Demo code written to help @Zac-HD develop improved datetime strategies for Hypothesis.

datetime_predicates.py

# Released under MPL 2.0 per https://github.com/HypothesisWorks/hypothesis/blob/master/LICENSE.txt
# and CC-0: https://creativecommons.org/publicdomain/zero/1.0/legalcode
#
# Feel free to contact Paul Ganssle if you'd like this released under other terms.

from datetime import datetime, timedelta, timezone
from dateutil import tz

ZERO = timedelta(0)


def _weird_offset(offset, otype):
    weirdnesses = set()
    if offset is not None and offset % timedelta(hours=1) != timedelta(0):
        weirdnesses.add(f"Non-integer {otype} hours")

        if offset % timedelta(minutes=1) != timedelta(0):
            weirdnesses.add(f"Non-integer {otype} minutes")

            if offset % timedelta(seconds=1) != timedelta(0):
                weirdnesses.add(f"Non-integer {otype} seconds")

    return weirdnesses


def datetime_weird(dt):
    weirdnesses = set()
    if tz.datetime_ambiguous(dt):
        weirdnesses.add("Ambiguous")

    if not tz.datetime_exists(dt):
        weirdnesses.add("Imaginary")

    weirdnesses |= _weird_offset(dt.utcoffset(), "utcoffset")
    weirdnesses |= _weird_offset(dt.dst(), "dst")

    tzname = dt.tzname()

    if tzname is not None:
        if len(tzname) != 3:
            weirdnesses.add("tzname length != 3")

        if not tzname.isalpha():
            weirdnesses.add("Non-alphabetic tzname")

        if not tzname.isascii():
            weirdnesses.add("Non-ASCII tzname")

    return weirdnesses


def transition_occurred(dt1, dt2):
    if dt1.utcoffset() != dt2.utcoffset():
        return True

    if dt1.dst() != dt2.dst():
        return True

    if dt1.tzname() != dt2.tzname():
        return True

    return False


NYC = tz.gettz("America/New_York")

def test_transitions():
    occurred = [
        (datetime(2019, 3, 9, tzinfo=NYC), datetime(2019, 3, 11, tzinfo=NYC)),
        (datetime(2019, 11, 2, tzinfo=NYC), datetime(2019, 11, 4, tzinfo=NYC)),
    ]

    for dt1, dt2 in occurred:
        assert transition_occurred(dt1, dt2), (dt1, dt2)

    didnt_occur = [
        (datetime(2019, 3, 1, tzinfo=NYC), datetime(2019, 3, 2, tzinfo=NYC)),
        (datetime(2019, 10, 1, tzinfo=NYC), datetime(2019, 10, 30, tzinfo=NYC)),
    ]

    for dt1, dt2 in didnt_occur:
        assert not transition_occurred(dt1, dt2), (dt1, dt2)

    print("Passed!")


def test_weirdness():
    cases = [
        (datetime(2019, 11, 3, 1, 30, tzinfo=NYC),
         {"Ambiguous"}),
        (datetime(2019, 3, 10, 2, 30, tzinfo=NYC),
         {"Imaginary"}),
        (datetime(1969, 1, 1, tzinfo=tz.gettz("Africa/Monrovia")),
         {"Non-integer utcoffset hours", "Non-integer utcoffset minutes"}),
        (datetime(2014, 1, 1, tzinfo=tz.gettz("America/Sao_Paulo")),
         {"Non-alphabetic tzname"}),
        (datetime(2014, 1, 1, tzinfo=tz.gettz("America/Sao_Paulo")),
         {"Non-alphabetic tzname"}),
        (datetime(2014, 1, 1, tzinfo=tz.gettz("Australia/Sydney")),
         {"tzname length != 3"}),
        (datetime(2014, 1, 1, tzinfo=timezone(timedelta(hours=1), "🐼")),
         {"Non-alphabetic tzname", "tzname length != 3", "Non-ASCII tzname"}),
    ]

    for dt, exp in cases:
        act = datetime_weird(dt)
        assert act == exp, f"{act} != {exp}"

    print("Passed")


print("Test weirdness:")
test_weirdness()

print("Test transitions:")
test_transitions()