import sys, pdb

def backward_references(pdb_session):
    initial = pdb_session
    while (hasattr(pdb_session, '_previous_sigint_handler') and
            pdb_session._previous_sigint_handler):
        prev = pdb_session._previous_sigint_handler.__self__
        if isinstance(prev, pdb.Pdb):
            if initial:
                yield initial
                initial = None
            yield prev
        pdb_session = prev

sessions = {}
i = 1
while i:
    pdb_session = pdb.Pdb()
    sessions[pdb_session] = i
    # Set a hard breakpoint here, this is equivalent to pdb.set_trace().
    pdb_session.set_trace(sys._getframe())
    rv = (' -> '.join('pdb %d' % sessions[s]
                      for s in backward_references(pdb_session)))
    if rv:
        print(rv)
    i += 1