[docs] [issue17259] Document round half to even rule for floats

[Mark Dickinson]

Mark Dickinson added the comment:

Indeed, in Python 3, round and new-style string formatting both do round-ties-to-even, by design.  Old-style formatting does whatever the underlying OS does, which is typically round-half-away-from-zero.

Python 2 is a bit more of a mess:  in 2.7, new-style formatting does round-ties-to-even, round does round-half-away-from-zero, and old-style formatting continues to do whatever the OS does, just as in Python 3.  And 2.6 is different again (and much more system dependent).

Agreed that this could be better documented for 'format'.  The documentation for the round function is already explicit on this, at least for Python 3.

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<http://bugs.python.org/issue17259>
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