babel.numbers.format_decimal() and babel.numbers.format_currency() have a built-in banker rounding implemented. See:

If we need to localize numbers without rounding, we can't use these methods.

The minimal default decimal precision per method are:

• babel.numbers.format_decimal() => 3 digits after the dot
• babel.numbers.format_currency() => 2 digits after the dot

Proof:

>>> import babel
>>> set([babel.Locale.parse(l).decimal_formats._data[None].frac_prec
...      for l in babel.localedata.locale_identifiers()])
set([(0, 6), (0, 3)])
>>>

>>> import babel
>>> set([babel.Locale.parse(l).currency_formats._data[None].frac_prec
...      for l in babel.localedata.locale_identifiers()])
set([(2, 2)])
>>>

So if you have monetary amounts to localize, and they're already rounded to 2 trailing digits, then you're lucky. Using any of the format_*() methods will have no side effects.

But for other numbers with higher precision, using format_*() as-is is dangerous. It will introduce unwanted rounding. For example, calling format_currency(0.9999, 'EUR', locale='fr') will return 1,00 €. I expect here to get the pristine 0,9999 € string.

I think there must be a clean and documented way to bypass arbitrary rounding when localizing numbers.