Use stack property to return a modified view of the stack in post_mortem

If we're in post_mortem, accessing the stack will return the bottom
frame instead of an empty stack. By doing this, users can expand
variables in post_mortem.

pudb/debugger.py

@@ -200,6 +200,14 @@ def __init__(self, stdin=None, stdout=None, term_size=None, steal_output=False,
         for bpoint_descr in load_breakpoints():
             self.set_break(*bpoint_descr)
 
+    @property
+    def stack(self):
+        if self.post_mortem:
+            # Return the bottom frame so the user can expand variables in post_mortem
+            return [(self.bottom_frame, self.bottom_frame.f_lineno)]
+        else:
+            return self._stack
+
     # These (dispatch_line and set_continue) are copied from bdb with the
     # patch from https://bugs.python.org/issue16482 applied. See
     # https://github.com/inducer/pudb/pull/90.
@@ -401,17 +409,17 @@ def interaction(self, frame, exc_tuple=None, show_exc_dialog=True):
         if not found_bottom_frame and not self.post_mortem:
             return
 
-        self.stack, index = self.get_shortened_stack(frame, tb)
+        self._stack, index = self.get_shortened_stack(frame, tb)
 
         if self.post_mortem:
-            self.stack.append((self.bottom_frame, self.bottom_frame.f_lineno))
             index = len(self.stack)-1
 
         self.set_frame_index(index)
 
         self.ui.call_with_ui(self.ui.interaction, exc_tuple,
                 show_exc_dialog=show_exc_dialog)
 
+
     def get_stack_situation_id(self):
         return str(id(self.stack[self.curindex][0].f_code))