在Python中'适当'舍入到小数点后3位

I may be missing something essential, but I can't figure out a way to 'properly' round floats/Decimals in Python (2.7), at least to three decimal places. By 'properly' I mean that 1.2225 should round to 1.223, and 1.2224 should round to 1.222. I may be missing something essential, but I can