[Python-Dev] SWIG (was Re: Ctypes and the stdlib)
dave at dabeaz.com
Mon Aug 29 14:41:23 CEST 2011
On Mon, Aug 29, 2011 at 12:27 PM, Guido van Rossum <guido at python.org> wrote:
> I wonder if for
> this particular purpose SWIG isn't the better match. (If SWIG weren't
> universally hated, even by its original author. :-)
Hate is probably a strong word, but as the author of Swig, let me chime in here ;-). I think there are probably some lessons to be learned from Swig.
As Nick noted, Swig is best suited when you have control over both sides (C/C++ and Python) of whatever code you're working with. In fact, the original motivation for Swig was to give application programmers (scientists in my case), a means for automatically generating the Python bindings to their code. However, there was one other important assumption--and that was the fact that all of your "real code" was going to be written in C/C++ and that the Python scripting interface was just an optional add-on (perhaps even just a throw-away thing). Keep in mind, Swig was first created in 1995 and at that time, the use of Python (or any similar language) was a pretty radical idea in the sciences. Moreover, there was a lot of legacy code that people just weren't going to abandon. Thus, I always viewed Swig as a kind of transitional vehicle for getting people to use Python who might otherwise not even consider it. Getting back to Nick's point though, to really use Swig effectively, it was always known that you might have to reorganize or refactor your C/C++ code to make it more Python friendly. However, due to the automatic wrapper generation, you didn't have to do it all at once. Basically your code could organically evolve and Swig would just keep up with whatever you were doing. In my projects, we'd usually just tuck Swig away in some Makefile somewhere and forget about it.
One of the major complexities of Swig is the fact that it attempts to parse C/C++ header files. This very notion is actually a dangerous trap waiting for anyone who wants to wander into it. You might look at a header file and say, well how hard could it be to just grab a few definitions out of there? I'll just write a few regexs or come up with some simple hack for recognizing function definitions or something. Yes, you can do that, but you're immediately going to find that whatever approach you take starts to break down into horrible corner cases. Swig started out like this and quickly turned into a quagmire of esoteric bug reports. All sorts of problems with preprocessor macros, typedefs, missing headers, and other things. For awhile, I would get these bug reports that would go something like "I had this C++ class inside a namespace with an abstract method taking a typedef'd const reference to this smart pointer ..... and Swig broke." Hell, I can't even understand the bug report let alone know how to fix it. Almost all of these bugs were due to the fact that Swig started out as a hack and didn't really have any kind of solid conceptual foundation for how it should be put together.
If you flash forward a bit, from about 2001-2004 there was a very serious push to fix these kinds of issues. Although it was not a complete rewrite of Swig, there were a huge number of changes to how it worked during this time. Swig grew a fully compatible C++ preprocessor that fully supported macros A complete C++ type system was implemented including support for namespaces, templates, and even such things as template partial specialization. Swig evolved into a multi-pass compiler that was doing all sorts of global analysis of the interface. Just to give you an idea, Swig would do things such as automatically detect/wrap C++ smart pointers. It could wrap overloaded C++ methods/function. Also, if you had a C++ class with virtual methods, it would only make one Python wrapper function and then reuse across all wrapped subclasses.
Under the covers of all of this, the implementation basically evolved into a sophisticated macro preprocessor coupled with a pattern matching engine built on top of the C++ type system. For example, you could write patterns that matched specific C++ types (the much hated "typemap" feature) and you could write patterns that matched entire C++ declarations. This whole pattern matching approach had a huge power if you knew what you were doing. For example, I had a graduate student working on adding "contracts" to Swig--something that was being funded by a NSF grant. It was cool and mind boggling all at once.
In hindsight however, I think the complexity of Swig has exceeded anyone's ability to fully understand it (including my own). For example, to even make sense of what's happening, you have to have a pretty solid grasp of the C/C++ type system (easier said than done). Couple that with all sorts of crazy pattern matching, low-level code fragments, and a ton of macro definitions, your head will literally explode if you try to figure out what's happening. So far as I know, recent versions of Swig have even combined all of this type-pattern matching with regular expressions. I can't even fathom it.
Sadly, my involvement was Swig was an unfortunate casualty of my academic career biting the dust. By 2005, I was so burned out of working on it and so sick of what I was doing, I quite literally put all of my computer stuff aside to go play in a band for a few years. After a few years, I came back to programming (obviously), but not to keep working on the same stuff. In particularly, I will die quite happy if I never have to look at another line of C++ code ever again. No, I would much rather fling my toddlers, ride my bike, play piano, or do just about anything than ever do that again. Although I still subscribe the Swig mailing lists and watch what's happening, I'm not active with it at the moment.
I've sometimes thought it might be interesting to create a Swig replacement purely in Python. When I work on the PLY project, this is often what I think about. In that project, I've actually built a number of the parsing tools that would be useful in creating such a thing. The only catch is that when I start thinking along these lines, I usually reach a point where I say "nah, I'll just write the whole application in Python."
Anyways, this is probably way more than anyone wants to know about Swig. Getting back to the original topic of using it to make standard library modules, I just don't know. I think you probably could have some success with an automatic code generator of some kind. I'm just not sure it should take the Swig approach of parsing C++ headers. I think you could do better.
P.S. By the way, if people want to know a lot more about Swig internals, they should check out the PyCon 2008 presentation I gave about it. http://www.dabeaz.com/SwigMaster/
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