Strange behavior of datetime rounding between python 2 & 3

Question

In python 2, we have:

>>> datetime.datetime.utcfromtimestamp(1000000000005.0/1000.0)
datetime.datetime(2001, 9, 9, 1, 46, 40, 5000)

But in python 3, we have:

>>> datetime.datetime.utcfromtimestamp(1000000000005.0/1000.0)
datetime.datetime(2001, 9, 9, 1, 46, 40, 4999)

What is the cause of this strange rounding behavior and is it intended? Isn't 1000000000005.0 still within the range of doubles with a few digits to spare?

Answer 1

Below I have essentially included utcfromtimestamp (I modified it slightly to standalone).

In Python 2:

import time, datetime
def utcfromtimestamp(t):
    y, m, d, hh, mm, ss, weekday, jday, dst = time.gmtime(t)
    us = int((t % 1.0) * 1000000)
    ss = min(ss, 59)
    return datetime.datetime(y, m, d, hh, mm, ss, us)

In Python 3:

import time, datetime
def utcfromtimestamp(t):
    t, frac = divmod(t, 1.0)
    us = int(frac * 1e6)
    if us == 1000000:
        t += 1
        us = 0
    y, m, d, hh, mm, ss, weekday, jday, dst = time.gmtime(t)
    ss = min(ss, 59)
    return datetime.datetime(y, m, d, hh, mm, ss, us)

(The input 1000000000005.0/1000.0 is calculated to 1000000000.005.)

In my standalone version:

Python 2 uses the modulus operator % to determine if the input is a whole number or a fraction. The statement (t % 1.0) * 1000000 then multiplies the fraction (in our case 0.004999995231628418) by 1000000. This returns 4999.995231628418, which is rounded down to 4999 by int.

Python 3 uses divmod to return the whole number (t) 1000000000.0 and the fraction (frac) 0.005. Instead of returning this, it returns t as 1000000000 and frac as 0.004999995231628418. It then goes to calculating us using frac * 1e6. This is multiplying your 0.004999995231628418 by 1000000 resulting in 4999.995231628418, which is rounded down to 4999 by int.

There is no real difference in the methods used. Both are accurate and return the same result. My conclusion is that Python 2 is rounding microseconds up, while Python 3 is rounding them down.