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In the analogous C operations, ftell (analogous to .tell) actually causes the underlying file descriptor's position (analogous to the raw stream's position) to be reset to be at the same value that ftell has returned. Which means, yes, that you lose the benefits of buffering if you're so foolish as to call ftell after every read. But in this case the sequence "read / tell / truncate" would be analogous to "fread(f) / ftell(f) / ftruncate(fileno(f))
Though, the fact that fread operates on the FILE * whereas truncate operates on a file descriptor serves as a red flag to C programmers... arguably since this is not the case with Python, truncate on a buffered stream should implicitly include this same "reset underlying position" operation before actually performing the truncate.