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

This infrastructure applies to Python packages that use the standard
Python setuptools mechanism as their build system, generally
recognizable by the usage of a +setup.py+ script.

[[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-3c
11: PYTHON_FOO_LICENSE_FILES = LICENSE
12: PYTHON_FOO_ENV = SOME_VAR=1
13: PYTHON_FOO_DEPENDENCIES = libmad
14: PYTHON_FOO_SETUP_TYPE = distutils
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 +distutils+ Python build system is used. The two
supported ones are +distutils+ and +setuptools+.

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+).

In their +Config.in+ file, they should depend on +BR2_PACKAGE_PYTHON+
so that when Buildroot will enable Python 3 usage for modules, we will
be able to enable Python modules progressively on Python 3.

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: +PYTHON_FOO_VERSION+,
+PYTHON_FOO_SOURCE+, +PYTHON_FOO_PATCH+, +PYTHON_FOO_SITE+,
+PYTHON_FOO_SUBDIR+, +PYTHON_FOO_DEPENDENCIES+, +PYTHON_FOO_LICENSE+,
+PYTHON_FOO_LICENSE_FILES+, +PYTHON_FOO_INSTALL_STAGING+, etc.

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-setuptools+ and/or
   +host-distutilscross+ dependencies to +PYTHON_FOO_DEPENDENCIES+ for
   setuptools-based packages, since these are 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 two supported values are +distutils+ and
  +setuptools+. If you don't know which one is used in your package,
  look at the +setup.py+ file in your package source code, and see
  whether it imports things from the +distutils+ module or the
  +setuptools+ module.

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_ENV+, to specify additional environment variables to
  pass to the Python +setup.py+ script (for both the build and install
  steps). Note that the infrastructure is automatically passing
  several standard variables, defined in +PKG_PYTHON_DISTUTILS_ENV+
  (for distutils target packages), +HOST_PKG_PYTHON_DISTUTILS_ENV+
  (for distutils host packages), +PKG_PYTHON_SETUPTOOLS_ENV+ (for
  setuptools target packages) and +HOST_PKG_PYTHON_SETUPTOOLS_ENV+
  (for setuptools host packages).

* +PYTHON_FOO_BUILD_OPTS+, to specify additional options to pass to the
  Python +setup.py+ script during the build step. For target distutils
  packages, the +PKG_PYTHON_DISTUTILS_BUILD_OPTS+ options are already
  passed automatically by the infrastructure.

* +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 during the target installation step,
  the staging installation step or the host installation,
  respectively. Note that the infrastructure is automatically passing
  some options, defined in +PKG_PYTHON_DISTUTILS_INSTALL_TARGET_OPTS+
  or +PKG_PYTHON_DISTUTILS_INSTALL_STAGING_OPTS+ (for target distutils
  packages), +HOST_PKG_PYTHON_DISTUTILS_INSTALL_OPTS+ (for host
  distutils packages), +PKG_PYTHON_SETUPTOOLS_INSTALL_TARGET_OPTS+ or
  +PKG_PYTHON_SETUPTOOLS_INSTALL_STAGING_OPTS+ (for target setuptools
  packages) and +HOST_PKG_PYTHON_SETUPTOOLS_INSTALL_OPTS+ (for host
  setuptools packages).

* +HOST_PYTHON_FOO_NEEDS_HOST_PYTHON+, to define the host python
  interpreter. The usage of this variable is limited to host
  packages. The two supported value are +python2+ and +python3+. It
  will ensure the right host python package is available and will
  invoke it for the build. If some build steps are overloaded, the
  right python interpreter must be explicitly called in the commands.

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 +support/scripts+ 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 :

-----------------------
./support/script/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:

-----------------------
./support/script/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:

-----------------------
./support/script/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))
------------------------