'''
использование двух менеджеров контекста одновременно
'''
# обычный способ
try:
    with open(source) as fin:
        with open(target, "w") as fout:
            for line in fin:
                fout.write(process(line))
except EnvironmentError as err:
    print(err)
# оптимизированный способ
source = "/tmp/source.txt"
target = "/tmp/target.txt"
try:
    with open(source) as fin, open(target, 'w') as fout:
        for line in fin:
            fout.write(line)
except EnvironmentError as err:
    print(err)

with open('source.txt', 'r') as a, open('target.txt', 'w') as b: