- New function sys.call_tracing() allows pdb to debug code

recursively.
- pdb has a new command, "debug", which lets you step through
  arbitrary code from the debugger's (pdb) prompt.

Python/ceval.c

@@ -3024,6 +3024,24 @@ call_trace(Py_tracefunc func, PyObject *obj, PyFrameObject *frame,
 	return result;
 }
 
+PyObject *
+_PyEval_CallTracing(PyObject *func, PyObject *args)
+{
+	PyFrameObject *frame = PyEval_GetFrame();
+	PyThreadState *tstate = frame->f_tstate;
+	int save_tracing = tstate->tracing;
+	int save_use_tracing = tstate->use_tracing;
+	PyObject *result;
+
+	tstate->tracing = 0;
+	tstate->use_tracing = ((tstate->c_tracefunc != NULL)
+			       || (tstate->c_profilefunc != NULL));
+	result = PyObject_Call(func, args, NULL);
+	tstate->tracing = save_tracing;
+	tstate->use_tracing = save_use_tracing;
+	return result;
+}
+
 static int
 maybe_call_line_trace(Py_tracefunc func, PyObject *obj, 
 		      PyFrameObject *frame, int *instr_lb, int *instr_ub)

Python/sysmodule.c

@@ -17,6 +17,7 @@ Data members:
 #include "Python.h"
 #include "compile.h"
 #include "frameobject.h"
+#include "eval.h"
 
 #include "osdefs.h"
 
@@ -609,6 +610,23 @@ sys_getframe(PyObject *self, PyObject *args)
 	return (PyObject*)f;
 }
 
+PyDoc_STRVAR(call_tracing_doc,
+"call_tracing(func, args) -> object\n\
+\n\
+Call func(*args), while tracing is enabled.  The tracing state is\n\
+saved, and restored afterwards.  This is intended to be called from\n\
+a debugger from a checkpoint, to recursively debug some other code."
+);
+
+static PyObject *
+sys_call_tracing(PyObject *self, PyObject *args)
+{
+	PyObject *func, *funcargs;
+	if (!PyArg_ParseTuple(args, "OO:call_tracing", &func, &funcargs))
+		return NULL;
+	return _PyEval_CallTracing(func, funcargs);
+}
+
 PyDoc_STRVAR(callstats_doc,
 "callstats() -> tuple of integers\n\
 \n\
@@ -700,6 +718,7 @@ static PyMethodDef sys_methods[] = {
 	{"setrecursionlimit", sys_setrecursionlimit, METH_VARARGS,
 	 setrecursionlimit_doc},
 	{"settrace",	sys_settrace, METH_O, settrace_doc},
+	{"call_tracing", sys_call_tracing, METH_VARARGS, call_tracing_doc},
 	{NULL,		NULL}		/* sentinel */
 };