Why does str(float) return more digits in Python 3 than Python 2?
In Python 2.7, repr of a float returns the nearest decimal number up to 17 digits long; this is precise enough to uniquely identify each possible IEEE floating point value. str of a float worked similarly, except that it limited the result to 12 digits; for most purposes this is a more reasonable result, and insulates you from the slight differences between binary and decimal representation.