Oh. I also got this surprising (Pdb) prompt when running "python -m test" on Windows.

The prompt doesn't display any output, I only get the prompt display 

(...)
0:34:05 load avg: 1.06 [300/426/2] test_select
0:34:08 load avg: 1.03 [301/426/2] test_selectors
0:34:13 load avg: 0.94 [302/426/2] test_set
0:34:24 load avg: 0.80 [303/426/2] test_setcomps
0:34:26 load avg: 0.78 [304/426/2] test_shelve
0:34:30 load avg: 0.74 [305/426/2] test_shlex
0:34:32 load avg: 0.70 [306/426/2] test_shutil
0:34:36 load avg: 0.66 [307/426/2] test_signal
(Pdb) help
(Pdb) l
(Pdb) ?
(Pdb) help
(Pdb) where
(Pdb) down
(Pdb) p 1
(Pdb) p 2
(Pdb)  


*But* I managed to get more info about that issue:

(Pdb) f=open("debug.txt", "w")
(Pdb) import traceback
(Pdb) traceback.print_stack(file=f)
(Pdb) f.flush()

C:\vstinner\python\master\build\test_python_6260æ\debug.txt content:

(...)
  File "C:\vstinner\python\master\lib\test\libregrtest\runtest.py", line 154, in _runtest
    result = _runtest_inner(ns, test_name,
(...)
  File "C:\vstinner\python\master\lib\unittest\case.py", line 549, in _callTestMethod
    method()
  File "C:\vstinner\python\master\lib\test\test_signal.py", line 1345, in test_sigint
    signal.raise_signal(signal.SIGINT)
  File "C:\vstinner\python\master\lib\unittest\case.py", line 216, in __exit__
    def __exit__(self, exc_type, exc_value, tb):
  File "C:\vstinner\python\master\lib\bdb.py", line 92, in trace_dispatch
    return self.dispatch_call(frame, arg)
  File "C:\vstinner\python\master\lib\bdb.py", line 136, in dispatch_call
    self.user_call(frame, arg)
  File "C:\vstinner\python\master\lib\pdb.py", line 252, in user_call
    self.interaction(frame, None)
  File "C:\vstinner\python\master\lib\pdb.py", line 357, in interaction
    self._cmdloop()
  File "C:\vstinner\python\master\lib\pdb.py", line 322, in _cmdloop
    self.cmdloop()
  File "C:\vstinner\python\master\lib\cmd.py", line 138, in cmdloop
    stop = self.onecmd(line)
  File "C:\vstinner\python\master\lib\pdb.py", line 423, in onecmd
    return cmd.Cmd.onecmd(self, line)
  File "C:\vstinner\python\master\lib\cmd.py", line 216, in onecmd
    return self.default(line)
  File "C:\vstinner\python\master\lib\pdb.py", line 381, in default
    exec(code, globals, locals)
  File "<stdin>", line 1, in <module>


Using debug.txt, I also see that in test_signal, sys.gettrace() = <bound method Bdb.trace_dispatch of <pdb.Pdb object at 0x0000029D85AB3570>>.

Lib/test/libregrtest/save_env.py is supposed to mark a test as "env changed" (ENV_CHANGED) if sys.gettrace() is changed between two tests.