// -*- mode:doc; -*-
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=== Infrastructure for Python packages

This infrastructure applies to Python packages that use the standard
Python setuptools, pep517, flit or maturin mechanisms as their build
system, generally recognizable by the usage of a +setup.py+ script or
+pyproject.toml+ file.

[[python-package-tutorial]]

==== +python-package+ tutorial

First, let's see how to write a +.mk+ file for a Python package,
with an example :

----
01: ################################################################################
02: #
03: # python-foo
04: #
05: ################################################################################
06:
07: PYTHON_FOO_VERSION = 1.0
08: PYTHON_FOO_SOURCE = python-foo-$(PYTHON_FOO_VERSION).tar.xz
09: PYTHON_FOO_SITE = http://www.foosoftware.org/download
10: PYTHON_FOO_LICENSE = BSD-3-Clause
11: PYTHON_FOO_LICENSE_FILES = LICENSE
12: PYTHON_FOO_ENV = SOME_VAR=1
13: PYTHON_FOO_DEPENDENCIES = libmad
14: PYTHON_FOO_SETUP_TYPE = setuptools
15:
16: $(eval $(python-package))
----

On line 7, we declare the version of the package.

On line 8 and 9, we declare the name of the tarball (xz-ed tarball
recommended) and the location of the tarball on the Web. Buildroot
will automatically download the tarball from this location.

On line 10 and 11, we give licensing details about the package (its
license on line 10, and the file containing the license text on line
11).

On line 12, we tell Buildroot to pass custom options to the Python
+setup.py+ script when it is configuring the package.

On line 13, we declare our dependencies, so that they are built
before the build process of our package starts.

On line 14, we declare the specific Python build system being used. In
this case the +setuptools+ Python build system is used. The five
supported ones are +flit+, +pep517+, +setuptools+, +setuptools-rust+
and +maturin+.

Finally, on line 16, we invoke the +python-package+ macro that
generates all the Makefile rules that actually allow the package to be
built.

[[python-package-reference]]

==== +python-package+ reference

As a policy, packages that merely provide Python modules should all be
named +python-<something>+ in Buildroot. Other packages that use the
Python build system, but are not Python modules, can freely choose
their name (existing examples in Buildroot are +scons+ and
+supervisor+).

The main macro of the Python package infrastructure is
+python-package+. It is similar to the +generic-package+ macro. It is
also possible to create Python host packages with the
+host-python-package+ macro.

Just like the generic infrastructure, the Python infrastructure works
by defining a number of variables before calling the +python-package+
or +host-python-package+ macros.

All the package metadata information variables that exist in the
xref:generic-package-reference[generic package infrastructure] also
exist in the Python infrastructure.

Note that:

 * It is not necessary to add +python+ or +host-python+ in the
   +PYTHON_FOO_DEPENDENCIES+ variable of a package, since these basic
   dependencies are automatically added as needed by the Python
   package infrastructure.

 * Similarly, it is not needed to add +host-python-setuptools+ to
   +PYTHON_FOO_DEPENDENCIES+ for setuptools-based packages, since it's
   automatically added by the Python infrastructure as needed.

One variable specific to the Python infrastructure is mandatory:

* +PYTHON_FOO_SETUP_TYPE+, to define which Python build system is used
  by the package. The five supported values are +flit+, +pep517+ and
  +setuptools+, +setuptools-rust+ and +maturin+. If you don't know
  which one is used in your package, look at the +setup.py+ or
  +pyproject.toml+ file in your package source code, and see whether
  it imports things from the +flit+ module or the +setuptools+
  module. If the package is using a +pyproject.toml+ file without any
  build-system requires and with a local in-tree backend-path one
  should use +pep517+.

A few additional variables, specific to the Python infrastructure, can
optionally be defined, depending on the package's needs. Many of them
are only useful in very specific cases, typical packages will
therefore only use a few of them, or none.

* +PYTHON_FOO_SUBDIR+ may contain the name of a subdirectory inside the
  package that contains the main +setup.py+ or +pyproject.toml+ file.
  This is useful, if for example, the main +setup.py+ or +pyproject.toml+
  file is not at the root of the tree extracted by the tarball. If
  +HOST_PYTHON_FOO_SUBDIR+ is not specified, it defaults to
  +PYTHON_FOO_SUBDIR+.

