'Returns a double value from a v1 Value.
Args:
v1_value: an googledatastore.Value
Returns:
a double
Raises:
InvalidConversionError: if the value doesn\'t contain a double value'

def __get_v1_double_value(self, v1_value):

    check_conversion(v1_value.HasField('double_value'), 'Value    does    not    contain    a    double    value.')
    return v1_value.double_value
