posix_time.py

#!/usr/bin/env python
"""Test POSIX definition of "seconds since Epoch".

Assert that posix_time(), calendar.timegm(), and timegm() (mktime() in
UTC) behave the same. Check `datetime.timestamp()` and the explicit
formula (d - epoch) too.

"""
import calendar
import itertools
import os
import time
from contextlib import contextmanager
from datetime import datetime, date, time as datetime_time

try:
    from datetime import timezone
except ImportError:
    timezone = None


def posix_time(utc_time_tuple):
    """seconds since Epoch as defined by POSIX."""
    tm_year = utc_time_tuple.tm_year - 1900
    tm_yday = utc_time_tuple.tm_yday - 1
    tm_hour = utc_time_tuple.tm_hour
    tm_min = utc_time_tuple.tm_min
    tm_sec = utc_time_tuple.tm_sec
    # http://pubs.opengroup.org/stage7tc1/basedefs/V1_chap04.html#tag_04_15
    return (tm_sec + tm_min*60 + tm_hour*3600 + tm_yday*86400 +
            (tm_year-70)*31536000 + ((tm_year-69)//4)*86400 -
            ((tm_year-1)//100)*86400 + ((tm_year+299)//400)*86400)


@contextmanager
def local_timezone(zonename):
    #NOTE: it manipulates global state
    oldname = os.environ.get('TZ')
    try:
        os.environ['TZ'] = zonename
        time.tzset()
        yield
    finally:
        if oldname is None:
           del os.environ['TZ']
        else:
           os.environ['TZ'] = oldname
        time.tzset()


def timegm(utc_time_tuple):
    with local_timezone('UTC'):
        return time.mktime(utc_time_tuple)

def eq(a, b, err):
    return abs(a - b) < err

if __name__ == "__main__":
    seconds_since_epoch = time.time() #NOTE: assume POSIX epoch (fail with TZ=right/UTC)
    tt = time.gmtime(seconds_since_epoch)

    print(posix_time(tt))

    assert posix_time(tt) == int(seconds_since_epoch)
    assert calendar.timegm(tt) == int(seconds_since_epoch)
    assert timegm(tt) == int(seconds_since_epoch)
    if timezone:
        aware_dt = datetime.fromtimestamp(seconds_since_epoch, timezone.utc)
        assert eq(aware_dt.timestamp(), seconds_since_epoch, 1e-6)

    for year, month in itertools.product(range(1970, 1970+401), range(1, 13)):
        utc_d = date(year, month, 1)
        seconds_since_epoch = (utc_d - date(1970, 1, 1)).days*86400
        tt = utc_d.timetuple()
        assert posix_time(tt) == seconds_since_epoch
        assert calendar.timegm(tt) == seconds_since_epoch
        assert timegm(tt) == seconds_since_epoch
        if timezone:
            aware_dt = datetime.combine(utc_d, datetime_time(tzinfo=timezone.utc))
            assert aware_dt.timestamp() == seconds_since_epoch