Boost C++ Libraries

...one of the most highly regarded and expertly designed C++ library projects in the world. Herb Sutter and Andrei Alexandrescu, C++ Coding Standards

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Creating Packages

A Python package is a collection of modules that provide to the user a certain functionality. If you're not familiar on how to create packages, a good introduction to them is provided in the Python Tutorial.

But we are wrapping C++ code, using Boost.Python. How can we provide a nice package interface to our users? To better explain some concepts, let's work with an example.

We have a C++ library that works with sounds: reading and writing various formats, applying filters to the sound data, etc. It is named (conveniently) sounds. Our library already has a neat C++ namespace hierarchy, like so:

    sounds::core
    sounds::io
    sounds::filters

We would like to present this same hierarchy to the Python user, allowing him to write code like this:

    import sounds.filters
    sounds.filters.echo(...) ##echo is a C++ function

The first step is to write the wrapping code. We have to export each module separately with Boost.Python, like this:

    /* file core.cpp */
    BOOST_PYTHON_MODULE(core)
    {
        /* export everything in the sounds::core namespace */
        ...
    } 
        
    /* file io.cpp */
    BOOST_PYTHON_MODULE(io)
    {
        /* export everything in the sounds::io namespace */
        ...
    } 

    /* file filters.cpp */
    BOOST_PYTHON_MODULE(filters)
    {
        /* export everything in the sounds::filters namespace */
        ...
    } 

Compiling these files will generate the following Python extensions: core.pyd, io.pyd and filters.pyd.

The extension .pyd is used for python extension modules, which are just shared libraries. Using the default for your system, like .so for Unix and .dll for Windows, works just as well.

Now, we create this directory structure for our Python package:

    sounds/
        __init__.py
        core.pyd
        filters.pyd
        io.pyd

The file __init__.py is what tells Python that the directory sounds/ is actually a Python package. It can be a empty file, but can also perform some magic, that will be shown later.

Now our package is ready. All the user has to do is put sounds into his PYTHONPATH and fire up the interpreter:

    >>> import sounds.io
    >>> import sounds.filters
    >>> sound = sounds.io.open('file.mp3')
    >>> new_sound = sounds.filters.echo(sound, 1.0)

Nice heh?

This is the simplest way to create hierarchies of packages, but it is not very flexible. What if we want to add a pure Python function to the filters package, for instance, one that applies 3 filters in a sound object at once? Sure, you can do this in C++ and export it, but why not do so in Python? You don't have to recompile the extension modules, plus it will be easier to write it.

If we want this flexibility, we will have to complicate our package hierarchy a little. First, we will have to change the name of the extension modules:

    /* file core.cpp */
    BOOST_PYTHON_MODULE(_core)
    {
        ...
        /* export everything in the sounds::core namespace */
    }  

Note that we added an underscore to the module name. The filename will have to be changed to _core.pyd as well, and we do the same to the other extension modules. Now, we change our package hierarchy like so:

    sounds/
        __init__.py
        core/
            __init__.py
            _core.pyd
        filters/
            __init__.py
            _filters.pyd
        io/
            __init__.py
            _io.pyd

Note that we created a directory for each extension module, and added a __init__.py to each one. But if we leave it that way, the user will have to access the functions in the core module with this syntax:

    >>> import sounds.core._core
    >>> sounds.core._core.foo(...)

which is not what we want. But here enters the __init__.py magic: everything that is brought to the __init__.py namespace can be accessed directly by the user. So, all we have to do is bring the entire namespace from _core.pyd to core/__init__.py. So add this line of code to sounds/core/__init__.py:

    from _core import *

We do the same for the other packages. Now the user accesses the functions and classes in the extension modules like before:

    >>> import sounds.filters
    >>> sounds.filters.echo(...)

with the additional benefit that we can easily add pure Python functions to any module, in a way that the user can't tell the difference between a C++ function and a Python function. Let's add a pure Python function, echo_noise, to the filters package. This function applies both the echo and noise filters in sequence in the given sound object. We create a file named sounds/filters/echo_noise.py and code our function:

    import _filters
    def echo_noise(sound):
        s = _filters.echo(sound)
        s = _filters.noise(sound)
        return s

Next, we add this line to sounds/filters/__init__.py:

    from echo_noise import echo_noise

And that's it. The user now accesses this function like any other function from the filters package:

    >>> import sounds.filters
    >>> sounds.filters.echo_noise(...)