How to use the pypuppetdb.QueryBuilder.FunctionOperator function in pypuppetdb

def test_sum_function(self):
        assert str(FunctionOperator('sum', 'memoryfree_mb')) == \
            '["function", "sum", "memoryfree_mb"]'
        assert repr(FunctionOperator('sum', 'memoryfree_mb')) == \
            'Query: ["function", "sum", "memoryfree_mb"]'
        assert str(FunctionOperator('sum', 'memoryfree_mb')) == \
            '["function", "sum", "memoryfree_mb"]'

        with pytest.raises(APIError):
            FunctionOperator("sum")

def test_max_function(self):
        assert str(FunctionOperator('max', 'facterversion')) == \
            '["function", "max", "facterversion"]'
        assert repr(FunctionOperator('max', 'facterversion')) == \
            'Query: ["function", "max", "facterversion"]'
        assert str(FunctionOperator('max', 'facterversion')) == \
            '["function", "max", "facterversion"]'

        with pytest.raises(APIError):
            FunctionOperator("max")

def test_min_function(self):
        assert str(FunctionOperator('min', 'kernelversion')) == \
            '["function", "min", "kernelversion"]'
        assert repr(FunctionOperator('min', 'kernelversion')) == \
            'Query: ["function", "min", "kernelversion"]'
        assert str(FunctionOperator('min', 'kernelversion')) == \
            '["function", "min", "kernelversion"]'

        with pytest.raises(APIError):
            FunctionOperator("min")

def test_sum_function(self):
        assert str(FunctionOperator('sum', 'memoryfree_mb')) == \
            '["function", "sum", "memoryfree_mb"]'
        assert repr(FunctionOperator('sum', 'memoryfree_mb')) == \
            'Query: ["function", "sum", "memoryfree_mb"]'
        assert str(FunctionOperator('sum', 'memoryfree_mb')) == \
            '["function", "sum", "memoryfree_mb"]'

        with pytest.raises(APIError):
            FunctionOperator("sum")

def test_avg_function(self):
        assert str(FunctionOperator('avg', 'uptime')) == \
            '["function", "avg", "uptime"]'
        assert repr(FunctionOperator('avg', 'uptime')) == \
            'Query: ["function", "avg", "uptime"]'
        assert str(FunctionOperator('avg', 'uptime')) == \
            '["function", "avg", "uptime"]'

        with pytest.raises(APIError):
            FunctionOperator("avg")

def test_with_add_function_operator(self):
        op = ExtractOperator()

        op.add_field(FunctionOperator('to_string',
                                      'producer_timestamp',
                                      'FMDAY'))
        op.add_field(FunctionOperator('count'))
        op.add_group_by(FunctionOperator('to_string',
                                         'producer_timestamp',
                                         'FMDAY'))

        assert str(op) == '["extract", '\
            '[["function", "to_string", "producer_timestamp", "FMDAY"], '\
            '["function", "count"]], '\
            '["group_by", '\
            '["function", "to_string", "producer_timestamp", "FMDAY"]]]'
        assert repr(op) == 'Query: ["extract", '\
            '[["function", "to_string", "producer_timestamp", "FMDAY"], '\
            '["function", "count"]], '\
            '["group_by", '\
            '["function", "to_string", "producer_timestamp", "FMDAY"]]]'
        assert str(op) == '["extract", ' \
                          '[["function", "to_string", "producer_timestamp", "FMDAY"], '\

def test_with_add_function_operator(self):
        op = ExtractOperator()

        op.add_field(FunctionOperator('to_string',
                                      'producer_timestamp',
                                      'FMDAY'))
        op.add_field(FunctionOperator('count'))
        op.add_group_by(FunctionOperator('to_string',
                                         'producer_timestamp',
                                         'FMDAY'))

        assert str(op) == '["extract", '\
            '[["function", "to_string", "producer_timestamp", "FMDAY"], '\
            '["function", "count"]], '\
            '["group_by", '\
            '["function", "to_string", "producer_timestamp", "FMDAY"]]]'
        assert repr(op) == 'Query: ["extract", '\
            '[["function", "to_string", "producer_timestamp", "FMDAY"], '\
            '["function", "count"]], '\
            '["group_by", '\
            '["function", "to_string", "producer_timestamp", "FMDAY"]]]'
        assert str(op) == '["extract", ' \
                          '[["function", "to_string", "producer_timestamp", "FMDAY"], '\

def test_min_function(self):
        assert str(FunctionOperator('min', 'kernelversion')) == \
            '["function", "min", "kernelversion"]'
        assert repr(FunctionOperator('min', 'kernelversion')) == \
            'Query: ["function", "min", "kernelversion"]'
        assert str(FunctionOperator('min', 'kernelversion')) == \
            '["function", "min", "kernelversion"]'

        with pytest.raises(APIError):
            FunctionOperator("min")

def test_max_function(self):
        assert str(FunctionOperator('max', 'facterversion')) == \
            '["function", "max", "facterversion"]'
        assert repr(FunctionOperator('max', 'facterversion')) == \
            'Query: ["function", "max", "facterversion"]'
        assert str(FunctionOperator('max', 'facterversion')) == \
            '["function", "max", "facterversion"]'

        with pytest.raises(APIError):
            FunctionOperator("max")

def _build_query(env, start, end, certname=None):
    """Build a extract query with optional certname and environment."""
    query = ExtractOperator()
    query.add_field(FunctionOperator('count'))
    query.add_field('status')
    subquery = AndOperator()
    subquery.add(GreaterEqualOperator('producer_timestamp', start))
    subquery.add(LessOperator('producer_timestamp', end))
    if certname is not None:
        subquery.add(EqualsOperator('certname', certname))
    if env != '*':
        subquery.add(EqualsOperator('environment', env))
    query.add_query(subquery)
    query.add_group_by("status")
    return query