[Python] datetime: 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

import pytz

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)
print(dt_aware_kst)

Asia/Seoul
2020-03-01 00:00:00+09:00

Timezone Assigment to Naive Object

Naive object to Aware object

replace 로 naive 객체를 Asia/Seoul timezone 을 가진 aware 객체로 변경시 KST 가 아닌 LMT 기준이 된다.

from datetime import datetime

import pytz

# 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) => LMT(Not KST)
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>)

Timezone Conversion

naive 객체에 astimezone 적용시, runtime 의 기본 timezone 기준으로 변경된다.
(Runtime 환경에 따라 값이 달라질 수 있다.)

timezone 변경이 필요한 경우, timezone 이 명시적인 aware 객체로 변환 후, 새로운 timezone 변경하는 것을 권장

import time
from datetime import datetime

import pytz

# 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_utc 이 -9 hours 가 된 것을 확인할 수 있다.

Comparison of datetime

datetime 객체의 대소비교는 동일한 종류의 객체끼리만 가능하다.

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

Case 1. Same objects

from datetime import datetime

import pytz

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

Case 2. Naive & Aware

from datetime import datetime

import pytz

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