tell() tracks file position

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io.StringIO lets you treat a string as a file - useful for testing file-reading code without touching the filesystem. f.read(2) reads 'ab' and advances the pointer to position 2. f.tell() returns 2. The next f.read(2) reads the next two characters: 'cd', moving the pointer to 4. tell() and seek() work identically on StringIO and BytesIO as they do on real file objects. This is why StringIO is the standard way to write unit tests for functions that accept file-like objects - you can control position, rewind, and inspect the result without any disk I/O.