building.rst

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__ http://www.boost.orgdoc/html/bbv2/advanced.html#bbv2.advanced.configuration

.. Admonition:: Users of Unix-Variant OSes

   If you are using a unix-variant OS and you ran Boost's
   ``configure`` script, it may have generated a
   ``user-config.jam`` for you. [#overwrite]_ If your ``configure``\
   /\ ``make`` sequence was successful and Boost.Python binaries
   were built, your ``user-config.jam`` file is probably already
   correct.

If you have one fairly “standard” python installation for your
platform, you might not need to do anything special to describe it.  If
you haven't configured python in ``user-config.jam`` (and you don't
specify ``--without-python`` on the Boost.Build command line),
Boost.Build will automatically execute the equivalent of ::

  import toolset : using ; 
  using python ;

which automatically looks for Python in the most likely places.
However, that only happens when using the Boost.Python project file
(e.g. when referred to by another project as in the quickstart_
method).  If instead you are linking against separately-compiled
Boost.Python binaries, you should set up a ``user-config.jam`` file
with at least the minimal incantation above.

Python Configuration Parameters
-------------------------------

If you have several versions of Python installed, or Python is
installed in an unusual way, you may want to supply any or all of
the following optional parameters to ``using python``.

version
  the version of Python to use.  Should be in Major.Minor
  format, for example, ``2.3``.  Do not include the subminor
  version (i.e. *not* ``2.5.1``).  If you have multiple Python
  versions installed, the version will usually be the only
  configuration argument required.

cmd-or-prefix
  preferably, a command that invokes a Python interpreter.
  Alternatively, the installation prefix for Python libraries and
  header files.  Only use the alternative formulation if there is
  no appropriate Python executable available.

includes
  the ``#include`` paths for Python headers.  Normally the correct
  path(s) will be automatically deduced from ``version`` and/or
  ``cmd-or-prefix``.  

libraries
  the path to Python library binaries.  On MacOS/Darwin,
  you can also pass the path of the Python framework.  Normally the
  correct path(s) will be automatically deduced from ``version``
  and/or ``cmd-or-prefix``. 

condition
  if specified, should be a set of Boost.Build
  properties that are matched against the build configuration when
  Boost.Build selects a Python configuration to use.  See examples
  below for details.

extension-suffix
  A string to append to the name of extension
  modules before the true filename extension.  You almost certainly
  don't need to use this.  Usually this suffix is only used when
  targeting a Windows debug build of Python, and will be set
  automatically for you based on the value of the
  |python-debugging|_ feature.  However, at least one Linux
  distribution (Ubuntu Feisty Fawn) has a specially configured
  `python-dbg`__ package that claims to use such a suffix.

.. |python-debugging| replace:: ``<python-debugging>``

__ https://wiki.ubuntu.com/PyDbgBuilds


Examples
--------

Note that in the examples below, case and *especially whitespace* are
significant.

- If you have both python 2.5 and python 2.4 installed,
  ``user-config.jam`` might contain::
  
    using python : 2.5 ;  # Make both versions of Python available 

    using python : 2.4 ;  # To build with python 2.4, add python=2.4 
                          # to your command line.

  The first version configured (2.5) becomes the default.  To build
  against python 2.4, add ``python=2.4`` to the ``bjam`` command line.

- If you have python installed in an unusual location, you might
  supply the path to the interpreter in the ``cmd-or-prefix``
  parameter::

    using python : : /usr/local/python-2.6-beta/bin/python ;

- If you have a separate build of Python for use with a particular
  toolset, you might supply that toolset in the ``condition``
  parameter::

    using python ;  # use for most toolsets
    
    # Use with Intel C++ toolset
    using python 
         : # version
         : c:\\Devel\\Python-2.5-IntelBuild\\PCBuild\\python # cmd-or-prefix
         : # includes
         : # libraries
         : <toolset>intel # condition
         ;


