Are nested breakpoints possible in pdb--if not, what prevents them?

Question

Following a tutorial on Python's debugger, I used pdb.set_trace() to interrupt the sample code. It worked, but what if you are at the interactive prompt and want another nested breakpoint?

(Pdb) def test(): pdb.set_trace(); print "don't print this yet"
(Pdb) test()
don't print this yet

It didn't stop. Is the debugger fundamentally "one deep"? e.g. is this is a limitation of Python's hooks, or just something pdb doesn't choose to do?

Answer 1

is this is a limitation of Python's hooks, or just something pdb doesn't choose to do?

It appears to be a limitation of the hooks.

I did a test to see what was being called and what wasn't (putting print statements in /usr/lib/python2.7/bdb.py)

Quick inspection finds set_trace in pdb.py:

def set_trace():
    Pdb().set_trace(sys._getframe().f_back)

That calls set_trace in bdb.py

def set_trace(self, frame=None):
    """Start debugging from `frame`.
    If frame is not specified, debugging starts from caller's frame.
    """
    if frame is None:
        frame = sys._getframe().f_back
    self.reset()
    while frame:
        frame.f_trace = self.trace_dispatch
        self.botframe = frame
        frame = frame.f_back
    self.set_step()
    sys.settrace(self.trace_dispatch)

This sets up a callback to trace_dispatch, also in bdb.py. The sys.settrace code itself is perhaps in threading.py:

def settrace(func):
    global _trace_hook
    _trace_hook = func

GitHub search finds no more references for _trace_hook, so presumably that's picked up magically in C code somewhere.

When test() is called, it turns out the sys.settrace() call is made...but then the call to trace_dispatch() does not happen.