'Assign the value for a field.'

def __setitem__(self, key, value):

    if (key in self._fields):
        source = self._values_fixed
    else:
        source = self._values_flex
    value = self._type(key).normalize(value)
    old_value = source.get(key)
    source[key] = value
    if (self._always_dirty or (old_value != value)):
        self._dirty.add(key)
