01·DataFrame and Series — the basics

Pitfall: df[df.age > 30]["score"] = 0 — what is wrong?

• Chained indexing — df[df.age > 30] may return a view or a copy, and the following ["score"] = 0 mutates that intermediate — often not the original df. Pandas emits SettingWithCopyWarning. Correct form: df.loc[df.age > 30, "score"] = 0.
• The > operator does not work on integers.
• The resulting DataFrame is sorted descending.
• Pandas does not support modifying many rows at once.