Yes. I'm fairly convinced about the bug part; it's the gratuitous breakage part that worries me. It wouldn't surprise me at all to find that there's someone out there who wants all their numbers to be written out to JSON with exactly 6 places after the point, and subclasses float just for that purpose. (I'm not for a moment suggesting that this is a good idea, but there's a big difference between the set of Python code that *should* have been written and the set of Python code that *has* been written. :-)

>>> class MyFloat(float):
...     def __repr__(self):
...         return '{:.6f}'.format(self)
... 
>>> import math, json
>>> json.dumps(MyFloat(math.pi))
'3.141593'