Christmas Bird Count history report system

An instance of this class represents the second heading row, showing the total party-hours of effort for each column (where known). For general remarks about the design of this class, see Section 40, “class NumberCell: Base class for numeric table cells”.

# - - - - -   c l a s s   P a r t y H o u r s C e l l

class PartyHoursCell(NumberCell):
    '''Displays a total party-hours of effort in a heading cell.

      Exports:
        PartyHoursCell(value):
          [ if value is None ->
              return a new PartyHoursCell with no value
            else ->
              return a new PartyHoursCell with value (value) ]
        .add(value):
          [ value is a float ->
              self  :=  self with value added to its value ]

      State/Invariants:
        ._total:
          [ if no values have been added -> None
            else -> sum of all values added to self so far ]
    '''
    __slots__ = ('_total')