or many versions including the default) as described
	  in <a class="xref" href="module_packages.html#package_names" title="3.3. Module Package Names">Section 3.3, “Module Package Names”</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 <code class="literal">Depends:
	  python3 (&gt;= 3.<em class="replaceable"><code>Y</code></em>)</code> and any
	  corresponding <code class="literal">python3-<em class="replaceable"><code>foo</code></em></code> packages.
	</p><p>
	  If any Python 2 packages remain, the correct dependencies are <code class="literal">Depends:
	  python2 (&gt;= 2.<em class="replaceable"><code>Y</code></em>)</code> and any
	  corresponding <code class="literal">python2-<em class="replaceable"><code>foo</code></em></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. <code class="literal">Depends:
	  python3 (&lt;&lt; <em class="replaceable"><code>X</code></em>.<em class="replaceable"><code>Y</code></em>)</code>.
	</p></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="provides"></a>3.6. Provides</h2></div></div></div><p>
	  Binary packages that declare Provides dependencies of the form
	  <code class="literal">python<em class="replaceable"><code>X</code></em>.<em class="replaceable"><code>Y</code></em>-<em class="replaceable"><code>foo</code></em></code>
	  were never supported for Python 3. They should be removed in
	  the normal course of package updates. Future provision of
	  values for the substituation variable <code class="literal">python:Provides</code>
	  is not guaranteed.
	</p></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="byte_compilation"></a>3.7. Modules Byte-Compilation</h2></div></div></div><p>
	  If a binary package provides any binary-independent modules
	  (<code class="filename"><em class="replaceable"><code>foo</code></em>.py</code> files), the corresponding
	  byte-compiled modules (<code class="filename"><em class="replaceable"><code>foo</code></em>.pyc</code> files) and
	  optimized modules (<code class="filename"><em class="replaceable"><code>foo</code></em>.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 class="filename"><em class="replaceable"><code>foo</code></em>.pyc</code> and
	  <code class="filename"><em class="replaceable"><code>foo</code></em>.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 class="filename">/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 class="filename">*.rtupdate</code> script.
	</p></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="python.html">Prev</a> </td><td width="20%" align="center"> </td><td width="40%" align="right"> <a accesskey="n" href="programs.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Chapter 2. Python Packaging </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> Chapter 4. Python Programs</td></tr></table></div></body></html>