For loop is only storing the last value in colum

Question

I am trying to pull the week number given a date and then add that week number to the corresponding row in a pandas/python dataframe.

When I run a for loop it is only storing the last calculated value instead of recording each value.

I've tried .append but haven't been able to get anything to work.

import datetime
from datetime import date

for i in df.index:
    
    week_number = date(df.year[i],df.month[i],df.day[i]).isocalendar()
    df['week'] = (week_number[1])

Expected values:

day month  year  week  
 8    12  2021    49  
19    12  2021    50  
26    12  2021    51  

Values I'm getting:
day month  year  week  
 8    12  2021    51  
19    12  2021    51  
26    12  2021    51  

Answer 1

You can simply use Pandas .apply method to make it a one-liner:

df["week"] = df.apply(lambda x: date(x.year, x.month, x.day).isocalendar()[1], axis=1)

Answer 2

You need to assign it back at the correct corresponding, i, position. Using loc, should help:

for i in df.index:
    week_number = date(df.year[i],df.month[i],df.day[i]).isocalendar()
    df.loc[i,'week'] = (week_number[1])

prints back:

print(df)

   day  month  year  week
0    8     12  2021    49
1   19     12  2021    50
2   26     12  2021    51