[Solved] How can I make the efficent for loop in Python?

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Assuming s.subjects is None or some other False value when there are no subjects, and likewise for books

for s in students:
    for subject in s.subjects or []:
        for book in subject.books or []:
            writer.writerow(s.name, s.class, subject.name, book.name)

More generally, you can write

for s in students:
    for subject in s.subjects if <condition> else []:
        for book in subject.books if <condition> else []:
            writer.writerow(s.name, s.class, subject.name, book.name)

Where <condition> is whatever expression makes sense

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solved How can I make the efficent for loop in Python?