What causes SettingWithCopyWarning?

It appears when you assign into something that may be a view of another DataFrame rather than an independent object — typically after filtering.

subset = df[df['region'] == 'East'] followed by subset['flag'] = 1 triggers it, because pandas cannot tell whether the assignment affects df as well.

Two fixes: add .copy() when you intend an independent object, or assign directly into the original with df.loc[df['region'] == 'East', 'flag'] = 1.

It is a warning rather than an error, which is precisely why it is worth understanding — ignoring it means your assignment may silently not do what you expect.