seek(0) after consuming the file

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Every file object maintains an internal position pointer. After f.read() consumes the whole file, the pointer sits at the end. A second f.read() at that position returns an empty string - there's nothing left to read. f.seek(0) moves the pointer back to byte 0 (the start), so the second f.read() sees the full file again and returns 'hello'. This is the seek/tell pattern. f.tell() returns the current byte position; f.seek(n) moves to byte n; f.seek(0, 2) moves to the end. Where does this matter in practice? Anywhere you need to read a file twice without reopening it - for example, sniffing the first few bytes to detect a file format, then rewinding to parse the whole thing. It also shows up in testing when you create an in-memory file with io.BytesIO() or io.StringIO() and need to rewind before reading back what you wrote.