# Abstract

This document describes the packaging of Python within the Debian GNU/Linux distribution and the policy requirements for packaged Python programs and modules.

# Contents

## 1. Copyright

1999-2021, Software in the Public Interest

Authors:

  - Neil Schemenauer <nas@debian.org>

  - Matthias Klose <doko@debian.org>

  - Gregor Hoffleit <flight@debian.org>

  - Josselin Mouette <joss@debian.org>

  - Joe Wreschnig <piman@debian.org>

  - Loïc Minier <lool@debian.org>

  - Scott Kitterman <scott@kitterman.com>

  - Barry Warsaw <barry@debian.org>

  - Ben Finney <ben+debian@benfinney.id.au>

  - Neil Williams <codehelp@debian.org>

  - Stefano Rivera <stefanor@debian.org>

This manual is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version.

This is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

A copy of the GNU General Public License version 2 is available as `/usr/share/common-licences/GPL-2` in the Debian GNU/Linux system, or on the World Wide Web at GNU General Public License, version 2.

You can also obtain it by writing to the Free Software Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301, USA.

## 2. Completing the move to Python 3

Debian has previously supported two Python stacks, one for Python 3 and one for Python 2. The goal for Debian is to reduce this to one stack, dropping the Python 2 stack and interpreter for the Bullseye release.

PEP 404 states that no more major Python 2 releases are planned, although the latest released minor version 2.7 will see some extended support, documented in PEP 466.

Packages in Debian should use Python 3. New packages must use Python 3 from the initial upload, new upstream versions for existing packages must use Python 3. If Python 2 is still supported in Bullseye, selected packages may continue using Python 2 until Python 3 support is available for those packages. Please discuss all use of Python 2 on the debian-python mailing list before uploading.

1. Applications should use Python 3 and must not be packaged for Python 2 as well. If an application supports only Python 2, the application may need to be removed from Debian so that it does not block removal of other Python 2 packages.

2. Python libraries need to support Python 3 and new versions must be packaged for Python 3. Existing Python 2 libraries must not be dropped before the last reverse dependency is removed. New Python 2 libraries must not be introduced.

3. Python 3 should be used for the packaging if the packaging scripts use Python.

### 2.1. Removal of the unversioned packages

Starting with the Debian 11 release (bullseye), the binary packages `python`, `python-minimal`, `python-dev`, `python-dbg` and `python-doc` are removed. No package in the archive must use any of these packages as build dependencies, dependencies, recommendations or suggestions.

### 2.2. Unversioned python commands

For the Debian 11 release (bullseye), the `/usr/bin/python` command is provided in the `python-is-python2` package (pointing to `/usr/bin/python2`). The `/usr/bin/python-config` and `/usr/bin/pydoc` commands are provided in the `python-dev-is-python2` package. These package are not installed by default for new installations, but only for upgrades from the Debian 10 release (buster). These packages should be removed after an upgrade. These packages will not be part of the Debian 12 release (bookworm).

The packages `python-is-python3` and `python-dev-is-python3` provide the `/usr/bin/python`, `/usr/bin/python-config` and `/usr/bin/pydoc` commands pointing to Python3. These packages can be installed by developers and users to use the unversioned commands. NOTE: Locally installed software not yet ported to Python3 is likely to break when installing these packages.

The packages `python-is-python3`, `python-dev-is-python3`, `python-is-python2` and `python-dev-is-python2` must not be used as build dependencies, dependencies, recommendations or suggestions.

## 3. Python Packaging

### 3.1. Versions

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, for as long as it exists. 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 `pythonX.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`.

### 3.2. Main packages

For every Python version provided in Debian, the binary package `pythonX.Y` shall provide a complete distribution for deployment of Python scripts and applications. The package must ensure that the binary `/usr/bin/pythonX.Y` is provided.

Installation of `pythonX.Y` shall provide the modules of the upstream Python distribution with some exceptions.

Excluded are modules that cannot be included for licensing reasons, 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 and the `venv` module which depends on wheels to bootstrap pip). Modules that would interfere with system package management (for example `ensurepip`, when used outside virtual environments) are modified to print a message explaining the problem and recommending alternatives.

