import ctypes
ctypes.windll.kernel32.DebugBreak()
This used to be a handy way to attach a debugger to a running program, by way of JIT debugging.  Now ctypes catches and handles this exception so a debugger is never invoked.  Bummer.

With a release build of python, I have a similar behavior on both 2.5 and 2.7; the messages are different though:
2.5: WindowsError: [Error -2147483645] One or more arguments are invalid.
2.7: WindowsError: exception: breakpoint encountered

And with both versions, a debug build (python_d.exe) triggers the JIT debugger. What do you get exactly?

I _think_ that in our old 2.5 python (which had a backported ctypes from 2.6 to support 64 bits) we always got the JIT debugger i.e. with _ctypes.pyd and _ctypes_d.pyd.

This api, "DebugBreak" always invokes the JIT debugger, however the program was compiled (_NDEBUG, DEBUG, or not).
This is done by raising the breakpoint exception and apparently _ctypes.pyd is catching that exception and handling it, but only in release mode, while I think it shouldn't do.

This affects me too.

Here is a suggested patch.  SEH will now not catch BREAKPOINT exceptions.

New changeset b2e8392a8f77 by Kristján Valur Jónsson in branch '2.7':
Issue #10296 : Don't handle BreakPoint exceptions using
http://hg.python.org/cpython/rev/b2e8392a8f77

New changeset bc5778b488c1 by Kristján Valur Jónsson in branch '3.2':
Issue #10296 : Don't handle BreakPoint exceptions using
http://hg.python.org/cpython/rev/bc5778b488c1

New changeset 4c7eb717ea88 by Kristján Valur Jónsson in branch '3.3':
#10296: Merge to 3.3
http://hg.python.org/cpython/rev/4c7eb717ea88

New changeset 4f3fbdb4d748 by Kristján Valur Jónsson in branch 'default':
Issue #10296: Merge to default
http://hg.python.org/cpython/rev/4f3fbdb4d748