pdb docs need to contain a statement on threads/multithreaded debugging

[fcfd9df0-fe97-4eef-90a2-7e9380d47fbd]

BPO	23163
Nosy	@blueyed, @xdegaye, @bharel, @pablogsal
Files	pdb_thread.py

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields: ```python assignee = None closed_at = None created_at = labels = ['type-feature', 'library'] title = 'pdb docs need to contain a statement on threads/multithreaded debugging' updated_at = user = 'https://bugs.python.org/krichter' ``` bugs.python.org fields: ```python activity = actor = 'georg.brandl' assignee = 'none' closed = False closed_date = None closer = None components = ['Library (Lib)'] creation = creator = 'krichter' dependencies = [] files = ['37668'] hgrepos = [] issue_num = 23163 keywords = [] message_count = 7.0 messages = ['233409', '233816', '233819', '233860', '233861', '286377', '376467'] nosy_count = 6.0 nosy_names = ['blueyed', 'xdegaye', 'krichter', 'bar.harel', 'Attila-Mihaly Balazs', 'pablogsal'] pr_nums = [] priority = 'normal' resolution = None stage = None status = 'open' superseder = None type = 'enhancement' url = 'https://bugs.python.org/issue23163' versions = ['Python 3.5'] ```

[fcfd9df0-fe97-4eef-90a2-7e9380d47fbd]

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.

[490c593f-f636-409f-bb35-6abeb38a4595]

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)

[490c593f-f636-409f-bb35-6abeb38a4595]

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

[fcfd9df0-fe97-4eef-90a2-7e9380d47fbd]

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)

[fcfd9df0-fe97-4eef-90a2-7e9380d47fbd]

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.

[546a6683-2ac1-4017-a3ec-0692d8ee6932]

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).

[pablogsal]

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