在 python 中将 YYYYDDMM 更改为 YYYYMMDD

Question

I have a df with dates in a column converted to a datetime. the current format is YYYYDDMM. I need this converted to YYYYMMDD. I tried the below code but it does not change the format and still gives me YYYYDDMM. the end goal is to subtract 1 business day from the effective date but the format needs to be in YYYYMMDD to do this otherwise it subtracts 1 day from the M and not D. can someone help?

filtered_df['Effective Date'] = pd.to_datetime(filtered_df['Effective Date'])
# Effective Date = 20220408 (4th Aug 2022 for clarity) 
filtered_df['Effective Date new'] = filtered_df['Effective Date'].dt.strftime("%Y%m%d")
# Effective Date new = 20220408

desired output -- > Effective Date new = 20220804

Answer 1

I like to use the datetime library for this purpose. You can use strptime to convert a string into the datetime object and strftime to convert your datetime object to the new string.

from datetime import datetime
def change_date(row):
    row["Effective Date new"] = datetime.strptime(row["Effective Date"], "%Y%d%m").strftime("%Y%m%d")
    return row
df2 = df.apply(change_date, axis=1)

The output df2 will have Effective Date new as your new column.

Answer 2

By default, .to_datetime will interpret the input YYYYDDMM as YYYYMMDD, and therefore print the same thing with %Y%m%d as the format. You can fix this and make it properly parse days in the month greater than 12 by adding the dayfirst keyword argument.

filtered_df['Effective Date'] = pd.to_datetime(filtered_df['Effective Date'], dayfirst=True)