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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def calculate_variable(self, variable = None, period = None, use_baseline = False):
 """ Compute and return the variable values for period and baseline or reform tax_benefit_system """

        if use_baseline:
            assert self.baseline_simulation is not None, "self.baseline_simulation is None"
            simulation = self.baseline_simulation
        else:
            assert self.simulation is not None
            simulation = self.simulation

        tax_benefit_system = simulation.tax_benefit_system

        assert period is not None
        if not isinstance(period, periods.Period):
            period = periods.period(period)
        assert simulation is not None
        assert tax_benefit_system is not None

        assert variable in tax_benefit_system.variables, "{} is not a valid variable".format(variable)
        period_size_independent = tax_benefit_system.get_variable(variable).is_period_size_independent
        definition_period = tax_benefit_system.get_variable(variable).definition_period

        if period_size_independent is False and definition_period != u'eternity':
            values = simulation.calculate_add(variable, period = period)
        elif period_size_independent is True and definition_period == u'month' and period.size_in_months > 1:
            values = simulation.calculate(variable, period = period.first_month)
        elif period_size_independent is True and definition_period == u'month' and period.size_in_months == 1:
            values = simulation.calculate(variable, period = period)
        elif period_size_independent is True and definition_period == u'year' and period.size_in_months > 12:
            values = simulation.calculate(variable, period = period.start.offset('first-of', 'year').period('year'))
        elif period_size_independent is True and definition_period == u'year' and period.size_in_months == 12:
            values = simulation.calculate(variable, period = period)
        elif period_size_independent is True and definition_period == u'year':
            values = simulation.calculate(variable, period = period.this_year)
        elif definition_period == u'eternity':
            values = simulation.calculate(variable, period = period)
        else:
            values = None
        assert values is not None, 'Unspecified calculation period for variable {}'.format(variable)

        return values