[Python] datetime object - naive vs aware

Naive object vs Aware object

• datetime tzinfo: None -> naive object

from datetime import datetime


dt_naive = datetime(2020, 3, 1)  # naive object
print(dt_naive.tzinfo)

None

• datetime tzinfo 존재 -> aware object

from datetime import datetime


dt_naive = datetime(2020, 3, 1)  # naive object
dt_aware_kst = pytz.timezone("Asia/Seoul").localize(dt_naive)  # aware object
print(dt_aware_kst.tzinfo)

Asia/Seoul

Casting naive object to aware object

• replace 로 naive 객체를 “Asia/Seoul” timezone 을 가진 aware 객체로 변경시,
  KST 가 아닌 LMT 기준이 된다.
  일반적으로 KST를 사용하므로, pytz 의 localize 를 사용 권장

import pytz
from datetime import datetime


# Naive object(=> No timezone)
dt_naive = datetime(2020, 3, 1)  # naive object

# Aware object(=> have timezone)
## naive -> aware (using pytz localize) => Recommended
dt_aware_kst = pytz.timezone("Asia/Seoul").localize(dt_naive)
## naive -> aware (using replace)
dt_aware_lmt = dt_naive.replace(tzinfo=pytz.timezone("Asia/Seoul"))

print(f"{dt_aware_kst=}")
print(f"{dt_aware_lmt=}")

dt_aware_kst=datetime.datetime(2020, 3, 1, 0, 0, tzinfo=<DstTzInfo 'Asia/Seoul' KST+9:00:00 STD>)
dt_aware_lmt=datetime.datetime(2020, 3, 1, 0, 0, tzinfo=<DstTzInfo 'Asia/Seoul' LMT+8:28:00 STD>)

Converting timezone

naive 객체에 astimezone 적용시, runtime 의 기본 timezone 을 기준으로 변경된다.
즉, runtime 환경에 따라 값이 달라질 수 있다.
따라서 timezone이 명시적인 aware 객체로 변환 후 timezone 변경을 권장한다.

import pytz
import time
from datetime import datetime


# Naive object
dt_naive = datetime(2020, 3, 1)  # naive object
dt_naive_to_utc = dt_naive.astimezone(pytz.timezone("UTC")) # !Notice

# Aware object
## naive -> aware (using pytz localize)
dt_aware_kst = pytz.timezone("Asia/Seoul").localize(dt_naive)
## KST -> UTC
dt_aware_utc = dt_aware_kst.astimezone(pytz.timezone("UTC"))

print(f"Default timezone: {time.tzname}")
print(f"{dt_naive_to_utc=}")
print(f"{dt_aware_kst=}")
print(f"{dt_aware_utc=}")

Default timezone: ('KST', 'KST')
dt_naive_to_utc=datetime.datetime(2020, 2, 29, 15, 0, tzinfo=<UTC>)
dt_aware_kst=datetime.datetime(2020, 3, 1, 0, 0, tzinfo=<DstTzInfo 'Asia/Seoul' KST+9:00:00 STD>)
dt_aware_utc=datetime.datetime(2020, 2, 29, 15, 0, tzinfo=<UTC>)

• runtime 의 기본 timezone 이 “Asia/Seoul” 이기 때문에 dt_naive_to_utc이 -9 hours 가 된 것을 확인할 수 있다.

Comparison

datetime 객체의 대소비교는 동일한 종류의 객체끼리만 가능하다.

• ex) naive <= naive: Bool (가능)
• ex) aware <= aware: Bool (가능)
• ex) naive <= aware: Error (불가능)

Same objects

import pytz
from datetime import datetime


naive1 = datetime(2020, 3, 1)  # naive object
naive2 = datetime(2020, 4, 1)  # naive object
print(dt1 < dt2)

aware1 = pytz.timezone("Asia/Seoul").localize(naive1)  # aware object
aware2 = pytz.timezone("UTC").localize(naive2)  # aware object
print(aware1 < aware2)

True
True

Naive & Aware

import pytz
from datetime import datetime


naive = datetime(2020, 3, 1)  # naive object
aware = pytz.timezone("Asia/Seoul").localize(naive)  # aware object
print(naive < aware)

TypeError: can't compare offset-naive and offset-aware datetimes