open() modes: r, w, and a

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'r' (read) is the default mode. The file must already exist or you get FileNotFoundError. 'w' (write) is the destructive one - if the file exists it is immediately truncated to zero bytes before you write a single character. If the file doesn't exist, Python creates it. 'a' (append) is the safe write mode: existing content is preserved and the file pointer starts at the end, so every write adds to the bottom. A quick way to remember: 'w' wipes, 'a' adds. If you ever lose data by accidentally using 'w' when you meant 'a', you'll remember the difference permanently.