Собрать файлы в один

with open('Out2018.csv', 'w') as target:
    with open('sales.csv', 'r', encoding='utf8') as current:
        for i in current:
            a = readline(i)
            target.write(a, encoding='utf8')

with open('Out2018.csv', 'w', encoding='utf8') as target:
    with open('sales1.csv', 'r', encoding='utf8') as current:
        for i in current:
            target.write(i)
    with open('sales2.csv', 'r', encoding='utf8') as current:
        a = 0
        for i in current:
            if a == 0:
                a += 1
            else:
                target.write(i)

with open('Out2018.csv', 'w', encoding='utf8') as target:
    with open('sales01.csv', 'r', encoding='utf8') as current:
        for i in current:
            target.write(i)
    with open('sales02.csv', 'r', encoding='utf8') as current:
        a = 0
        for i in current: 
            if a == 0:
                a += 1
            else:
                target.write(i)
......