parse_duration returns None if passed a string with a negative number of seconds

[Filipe L B Correia]

For instance, parse_duration('172800') correctly returns timedelta(2) (2 days), while parse_duration('-172800') incorrectly returns None instead of timedelta(-2) (-2 days).

This particularly affects the DurationField in SQLite3, since it parse_duration is used to read the value from the database, and a negative duration is stored as a negative number.

I would suggest adding an optional minus sign in to the seconds pattern (maybe also for the hours and minutes?), as there is for the days pattern:

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'$'
)

Or have an alternative pattern:

negative_seconds_duration_re = re.compile(
    r'^'
    r'(?P<seconds>-\d+)'
    r'$'
)

If the current behavior is indeed the intended behavior for the parse_duration function, then one should fix the db.backends.base.operations.convert_durationfield_value function to handle a negative value for the DurationField.

[Jinank Jain]

I have added on PR could someone please review it. ​https://github.com/django/django/pull/7814

[Tim Graham]

Another ​PR from hardik1997. Jinank, for future reference, please observe if a ticket is assigned to prevent duplicate work. If a ticket goes without activity for weeks, feel free to reassign.

[Claude Paroz <claude@…>]

In f4c0eec7:

Fixed #27699 -- Added negative timedelta support to parse_duration()