[Cython] History of SWIG and applicability to Cython

[Robert Bradshaw]

On Mon, Aug 29, 2011 at 7:33 AM, Stefan Behnel <stefan_ml at behnel.de> wrote:
> Hi,
>
> here's an interesting history wrap-up of SWIG, by its original author.

Very interesting thread.

> One thing that stroke me when I read this (or rather when I was half-way
> through) was that it might be possible to use SWIG as a glue code generator
> for .pxd files and trivial wrapper code. Not sure how hard that would be -
> it does sound like such a complex system could also be somewhat tricky to
> extend ...

Yeah, I've thought about this before too. It's probably a project
worth pursuing (especially given all the swig wrapper and wrapped code
out there) but haven't ever looked seriously into it. Clang seems like
a more solid option long-term, and what I would pick if I were to
write a wrapper-generator from scratch (especially for C++). Of course
there's still all the messiness of pointer memory management, type
translation etc.

The idea of automatically parsing header files has been around for a
*long* time, and lots of people have come up with partial solutions
(e.g. "good enough" for their tasks) but solving the generic problem
can be quite hard. So they don't get lost, I've created a page at
http://wiki.cython.org/AutoPxd . Eventually it'd be nice if one such
solution became good enough to be the "standard" solution that
everyone put their weight behind. As for inclusion in Cython, I think
it's still to early to tell, but the a standalone tool to do the .h ->
.pxd/.pyx translation would be quite useful even if it remains a
separate project. (And the two could know about/interface with each
other, even if they weren't bundled together.)

- Robert


> -------- Original-Message --------
> Subject: SWIG (was Re: Ctypes and the stdlib)
> Date: Mon, 29 Aug 2011 07:41:23 -0500
> From: David Beazley <dave... at dabeaz.com>
> To: python-dev... at python.org
> CC: David Beazley <dave... at dabeaz.com>
>
> 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 alwa
>  ys 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 effectiv
>  ely, 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 g
>  o 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 underst
>  and 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 funct
>  ion 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.
>
> Cheers,
> Dave
>
> 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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