Passing a frame to pdb.Pdb.set_trace

[Rotwang]

Hi all, I've found something weird with pdb and I don't understand it. I 
want to define a function mydebugger() which starts the debugger in the 
caller's frame. The following is copied from IDLE with Python 2.7.3 
(I've since tried it with 3.3.0 and the same thing happens):


Python 2.7.3 (default, Feb 27 2014, 19:58:35)
[GCC 4.6.3] on linux2
Type "copyright", "credits" or "license()" for more information.
  >>> import pdb, sys
  >>> def f(x):
      mydebugger()


  >>> def mydebugger():
      frame = sys._getframe().f_back
      pdb.Pdb().set_trace(frame)


  >>> f(4)
--Return--
  > (2)f()->None
(Pdb) x
4


This is what I expect: sys._getframe().f_back gives f's frame, so the 
call to mydebugger() within f does approximately the same thing as if 
I'd just called pdb.set_trace() instead. But when I add another 
statement to mydebugger, this happens:


  >>> def mydebugger():
      frame = sys._getframe().f_back
      pdb.Pdb().set_trace(frame)
      print 'hmm'


  >>> f(4)
--Call--
  > /usr/lib/python2.7/idlelib/rpc.py(546)__getattr__()
-> def __getattr__(self, name):
(Pdb) x
*** NameError: name 'x' is not defined
(Pdb) w #Where am I?
    (1)()
    /usr/lib/python2.7/idlelib/run.py(97)main()
-> ret = method(*args, **kwargs)
    /usr/lib/python2.7/idlelib/run.py(298)runcode()
-> exec code in self.locals
    (1)()
    (2)f()
    (4)mydebugger()
  > /usr/lib/python2.7/idlelib/rpc.py(546)__getattr__()
-> def __getattr__(self, name):


The same thing happens if I define

     frame = sys._getframe().f_back.f_back

(i.e. the debugger starts in the same place) for example, though if I define

     frame = sys._getframe()

then the debugger starts in mydebugger's frame as I would expect. Also, 
whether it goes wrong depends on what the third line of mydebugger is; 
some kinds of statement consistently cause the problem and others 
consistently don't.

When I try the above simple code in the terminal rather than IDLE it 
works like it should, but in the more complicated version where I first 
noticed it it still goes wrong. Can anyone help?