[Howto] Compiling debug Python extensions for non-debug Python

[Mike C. Fletcher]

Every few months I get to working on a Python C extension (built with 
distutils) and discover a pointer error or the like where I'd like to be 
able to step into my DLL and see where the extension is going wonky. 

Now, the "proper" way to debug a DLL is apparently to build Python in 
debug mode (i.e. with the _d form dll), then build each and every 
extension I depend on (in debug mode), and then build each of my own 
extensions in debug mode.  That takes forever when there's 8 or 9 
complex-to-build extensions involved in the system generating the error, 
and is pointless when I could just print-debug in 1/100th of the time to 
fix the particular error.

Now, it so happens that there's no reason I know of to do all that 
rebuilding when all you want to do is to generate a program database and 
see what's going on *in your own extension*.  You can quite readily 
create a debuggable DLL linked against the non-debug Python and all its 
extensions with a 2 or 3 line change to msvccompiler.py in distutils.  I 
discovered that quite a while ago, but as the change is to a core Python 
file, I tend to forget about it and overwrite it when I upgrade Python 
and then spend a few hours re-discovering the exact switches, so, for my 
own memory, and for any other extension writers who want this kind of 
down-and-dirty debugging, here's the changes required:

In lib/distutils/msvccompiler.py:

        self.compile_options = [
            '/nologo',
            '/Od', # turn off optimisations (allow debug)
            '/MD', '/W3', '/GX',
            '/Zi', # create a pdb file for the object files we create, 
/Z7 works fine too
        ]
        self.ldflags_shared = [
            '/DLL', '/nologo', '/INCREMENTAL:YES',
            '/DEBUG', # do debug-mode linking
        ]

then rebuild, forcing a recompile to make sure everything's been built 
with the new flags.

There's probably some perfectly good reason not to do this as the 
default for distutils "debug" mode for building a PYD, but I find it 
*very* helpful, and hopefully others will as well.

Keywords:  Visual C MSVC++ VC++ Win32 debug extension dll pyd distutils

Enjoy all,
Mike

_______________________________________
  Mike C. Fletcher
  Designer, VR Plumber, Coder
  http://members.rogers.com/mcfletch/