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import pandas as pd
import datetime

def replace_week_start_date(date_str):
    date = datetime.datetime.strptime(str(date_str), "%Y%m%d")
    year = date.year
    month = date.month
    week_number = date.isocalendar()[1]
    
    if (month==1) and (week_number >= 52):  # 52 또는 53일 수 있음을 고려합니다.
        year -= 1
        week_number = 52 if week_number == 53 else week_number  # 53주차는 52주차로 변경
        start_date = datetime.datetime.strptime(f"{year}-W{week_number}-1", "%G-W%V-%u")
    else:
        start_date = datetime.datetime.strptime(f"{year}-W{week_number}-1", "%G-W%V-%u")
    
    return start_date


def replace_month_start_date(date_str):
    date = datetime.datetime.strptime(str(date_str), "%Y%m%d")
    year = date.year
    month = date.month

    start_date = datetime.datetime(year, month, 1)
    # start_date.strftime("%Y%m%d")
    return start_date

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# 예시 데이터 프레임 생성
data = {
    'YYYYMMDD': ['20200102', '20210104', '20210105','20210330']
}

df = pd.DataFrame(data)

# apply 함수를 사용하여 대체된 값을 적용
df['YYYYWK']= df['YYYYMMDD'].apply(replace_week_start_date)
df['YYYYMM']= df['YYYYMMDD'].apply(replace_month_start_date)

print(df)

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   YYYYMMDD     YYYYWK     YYYYMM
0  20200102 2019-12-30 2020-01-01
1  20210104 2021-01-04 2021-01-01
2  20210105 2021-01-04 2021-01-01
3  20210330 2021-03-29 2021-03-01