How to get the actual command that the Distutils compile() function uses?

Question

I have a Distutils setup.py script which uses new_compiler().compile() to compile test programs to make sure certain features (eg. MPI) are available on the system.

My problem is that there is one case where the compile() call results in a compiler error, but compiling the same little test program manually does not. The error is inside a standard header (mpi.h). All my actual source files also include this header but they compile fine! This particular check would be very useful to have, but I need to figure out why it's failing when it shouldn't be.

So my question is, how can I get the actual command used by ccompiler.compile()?

Answer 1

Adam (Wagner)'s comment is the right place to start: you have to hunt through the Distutils source code. There are a lot of levels of abstraction so you do have to trace the execution through several different files, but here's the gist:

• The package distutils.ccompiler includes an abstract class CCompiler which handles invoking the actual compiler. It has methods for the various tasks a compiler needs to perform: preprocess, compile, and link. CCompiler itself does not implement any of these methods except for compile, but even there it delegates the actual compiler invocation to the method _compile. So you need to check the subclass of CCompiler that is used on your platform and look at its implementation of _compile.
• There are several different subclasses of CCompiler, each implemented in its own package, but the "big two" are distutils.unixccompiler.UnixCCompiler (which invokes the native compiler on a UNIX-like system) and distutils.msvccompiler.MSVCCompiler (which invokes Visual Studio). Both of these use the function distutils.spawn.spawn to actually run the external process.
• In the source for distutils.spawn.spawn, you'll notice that each command is logged at the level INFO before it is run. But the catch is, this doesn't use Python's builtin logging system; rather it uses Distutils' custom logger implemented in distutils.log.

• If you now look at the source for distutils.log, you'll see that there is a function set_verbosity which sets the logging level. Unfortunately, it isn't tied to any of the other debugging infrastructure in Distutils (such as the DISTUTILS_DEBUG environment variable), so you'll need to manually call

  distutils.log.set_verbosity(1)
  

  somewhere in your setup script, before any compilation commands are run. Once you do that, all compilation commands (and who knows what other information) should be printed to standard output.