detail::error_string: handle call stacks that switch between C++ and Python multiple times

include/pybind11/cast.h

@@ -136,9 +136,19 @@ PYBIND11_NOINLINE inline std::string error_string() {
 
 
     PyErr_NormalizeException(&scope.type, &scope.value, &scope.trace);
     PyErr_NormalizeException(&scope.type, &scope.value, &scope.trace);
 
 
+#if PY_MAJOR_VERSION >= 3
+    if (scope.trace != nullptr)
+        PyException_SetTraceback(scope.value, scope.trace);
+#endif
+
     if (scope.trace) {
     if (scope.trace) {
-        PyFrameObject *frame = ((PyTracebackObject *) scope.trace)->tb_frame;
-        if (frame) {
+        PyTracebackObject *trace = (PyTracebackObject *) scope.trace;
+
+        /* Get the deepest trace possible */
+        while (trace->tb_next)
+            trace = trace->tb_next;
+
+        PyFrameObject *frame = trace->tb_frame;
         errorString += "\n\nAt:\n";
         errorString += "\n\nAt:\n";
         while (frame) {
         while (frame) {
             int lineno = PyFrame_GetLineNumber(frame);
             int lineno = PyFrame_GetLineNumber(frame);
@@ -148,7 +158,7 @@ PYBIND11_NOINLINE inline std::string error_string() {
                 handle(frame->f_code->co_name).cast<std::string>() + "\n";
                 handle(frame->f_code->co_name).cast<std::string>() + "\n";
             frame = frame->f_back;
             frame = frame->f_back;
         }
         }
-        }
+        trace = trace->tb_next;
     }
     }
 
 
     return errorString;
     return errorString;