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Due to the fact that `pdb` currently simply ignores breakpoints which are set and hit in another than the main thread the docs need to contain a statement on behavior in a multithreaded environment.

pdb does not ignore breakpoints which are set and hit in a non-main thread.
For example with the attached script:
$ python pdb_thread.py
> pdb_thread.py(5)foo()
-> lineno = 5
(Pdb) break 6
Breakpoint 1 at pdb_thread.py:6
(Pdb) continue
> pdb_thread.py(6)foo()
-> lineno = 6
(Pdb) threading.current_thread().name
'fooThread'
(Pdb)

The pdb documentation could make an explicit reference to the documentation of sys.settrace() where it is explained that the function is thread specific.

My initial description was imprecise. Clearification: The fact that in
 #!/usr/bin/python
 import threading
 def debugging():
 def __a_thread__():
 print("2")
 a_thread = threading.Thread(target=__a_thread__)
 print("1")
 if __name__ == "__main__":
 debugging()
when invoked with `python -m pdb` the breakpoint at line 7 is ignored, like in
 $ python -m pdb multithreaded_debugging.py 
 > /mnt/DATA/richter/examples/python/multithreaded_debugging/multithreaded_debugging.py(3)<module>()
 -> import threading
 (Pdb) break 7
 Breakpoint 1 at /mnt/DATA/richter/examples/python/multithreaded_debugging/multithreaded_debugging.py:7
 (Pdb) c
 1
 The program finished and will be restarted
 > /mnt/DATA/richter/examples/python/multithreaded_debugging/multithreaded_debugging.py(3)<module>()
 -> import threading
 (Pdb)

Sorry, I mean 
 #!/usr/bin/python
 import threading
 def debugging():
 def __a_thread__():
 print("2")
 a_thread = threading.Thread(target=__a_thread__)
 a_thread.start()
 a_thread.join()
 print("1")
 if __name__ == "__main__":
 debugging()
It's very uncommon to set the current debugging thread in the source.

Absolutely this. While it is good that there is at least some documentation (at sys.settrace), it is not very explicit nor is it self evident that one should look there (for example the PDB docs don't even mention settrace).

See also https://bugs.python.org/issue41571