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 `pythonX.Y-dev`.

Modules only used for building of Python modules (e.g. `distutils` and `lib2to3`) are split into separate packages. The `python3-venv` binary package depends on these.

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.Y` executable. The package must depend on the `python3.Y` package that installs the executable.

A `python3-full` binary package must ensure that the entire Python standard library is available, including all modules split into separate packages (but excluding modules excluded from Debian for licensing reasons). This package exists for the convenience of python developers, and must not be used in dependencies, recommendations and build dependencies by python module or application packages.

The version of the `python3` package must be greater than or equal to `3.Y` and lower than `3.Y+1`.

The `python` and `python-dbg` binary packages are to be removed for Bullseye. If any Python 2 packages remain in Bullseye, these must depend on `python2` or `python2-dbg`. The `python2` package must depend on the `python2.Y` package that installs the executable `/usr/bin/python2`. The version of the `python2` package must be greater than or equal to `2.Y` and lower than `2.Y+1`. The `python2` must not provide `/usr/bin/python`.

For as long as it remains supported, the `python` binary package must ensure that `/usr/bin/python2` is provided, as a symlink to the current `python2.Y` executable. The package must depend on the `python2.Y` package that installs the executable.

The `python` binary package must also ensure that `/usr/bin/python` is provided, as a symlink to the current `python2.Y` executable. See PEP 394 for details.

The version of the `python` package must be greater than or equal to `2.Y` and lower than `2.Y+1`.

### 3.3. Virtual packages for Python-version specific dependencies

The python3 package provides python3-supported-min (= {X}.{Y}) and python3-supported-max (= {X}.{Y}) virtual packages. This allows packages to declare dependencies of the form `python3-foo | python3-supported-min (>= 3.7)` or `python3-foo | python3-supported-max (<= 3.7)`, which would install python3-foo if any supported python3 version requires it.

### 3.4. Minimal packages

For every Python version provided in Debian, the binary package `pythonX.Y-minimal` might exist and should not be depended upon by other packages except the Python runtime packages themselves.

### 3.5. Python Interpreter

#### 3.5.1. Interpreter Name

The different Python major versions require different interpreters (see 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 for as long as this remains supported.

Python scripts should not specify `python` as the interpreter name even if they do not require any particular version of Python as the script would stop working upon removal of the Python 2 stack.

Python scripts that only work with a specific Python minor version must explicitly use the versioned interpreter name (`pythonX.Y`).

#### 3.5.2. Interpreter Location

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.Y` if it requires Python `3.Y`).

The preferred specification for the Python 2 interpreter is `/usr/bin/python2` (or `/usr/bin/python2.Y` if it requires Python `2.Y`).

Scripts requiring the default Python 2 version must not specify the interpreter `/usr/bin/python` as such scripts will fail when the unversioned interpreter binary `/usr/bin/python` is removed.

Maintainers should not override the Debian Python interpreter using `/usr/bin/env name`. This is not advisable as it bypasses Debian’s dependency checking and makes the package vulnerable to incomplete local installations of Python.

### 3.6. Module Path

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/pythonXY.zip` entry.

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.Y/dist-packages`, where `2.Y` is the Python 2 version.

A special directory is dedicated to public Python modules installed by the local administrator, `/usr/local/lib/python3/dist-packages` for all Python 3 versions, `/usr/local/lib/python2.Y/dist-packages` for Python 2.

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.Y/site-packages` is in the Python `2.Y` 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.

### 3.7. Hooks for updates to installed runtimes

The `python3` 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.

1. `/usr/share/python3/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 `pythonX.Y`) and the third and fourth argument are the old and new version of this packaged runtime if this runtime was already installed but unsupported.

2. `/usr/share/python3/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 `pythonX.Y`).

3. `/usr/share/python3/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 `pythonX.Y`), and the third argument is the new default runtime (for example `pythonX.Z`).

### 3.8. Documentation

Python documentation is split out in separate binary packages `pythonX.Y-doc`.

