SWIG (was Re: Ctypes and the stdlib)

On Mon, Aug 29, 2011 at 12:27 PM, Guido van Rossum <guido@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/