Existing code was producing invalid results:
a = 9876543212346789864323456.9543234578986432345678
>>> print "{:.13}".format(repr(a))
9.87654321234
>>> a
9.87654321234679e+24
The new converision should work fine:
>>> print "{:.13E}".format(a)
9.8765432123468E+24
Reported & fixed by ulrich1a