* +PYTHON_FOO_ENV+, to specify additional environment variables to
  pass to the Python +setup.py+ script (for setuptools packages) or
  the +support/scripts/pyinstaller.py+ script (for flit/pep517
  packages) for both the build and install steps. Note that the
  infrastructure is automatically passing several standard variables,
  defined in +PKG_PYTHON_SETUPTOOLS_ENV+ (for setuptools target
  packages), +HOST_PKG_PYTHON_SETUPTOOLS_ENV+ (for setuptools host
  packages), +PKG_PYTHON_PEP517_ENV+ (for flit/pep517 target packages)
  and +HOST_PKG_PYTHON_PEP517_ENV+ (for flit/pep517 host packages).

* +PYTHON_FOO_BUILD_OPTS+, to specify additional options to pass to
  the Python +setup.py+ script during the build step, this generally
  only makes sense to use for setuptools based packages as flit/pep517
  based packages do not pass these options to a +setup.py+ script but
  instead pass them to +support/scripts/pyinstaller.py+.

* +PYTHON_FOO_INSTALL_TARGET_OPTS+, +PYTHON_FOO_INSTALL_STAGING_OPTS+,
  +HOST_PYTHON_FOO_INSTALL_OPTS+ to specify additional options to pass
  to the Python +setup.py+ script (for setuptools packages) or
  +support/scripts/pyinstaller.py+ (for flit/pep517 packages) during
  the target installation step, the staging installation step or the
  host installation, respectively.

With the Python infrastructure, all the steps required to build and
install the packages are already defined, and they generally work well
for most Python-based packages. However, when required, it is still
possible to customize what is done in any particular step:

* By adding a post-operation hook (after extract, patch, configure,
  build or install). See xref:hooks[] for details.

* By overriding one of the steps. For example, even if the Python
  infrastructure is used, if the package +.mk+ file defines its own
  +PYTHON_FOO_BUILD_CMDS+ variable, it will be used instead of the
  default Python one. However, using this method should be restricted
  to very specific cases. Do not use it in the general case.

[[scanpypi]]

==== Generating a +python-package+ from a PyPI repository

If the Python package for which you would like to create a Buildroot
package is available on PyPI, you may want to use the +scanpypi+ tool
located in +utils/+ to automate the process.

You can find the list of existing PyPI packages
https://pypi.python.org[here].

+scanpypi+ requires Python's +setuptools+ package to be installed on
your host.

When at the root of your buildroot directory just do :

----
utils/scanpypi foo bar -o package
----

This will generate packages +python-foo+ and +python-bar+ in the package
folder if they exist on https://pypi.python.org.

Find the +external python modules+ menu and insert your package inside.
Keep in mind that the items inside a menu should be in alphabetical order.

Please keep in mind that you'll most likely have to manually check the
package for any mistakes as there are things that cannot be guessed by
the generator (e.g.  dependencies on any of the python core modules
such as BR2_PACKAGE_PYTHON_ZLIB).  Also, please take note that the
license and license files are guessed and must be checked. You also
need to manually add the package to the +package/Config.in+ file.

If your Buildroot package is not in the official Buildroot tree but in
a br2-external tree, use the -o flag as follows:

----
utils/scanpypi foo bar -o other_package_dir
----

This will generate packages +python-foo+ and +python-bar+ in the
+other_package_directory+ instead of +package+.

Option +-h+ will list the available options:

----
utils/scanpypi -h
----

[[python-package-cffi-backend]]

==== +python-package+ CFFI backend

C Foreign Function Interface for Python (CFFI) provides a convenient
and reliable way to call compiled C code from Python using interface
declarations written in C. Python packages relying on this backend can
be identified by the appearance of a +cffi+ dependency in the
+install_requires+ field of their +setup.py+ file.

Such a package should:

 * add +python-cffi+ as a runtime dependency in order to install the
compiled C library wrapper on the target. This is achieved by adding
+select BR2_PACKAGE_PYTHON_CFFI+ to the package +Config.in+.

----
config BR2_PACKAGE_PYTHON_FOO
        bool "python-foo"
        select BR2_PACKAGE_PYTHON_CFFI # runtime
----

 * add +host-python-cffi+ as a build-time dependency in order to
cross-compile the C wrapper. This is achieved by adding
+host-python-cffi+ to the +PYTHON_FOO_DEPENDENCIES+ variable.

----
################################################################################
#
# python-foo
#
################################################################################

...

PYTHON_FOO_DEPENDENCIES = host-python-cffi

$(eval $(python-package))
----