###  | # Boost.Python ## Building and Testing

## Contents

Requirements
Building Boost.Python
  Configuration
  Results
  Notes for Cygwin GCC Users
  Testing
Building your Extension Module
  The Easy Way
  Building your module outside the Boost project tree
Build Variants
Building Using the Microsoft Visual Studio IDE

## Requirements

Boost.Python version 2 requires Python 2.2 or newer. An unsupported archive of Boost.Python version 1, which works with versions of Python since 1.5.2, is available here.

## Building Boost.Python

Normally, Boost.Python extension modules must be linked with the `boost_python` shared library. In special circumstances you may want to link to a static version of the `boost_python` library, but if multiple Boost.Pythone extension modules are used together, it will prevent sharing of types across extension modules, and consume extra code space. To build `boost_python`, use Boost.Build in the usual way from the `libs/python/build` subdirectory of your boost installation (if you have already built boost from the top level this may have no effect, since the work is already done).

### Configuration

You may need to configure the following variables to point Boost.Build at your Python installation:

Variable Name | Semantics | Default | Notes
--- | --- | --- | ---
`PYTHON_ROOT` | The root directory of your Python installation | Windows: `c:/tools/python` Unix: `/usr/local` | On Unix, this is the `--with-prefix=` directory used to configure Python
`PYTHON_VERSION` | The The 2-part python Major.Minor version number | `2.2` | Be sure not to include a third number, e.g. not "`2.2.1`", even if that's the version you have.
`PYTHON_INCLUDES` | path to Python `#include` directories | Autoconfigured from `PYTHON_ROOT`
`PYTHON_LIB_PATH` | path to Python library object. | Autoconfigured from `PYTHON_ROOT`
`CYGWIN_PYTHON_[DEBUG_]VERSION` | The version of python being used under Cygwin. | $(PYTHON_VERSION) | Use only when building with Cygwin GCC. This and the following settings are useful when building with multiple toolsets on Windows, since Cygwin GCC requires a different build of Python.
`CYGWIN_PYTHON_[DEBUG_]ROOT` | unix-style path containing the `include/` directory containing `python$(CYGWIN_PYTHON_[DEBUG_]VERSION)/python.h`. | $(PYTHON_ROOT) | Use only when building with Cygwin GCC.
`CYGWIN_PYTHON_[DEBUG_]LIB_PATH` | path containing the user's Cygwin Python import lib `libpython$(CYGWIN_PYTHON_[DEBUG_]VERSION).dll.a` | Autoconfigured from `CYGWIN_PYTHON_ROOT` | Use only when building with Cygwin GCC.
`CYGWIN_PYTHON_[DEBUG_]DLL_PATH` | path containing the user's Cygwin Python dll (`libpython$(CYGWIN_PYTHON_[DEBUG_]VERSION).dll`) | `/bin` | Use only when building with Cygwin GCC.

### Notes for Cygwin GCC Users

If you are using Cygwin GCC to build extension modules, you must use a Cygwin build of Python. The regular Win32 Python installation that you can download from python.org will not work with your compiler because the dynamic linking conventions are different (you can use MinGW GCC if you want to build extension modules which are compatible with a stock Win32 Python). The Cygwin installer may be able to install an appropriate version of Python, or you can follow the traditional Unix installation process to build Python from source.

The special build configuration variables listed above as "Cygwin only" make it possible to use a regular Win32 build of bjam to build and test Boost.Python and Boost.Python extensions using Cygwin GCC and targeting a Cygwin build of Python.

### Results

The build process will create a `libs/python/build/bin-stage` subdirectory of the boost root (or of `$(ALL_LOCATE_TARGET)`, if you have set that variable), containing the built libraries. The libraries are actually built to unique directories for each toolset and variant elsewhere in the filesystem, and copied to the `bin-stage` directory as a convenience, so if you build with multiple toolsets at once, the product of later toolsets will overwrite that of earlier toolsets in `bin-stage`.

### Testing

To build and test Boost.Python, start from the `libs/python/test` directory and invoke

>     bjam -sTOOLS=toolset test

This will update all of the Boost.Python v1 test and example targets. The tests are relatively quiet by default. To get more-verbose output, you might try

>     bjam -sTOOLS=toolset -sPYTHON_TEST_ARGS=-v test

which will print each test's Python code with the expected output as it passes.

## Building your Extension Module

Though there are other approaches, the best way to build an extension module using Boost.Python is with Boost.Build. If you have to use another build system, you should use Boost.Build at least once with the "`-n`" option so you can see the command-lines it uses, and replicate them. You are likely to run into compilation or linking problems otherwise.