The binary package `python3-doc` will always provide the documentation for the default Debian Python 3 version. The binary package `python2-doc` will always provide the documentation for the default Debian Python 2 version, for as long as that remains supported.

TODO: Policy for documentation of third party packages.

## 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.

## 5. Python Programs

### 5.1. Interpreter directive (“Shebang”)

Executables written for interpretation by Python must use an appropriate interpreter directive, or “shebang”, as the first line of the program. This line should be of the form `#!interpreter_location`. See Interpreter Name for the interpreter name to use.

As noted in Interpreter Location, the form `#!/usr/bin/env interpreter_name` is deprecated.

### 5.2. Programs using the default Python

A package that installs a program that can be run by any version of Python 3 must declare a dependency on `python3`, with a versioned dependency if necessary.

A package that installs a program that can be run by any version of Python 2 must declare a dependency on `python2`, with a versioned dependency if necessary.

If the program needs the public Python module `foo`, the package must depend on the binary package that installs the `foo` module. See Module Package Names for the naming of packages that install public Python modules.

### 5.3. Programs Shipping Private Modules

A program that specifies `python3` as its interpreter may require its own private Python modules. These modules should be installed in `/usr/share/module`, or `/usr/lib/module` if the modules are architecture-dependent (e.g. extensions).

The rules explained in Modules Byte-Compilation apply to those private modules: the byte-compiled modules must not be shipped with the binary package, they should be generated in the package’s post-install script using the current default Python version, and removed in the pre-remove script. Modules should be byte-compiled using the current default Python version.

Programs that have private compiled extensions must either handle multiple version support themselves, or declare a tight dependency on the current Python version (e.g. `Depends: python3 (>= 3.5), python3 (<< 3.6)`.

### 5.4. Programs Using a Particular Python Version

A program which requires a specific minor version of Python must specify the versioned interpreter `pythonX.Y`. The package that installs the programs must also specify a dependency on `pythonX.Y` and on any packages that install necessary modules.

The notes on installation directories and byte-compilation for programs that support any version of Python also apply to programs supporting only a single Python version. Modules to be byte-compiled should use the same Python version as the package itself.

## 6. Programs Embedding Python

### 6.1. Building Embedded Programs

Any package that installs a program which embeds a Python interpreter must declare `Build-Depends` on `pythonX.Y-dev`, where `X.Y` is the Python version the program builds against. It should be the current default Python version unless the program does not work correctly with this version.

### 6.2. Embedded Python Dependencies

Dependencies for programs linking against the shared Python library will be automatically created by `dpkg-shlibdeps`. The `libpythonX.Y.so.Z` library the program is built against is provided by the `pythonX.Y` package.

## 7. Interaction with Locally Installed Python Versions

As long as you don’t install other versions of Python in your path, Debian’s Python versions won’t be affected by a new version.

If you install a different micro version of the version of Python you have got installed, you will need to be careful to install all the modules you use for that version of Python too.

## 1. Build Dependencies

Build dependencies for Python-dependent packages must be declared for every Python version that the package is built for.

The `python3-all-dev` should be used when building extensions for any or all Python 3 versions. The `python-all-dev` should be used when building extensions for any or all Python 2 versions. To build for a specific version or versions, declare `Build-Depends` on `pythonX.Y-dev`.

Some applications and pure Python modules may be able to avoid dependency on the `-dev` packages, and declare `Build-Depends` on the runtime environment only (`python3`, `python3-all`, `python2`, `python2-all`). A package that does not require the `-dev` packages must not declare `Build-Depends` on them.

Declare `Build-Depends` on at least:

    Build-Depends: python2.7
    Build-Depends: python2.6 (>= 2.6-1)
    Build-Depends: python (>= 2.6.6-9)
    Build-Depends: python-all

    Build-Depends: python2.7-dev
    Build-Depends: python3.5-dev (>= 3.5.1-1)
    Build-Depends: python-dev (>= 2.6.6-9)
    Build-Depends: python-all-dev
    Build-Depends: python3-all-dev (>= 3.2)

## 2. Packaging Tools

This section describes the various tools to help package Python programs and modules for Debian. Although none of these tools are mandatory, their use is strongly encouraged, as the above policy has been designed with them in mind (and vice versa). This appendix is just an overview. If you use these tools, you should read their full documentation.

