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

[krichter]

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:

assignee = None
closed_at = None
created_at = <Date 2015-01-04.10:21:23.812>
labels = ['type-feature', 'library']
title = 'pdb docs need to contain a statement on threads/multithreaded debugging'
updated_at = <Date 2020-09-19.19:01:34.616>
user = 'https://bugs.python.org/krichter'

bugs.python.org fields:

activity = <Date 2020-09-19.19:01:34.616>
actor = 'georg.brandl'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2015-01-04.10:21:23.812>
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']

[krichter]

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.

[xdegaye]

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)

[xdegaye]

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

[krichter]

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)

[krichter]

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.

[Attila-MihalyBalazs]

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