The following debugging session, run with python on the default branch, shows
that pdb does not stop in __del__ when it is invoked.

The reason is:

  - The destructor is not called when processing the 'c = 1' statement because
    foo frame.f_locals owns a reference to the C instance.
  - When the interpreter is about to invoke the trace function with the ensuing
    debug event, the call to PyFrame_LocalsToFast in call_trampoline causes the
    destructor to be invoked (as shown by gdb), and the destructor is not traced
    because at that time tstate->use_tracing is false.

This is confirmed by the fact that when 'c = 1' is replaced with
'c = 1; locals()' (on one single line so as not to trigger the trace function
before the call to locals()), then pdb stops in __del__.

===   foo.py   ================
class C:
    def __del__(self):
        print("Calling C destructor.")

def foo():
    c = C()
    import pdb; pdb.set_trace()
    c = 1

foo()
===============================
$ ./python /tmp/foo.py
> /tmp/foo.py(8)foo()
-> c = 1
(Pdb) step
Calling C destructor.
--Return--
> /tmp/foo.py(8)foo()->None
-> c = 1
(Pdb)
===============================