[DinoV]

VS 2012 added a new debugging framework for supporting debugging multiple code types at the same time. That model requires that the debugger work out of process - inspecting various data structures cross process and reporting them back to VS. That's different from how Python debuggers are usually built w/ sys.settrace and a debugger helper thread running in the process.

To accomplish that we require that we have symbols for the Python interpreter (which are typically available on python.org). The debug engine uses a combination of the native equivalent of sys.settrace, strategic breakpoints, and using the symbols to walk core Python data structures. There's a bunch of fun tricks for evaluating code when stopped at a Python frame and creating strings, ints, etc... when users type them into the watch window.

If you'd like to dig in more the source is at http://pytools.codeplex.com/SourceControl/latest under Python\Product\Debugger\DkmDebugger.

[ritonlajoie]

My understanding is that dual debugging works when your main process is oythin and you can the debug some C python module. Is ther a way to debug a C process loading python27.dll which then runs some pythin code using the oython-C api?

[int_19h]

Mixed-mode debugging works in both of these scenarios (remember that python.exe is really just a "C process loading python27.dll"). You don't need a Python project at all, you can just attach to a running process (whether it is python.exe or your own host) and start debugging. You can even do that if the host process hasn't actually loaded python##.dll yet, though of course Python breakpoints etc will only light up once that happens.

Here's a screenshot of a debug session with a custom host process written in C# running Python code that in turn uses a C++ extension module: http://i.imgur.com/IDPsWUu.png