Fix parse_duration() for some negative durations

[heikkimattila]

The ​https://docs.djangoproject.com/en/2.1/_modules/django/utils/dateparse/ defines:

standard_duration_re = re.compile(
    r'^'
    r'(?:(?P<days>-?\d+) (days?, )?)?'
    r'((?:(?P<hours>-?\d+):)(?=\d+:\d+))?'
    r'(?:(?P<minutes>-?\d+):)?'
    r'(?P<seconds>-?\d+)'
    r'(?:\.(?P<microseconds>\d{1,6})\d{0,6})?'
    r'$'
)

that doesn't match to negative durations, because of the <hours> definition final (lookahead) part does not have '-?' in it. The following will work:

    r'((?:(?P<hours>-?\d+):)(?=-?\d+:-?\d+))?'

(Thanks to Konstantin Senichev for finding the fix.)

[Tim Graham]

Please give an example valid that's not working. There are ​some tests for negative values.

[Simon Charette]

Right, this should have been fixed by #27699 which is included in 1.11.x.

[heikkimattila]

Example cases, can be discussed:

parse_duration('-00:01:01') => plus 61 seconds, so it is not -(00:01:01) but (-00):(+01):(+01)
parse_duration('00:-01:-01) => None , leading zeros will prevent parsing
parse_duration('-01:01') => minus 59 seconds
parse_duration('-01:-01') => minus 61 seconds

The fix presented would allow the second line to be parsed (which would help with generated durations).

And some instructions in the function/documentation/wiki would be useful, to clarify how the minus sign affects in duration.

[Tim Graham]

The fix from #27699 may not be entirely correct. I agree with your first and third examples. I'd expect a leading minus sign to negate the entire value so they would be minus 61 seconds. I think the second and fourth examples are invalid. I don't think a minus sign after a colon is valid.

[Simon Charette]

Thanks for the extra details. I agree with Tim that everything but a leading - seems like an invalid value that happened to work because of an inappropriate pattern as ​it was never tested.

[Tim Graham]

​PR

[Tim Graham <timograham@…>]

In 99fc5dc:

Fixed #30141 -- Fixed parse_duration() for some negative durations.

[Tim Graham <timograham@…>]

In 99fc5dc:

Fixed #30141 -- Fixed parse_duration() for some negative durations.