- If you have downloaded the Python sources and built both the
  normal and the “\ `python debugging`_\ ” builds from source on
  Windows, you might see::

    using python : 2.5 : C:\\src\\Python-2.5\\PCBuild\\python ;
    using python : 2.5 : C:\\src\\Python-2.5\\PCBuild\\python_d
      : # includes
      : # libs
      : <python-debugging>on ;

- You can set up your user-config.jam so a bjam built under Windows 
  can build/test both Windows and Cygwin_ python extensions.  Just pass
  ``<target-os>cygwin`` in the ``condition`` parameter
  for the cygwin python installation::

    # windows installation
    using python ;

    # cygwin installation
    using python : : c:\\cygwin\\bin\\python2.5 : : : <target-os>cygwin ;

  when you put target-os=cygwin in your build request, it should build
  with the cygwin version of python: [#flavor]_

    bjam target-os=cygwin toolset=gcc 

  This is supposed to work the other way, too (targeting windows
  python with a Cygwin_ bjam) but it seems as though the support in
  Boost.Build's toolsets for building that way is broken at the
  time of this writing.

- Note that because of `the way Boost.Build currently selects target
  alternatives`__, you might have be very explicit in your build
  requests.  For example, given::

    using python : 2.5 ; # a regular windows build
    using python : 2.4 : : : : <target-os>cygwin ;

  building with ::

    bjam target-os=cygwin

  will yield an error.  Instead, you'll need to write::

    bjam target-os=cygwin/python=2.4

.. _Cygwin: http://cygwin.com

__ http://zigzag.cs.msu.su/boost.build/wiki/AlternativeSelection

Choosing a Boost.Python Library Binary
======================================

If—instead of letting Boost.Build construct and link with the right
libraries automatically—you choose to use a pre-built Boost.Python
library, you'll need to think about which one to link with.  The
Boost.Python binary comes in both static and dynamic flavors.  Take
care to choose the right flavor for your application. [#naming]_

The Dynamic Binary
------------------

The dynamic library is the safest and most-versatile choice:

- A single copy of the library code is used by all extension
  modules built with a given toolset. [#toolset-specific]_

- The library contains a type conversion registry.  Because one
  registry is shared among all extension modules, instances of a
  class exposed to Python in one dynamically-loaded extension
  module can be passed to functions exposed in another such module.

The Static Binary
-----------------

It might be appropriate to use the static Boost.Python library in
any of the following cases:

- You are extending_ python and the types exposed in your
  dynamically-loaded extension module don't need to be used by any
  other Boost.Python extension modules, and you don't care if the
  core library code is duplicated among them.

- You are embedding_ python in your application and either:

  - You are targeting a Unix variant OS other than MacOS or AIX,
    where the dynamically-loaded extension modules can “see” the
    Boost.Python library symbols that are part of the executable.

  - Or, you have statically linked some Boost.Python extension
    modules into your application and you don't care if any
    dynamically-loaded Boost.Python extension modules are able to
    use the types exposed by your statically-linked extension
    modules (and vice-versa).

``#include`` Issues
===================

1. If you should ever have occasion to ``#include "python.h"``
   directly in a translation unit of a program using Boost.Python,
   use ``#include "boost/python/detail/wrap_python.hpp"`` instead.
   It handles several issues necessary for use with Boost.Python,
   one of which is mentioned in the next section.

2. Be sure not to ``#include`` any system headers before
   ``wrap_python.hpp``.  This restriction is actually imposed by
   Python, or more properly, by Python's interaction with your
   operating system.  See
   http://docs.python.org/ext/simpleExample.html for details.

.. _python-debugging:
.. _python debugging:

Python Debugging Builds
=======================

Python can be built in a special “python debugging” configuration
that adds extra checks and instrumentation that can be very useful
for developers of extension modules.  The data structures used by
the debugging configuration contain additional members, so **a
Python executable built with python debugging enabled cannot be
used with an extension module or library compiled without it, and
vice-versa.**

Since pre-built “python debugging” versions of the Python
executable and libraries are not supplied with most distributions
of Python, [#get-debug-build]_ and we didn't want to force our users
to build them, Boost.Build does not automatically enable python
debugging in its ``debug`` build variant (which is the default).
Instead there is a special build property called
``python-debugging`` that, when used as a build property, will
define the right preprocessor symbols and select the right
libraries to link with.

