This is coming from #2566, which noted that the xstrtod() method used by read_csv doesn't agree with standard numpy float conversion. I see that the priority is speed of parsing over complete accuracy (within 0.5 units in the last place, or ULP), but there are two issues here that (at least to me) actually appear to be buggy:
xstrtod() is off by more than 1 ULP, although these don't come up often.strtod(), but the error in conversion increases linearly as the number of supplied significant figures increases. With 30 significant figures, the error in conversion can potentially be over 7 ULP.Here is an IPython notebook analyzing the accuracy of xstrtod(). I think there are two problems here: xstrtod() keeps reading digits after the 17th, none of which should matter for conversion, and the scaling step at the end produces a compounded error by repeatedly multiplying/dividing by powers of 10. I have a solution for AstroPy that seems to fix these issues, so I can open a PR if it's agreed that xstrtod() should be changed.
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