observe triggers twice when setting value with a tuple

[krey]

import ipywidgets as widgets
import ipysheet

output = widgets.Output()
table = ipysheet.Sheet(rows=1, columns=2)
cell = ipysheet.Cell(value=("", ""), row_start=0, row_end=0, column_start=0, column_end=1,
                     squeeze_column=False, squeeze_row=True)
table.cells = cell,

def observer(change):
    with output:
        print(change)

cell.observe(lambda change: observer(change.new), names='value')

cell.value = ('a', 'b')

output

prints

('a', 'b')
['a', 'b']

This is probably a side-effect of serialization.

A simple workaround is to use lists, but this is somewhat inconsistent: Sheet.cells is a Tuple so I figured Cell.value would be as well.