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When you write pi.n(digits=3), you do not get the rational number 314/100. What you get is a floating point number $x$ with a guarantee that $|\pi - x| \le 0.01$ (I believe this number also "knows" how many digits you care about; that's why print shows exactly 3 digits). Because floating point numbers are internally stored in base 2, it is impossible to represent 314/100 exactly.