Undesirable / Incorrect behaviour TimeContainer

[UnravelSports]

The TimeContainer was added to store player positions, however it seems that (haven’t tested it with other data) when loading the open skillcorner data default match, I sometimes get an “empty” TimeContainer, something like this:

TimeContainer[Position]({'P1T00:00': Position(position_id=3, name='Left Center Back', coordinates=None)})
TimeContainer[Position]({'P1T00:00': Position(position_id=9, name='Left Midfield', coordinates=None)})
TimeContainer[Position]({'P1T00:00': Position(position_id=4, name='Right Center Back', coordinates=None)})
<TimeContainer>
<TimeContainer>
TimeContainer[Position]({'P1T00:00': Position(position_id=4, name='Right Center Back', coordinates=None)})
TimeContainer[Position]({'P1T00:00': Position(position_id=10, name='Right Midfield', coordinates=None)})
TimeContainer[Position]({'P1T00:00': Position(position_id=8, name='Center Midfield', coordinates=None)})

I don't think this should happen, if only because if I now try to call player.positions.at_start() it throws a KeyError because it has no self.items. Why are we raising a KeyError. It should instead be something like Position.NOT_SET (assuming this is the reason the TimeContainer is empty).

def at_start(self):
        """Return the value at the beginning of the match"""
        if not self.items:
            raise KeyError

        first_item: Time = self.items.keys()[0]

        tmp_period = first_item.period
        while tmp_period.prev_period:
            tmp_period = tmp_period.prev_period

        return self.value_at(Time.from_period(tmp_period, "start"))

Above is the code that raises the issue. Below is an MRE.

from kloppy import skillcorner

dataset = skillcorner.load_open_data(
    coordinates="secondspectrum", 
    include_empty_frames=False,
    limit=10
)

for frame in dataset:
    for pid in frame.players_data:
        print(pid.positions)