### 2.1. distutils

The standard Python `distutils` module has been modified in Debian to change the default installation directory of public Python modules and to add a new flag to the `install` command to override the default, `--install-layout=`.

Public Python modules installed with a modified distutils default to `/usr/local/lib/pythonX.Y/dist-packages` for Python 2.6 and later. This directory is seen by the system-provided Python 2.6.

When using a local Python installation, the default is `/usr/local/lib/pythonX.Y/site-packages` which is only seen by the local Python installation.

Using the `--install-layout=deb` flag to the `install` command of `setup.py` with a system-provided Python 2.6 or later versions, Python modules will be installed to `/usr/lib/pythonX.Y/dist-packages` which is only seen by the system-provided Python, not by a local installation.

### 2.2. setuptools

The related Python `setuptools` module has been modified in Debian along the same lines as `distutils`.

Upstream focus on developments and improvements for Python packaging tools has largely shifted away from `distutils` and to `setuptools`. They offer a similar API and at some point in the future, `setuptools` may fully replace `distutils` in Debian package builds.

### 2.3. dh-python

`dh-python` provides extensions for `debhelper` to make it easier to package Python modules and extensions. They calculate Python dependencies, add maintainer scripts to byte compile files, etc. Their use is not mandatory, but they are recommended by the Debian Python maintainers. See man dh_python3 for details.

### 2.4. pybuild

Pybuild is a Debian Python specific build system that invokes various build systems for requested Python versions in order to build modules and extensions. It supports automatically building for multiple Python versions.

### 2.5. CDBS

The CDBS `python-distutils.mk` class helps packaging of distutils based Python packages.

### 2.6. python-support (removed)

`python-support` provided another way to manage Python modules. It has been removed from Debian Stretch and later releases.

### 2.7. python-central (removed)

`python-central` provided another way to manage Python modules. It has been removed from Debian Jessie and later releases.

## 3. Upgrade Procedure

This section describes the procedure for the upgrade when the default Python version is changed in the Debian `unstable` release, requiring recompilation of many Python-related packages.

1. Selected pre-releases and release candidates of new Python versions are uploaded to Debian `experimental` to support pre-transition work and testing.

2. Application and module maintainers make sourceful changes where needed to prepare for the new Python version when needed.

3. Have a long and heated discussion.

4. The Debian Python maintainer and module/application maintainers discuss the readiness for a new default Debian Python version and associated packaging/policy changes. Once there is some consensus, the Python maintainer announces the upgrade and uploads to `unstable`.

5. Upload of the Python core meta-packages `python3`, `python3-dev`, `python3-doc` and several `python3-module`, depending on the new `python3.Y`, `python3.Y-dev` and so on.

6. The Debian release team schedules rebuilds for packages that may need it. Packages that require additional manual work get updated and uploaded.

The necessary package builds are typically done in three phases in order to keep transitions as smooth as possible. For Python 3, there is no general need to update architecture all packages for a new Python 3 version. Only architecture any packages need to be rebuilt.

1. The new Python 3 version is added to supported versions and packages that support multiple Python 3 versions are binNMUed. They now support both the new and older Python 3 versions. This requires transition assistance from the release team in the form of a transition tracker and binNMU scheduling, but is not a transition that can cause entanglements with other transitions in Debian.

2. Once the default Python 3 version is changed, binNMUs are done for packages that only support one Python 3 version. Some transient uninstallability is unavoidable. This is a transition that can entangle other transitions in Debian and requires more careful coordination with the release team.

3. After the old Python 3 version is dropped from supported versions then packages with multi-version support are binNMUed again to remove support for the old Python 3 version. This is not a true transition and only needs a tracker and binNMU scheduling.

## 4. This document

Source of this document is `policy/` in git repository python3-defaults.

Propose changes to this policy on the debian-python mailing list for review.

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