### The Easy Way

Until Boost.Build v2 is released, cross-project build dependencies are not supported, so it works most smoothly if you add a new subproject to your boost installation. The `libs/python/example` subdirectory of your boost installation contains a minimal example (along with many extra sources). To copy the example subproject:

1. Create a new subdirectory in, `libs/python`, say `libs/python/my_project`.
2. Copy `libs/python/example/Jamfile` to your new directory.
3. Edit the Jamfile as appropriate for your project. You'll want to change the "`subproject`" rule invocation at the top, and the names of some of the source files and/or targets.

The instructions above for testing Boost.Python apply equally to your new extension modules in this subproject.

### Building your module outside the Boost project tree

If you can't (or don't wish to) modify your boost installation, the alternative is to create your own Boost.Build project. A similar example you can use as a starting point is available in `this archive`. You'll need to edit the Jamfile and Jamrules files, depending on the relative location of your Boost installation and the new project. Note that automatic testing of extension modules is not available in this configuration.

## Build Variants

Three variant configurations of all python-related targets are supported, and can be selected by setting the `BUILD` variable:

- `release` (optimization, `-DNDEBUG`)
- `debug` (no optimization `-D_DEBUG`)
- `debug-python` (no optimization, `-D_DEBUG -DBOOST_DEBUG_PYTHON`)

The first two variants of the `boost_python` library are built by default, and are compatible with the default Python distribution. The `debug-python` variant corresponds to a specially-built debugging version of Python. On Unix platforms, this python is 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 `PCBuild.dsw` Visual C++ 6.0 project in the `PCBuild` subdirectory of your Python distribution. Extension modules built with Python debugging enabled are not link-compatible with a non-debug build of Python. Since few people actually have a debug build of Python (it doesn't come with the standard distribution), the normal `debug` variant builds modules which are compatible with ordinary Python.

On many windows compilers, when extension modules are built with `-D_DEBUG`, Python defaults to force linking with a special debugging version of the Python DLL. Since this debug DLL isn't supplied with the default Python installation for Windows, Boost.Python uses `boost/python/detail/wrap_python.hpp` to temporarily undefine `_DEBUG` when `Python.h` is `#include`d - unless `BOOST_DEBUG_PYTHON` is defined.

If you want the extra runtime checks available with the debugging version of the library, `#define BOOST_DEBUG_PYTHON` to re-enable python debuggin, and link with the `debug-python` variant of `boost_python`.

If you do not `#define BOOST_DEBUG_PYTHON`, be sure that any source files in your extension module `#include <boost/python/detail/wrap_python.hpp>` instead of the usual `Python.h`, or you will have link incompatibilities.

## Building Using the Microsoft Visual Studio IDE

For the those of you who feel more comfortable in the IDE world, a workspace and project file have been included in the libs/python/build/VisualStudio subdirectory. It builds release and debug versions of the Boost.Python libraries and places them and the same directory as Jamfile build does, though the intermediate object files are placed in a different directory. The files have been created using Microsoft Visual C++ version 6, but they should work for later versions as well. You will need to tell the IDE where to find the Python `Include/` and `Libs/` directories. Under Tools>Options>Directories, add an entry for the Python include dir (i.e. `c:/Python22/Include`), and one for the Lib (i.e. `c:/Python/Libs`. Make sure it is `Libs` with an "`s`" and not just `Lib`).

### Using the IDE for your own projects

Building your own projects using the IDE is slightly more complicated. Firstly, you need to make sure that the project you create as the right kind. It should be a "Win32 Dynamic-Link Library". The default one that Visual Studio 6 creates needs some modifications: turn on RTTI, and change the debug and release builds to use the respective debug and release Multithreaded DLL versions. You should probably turn off incremental linking too -- I believe it a bit flaky. If you do this, then change the "Debug Info" to "Program Database" to get rid of the Edit and Continue warning.

You'll need to add the Boost root directory under Tools>Options>Directories to get your code compiling. To make it link, add the above `boost_python.dsp` file to your workspace, and make your project depend upon it (under Project>Dependencies). You should be able to build now.

Lastly, go to the Project Settings>Debug Page and add the `Python.exe` as the executable for the project. Set a startup directory, and make sure that your current project's output dll, the `boost_python.dll` and the `python22.dll` are on the current `PATH`. If you have a python script that tests your dll, then add it in the "Program Arguments". Now, if all went well, you should be able to hit the Run (F5) button, and debug your code.

> The Visual Studio project files are graciously contributed and maintained by Brett Calcott.

© Copyright David Abrahams 2002. Permission to copy, use, modify, sell and distribute this document is granted provided this copyright notice appears in all copies. This document is provided ``as is'' without express or implied warranty, and with no claim as to its suitability for any purpose.

Updated: 29 December, 2002 (David Abrahams)