On unix-variant platforms, the debugging versions of Python's data
structures will only be used if the symbol ``Py_DEBUG`` is defined.
On many windows compilers, when extension modules are built with
the preprocessor symbol ``_DEBUG``, Python defaults to force
linking with a special debugging version of the Python DLL.  Since
that symbol is very commonly used even when Python is not present,
Boost.Python temporarily undefines _DEBUG when Python.h
is #included from ``boost/python/detail/wrap_python.hpp`` - unless
``BOOST_DEBUG_PYTHON`` is defined.  The upshot is that if you want
“python debugging”and you aren't using Boost.Build, you should make
sure ``BOOST_DEBUG_PYTHON`` is defined, or python debugging will be
suppressed.

Testing Boost.Python
====================

To run the full test suite for Boost.Python, invoke ``bjam`` in the
``libs/python/test`` subdirectory of your Boost distribution.

Notes for MinGW (and Cygwin with -mno-cygwin) GCC Users
=======================================================

If you are using a version of Python prior to 2.4.1 with a MinGW
prior to 3.0.0 (with binutils-2.13.90-20030111-1), you will need to
create a MinGW-compatible version of the Python library; the one
shipped with Python will only work with a Microsoft-compatible
linker. Follow the instructions in the “Non-Microsoft” section of
the “Building Extensions: Tips And Tricks” chapter in `Installing
Python Modules`__ to create ``libpythonXX.a``, where ``XX``
corresponds to the major and minor version numbers of your Python
installation.

__ http://www.python.org/doc/current/inst/index.html

-----------------------------

.. [#2.2] Note that although we tested earlier versions of
   Boost.Python with Python 2.2, and we don't *think* we've done
   anything to break compatibility, this release of Boost.Python
   may not have been tested with versions of Python earlier than
   2.4, so we're not 100% sure that python 2.2 and 2.3 are
   supported.

.. [#naming] Information about how to identify the
   static and dynamic builds of Boost.Python:

   * `on Windows`__
   * `on Unix variants`__

   __ ../../../more/getting_started/windows.html#library-naming
   __ ../../../more/getting_started/unix-variants.html#library-naming

.. [#toolset-specific] Because of the way most \*nix platforms
   share symbols among dynamically-loaded objects, I'm not certain
   that extension modules built with different compiler toolsets
   will always use different copies of the Boost.Python library
   when loaded into the same Python instance.  Not using different
   libraries could be a good thing if the compilers have compatible
   ABIs, because extension modules built with the two libraries
   would be interoperable.  Otherwise, it could spell disaster,
   since an extension module and the Boost.Python library would
   have different ideas of such things as class layout. I would
   appreciate someone doing the experiment to find out what
   happens.

.. [#overwrite] ``configure`` overwrites the existing
   ``user-config.jam`` in your home directory
   (if any) after making a backup of the old version.

.. [#flavor] Note that the ``<target-os>cygwin`` feature is
   different from the ``<flavor>cygwin`` subfeature of the ``gcc``
   toolset, and you might need handle both explicitly if you also
   have a MinGW GCC installed.

.. [#home-dir] Windows users, your home directory can be
   found by typing::

     ECHO %HOMEDRIVE%%HOMEPATH%

   into a `command prompt`_ window.

.. [#get-debug-build] On Unix and similar platforms, a debugging
   python and associated libraries are built by adding
   ``--with-pydebug`` when configuring the Python build. On
   Windows, the debugging version of Python is generated by
   the "Win32 Debug" target of the Visual Studio project in the
   PCBuild subdirectory of a full Python source code distribution.