Embedding Python in D

A while back I began spending some time acquainting myself with D. For those who don't know, D is a relatively new system programming language with all kinds of interesting features like very flexible garbage collection, mixins and an easy-to-use yet powerful template system. D was created by Walter Bright, who also created the first native C++ compiler and Empire (of all things).

Anyhow, one of the things that I really like about D is that it is maintains link compatibility with C ¹. This means that D code can make calls into libraries like those from GTK in the same way that C++ code can. This mitigates the need or the desire to rewrite useful tools. Many times simple binding layers can be written to expose C-style APIs in D-style ways.

In my case, I'm writing an application which uses the GTK bindings for D, GtkD, and embeds a Python interpreter. Creating the D header from Python.h turns out to be pretty easy with bcd, a tool which tries to automate most of the process ².

The first step is to run bcd against the header you want to use. For me this looked like:

./bcdgen /usr/include/python2.5/Python.h Python -C -A

In this case "-C" means "C-mode" (as opposed to C++-mode) and "-A" tells bcdgen to continue working on all includes recursively (since Python.h is really a meta-header). Unfortunately this gives me:

In file included from /usr/include/stdio.h:906,
              from /usr/include/python2.5/Python.h:32:
/usr/include/bits/stdio2.h: In function 'int sprintf(char*, const char*, ...)':
/usr/include/bits/stdio2.h:35: error: '__builtin_va_arg_pack' was not declared in this scope
/usr/include/bits/stdio2.h: In function 'int snprintf(char*, size_t, const char*, ...)':
...

This is a bug with Intrepid Ibex's version of gccxml. Luckily there are workarounds. Once bcdgen finishes it's work, it should create a bcd/Python/Python.d, which is where all the Python.h definitions landed. If you open it up and take a look, you can see bcdgen has converted them to D definitions. Way to go automated software development tools!

Generally some work is required before the D compiler accepts this file and the bcd page lists some general guidelines for this task. I'm familiar with Python's C interface, but definitely not intimate. This is a secret code that I'm using which means I spent a bit of time fixing weird D compiler errors. It wasn't bad though and I was was able to resolve most of the compile errors without much trouble. I'm my next post I'll write a little bit about that.

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