Help us understand the problem. What is going on with this article?

ファイルを読み込んで書き込む

test.txtというテキストファイルの中身が、
AAA
BBB
CCC
DDD
である場合に

s = """\
AAA
BBB
DDD
DDD
"""
with open ('test.txt', 'r+') as f:
    print(f.read())
    f.seek(0)
    f.write(s)
実行結果
AAA
BBB
CCC
DDD

でtest.txtというテキストファイルの中身は、
AAA
BBB
DDD
DDD
になる。

r+は読み込み+書き込み。
なので、
もしも、test.txtがないと、
読み込みができず、
エラーとなる。

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