polars python change date

Question

I have this dataframe made on the polars library of python :

from datetime import date
import polars as pl

my_df = pl.DataFrame({
    'Col1': [1, 1, 1, 1, 2, 2, 3, 3],
    'Col2': [date(2020, 1, 31), date(2020, 1, 31), date(2020, 1, 31),
             date(2020, 1, 31), date(2020, 1, 31), date(2020, 1, 31),
             date(2020, 1, 31), date(2020, 1, 31)],
})

I want the date in field 'col2' to change month every time the value in field 'col1' changes.

Could you please help me?

Thank you in advance for your answers.

Answer 1

Note that month=2 and 3 doesn't contain 31 days. Here's working script -

import polars as pl
import numpy as np

from datetime import date


df = pl.DataFrame({
    'Col1': [1, 1, 1, 1, 2, 2, 3, 3],
    'Col2': [date(2020, 1, 25), date(2020, 1, 25), date(2020, 1, 25),
             date(2020, 1, 25), date(2020, 1, 25), date(2020, 1, 25),
             date(2020, 1, 25), date(2020, 1, 25)],
})

df = df.with_columns(pl.Series('Col2_updated', list(map(lambda col1, col2: col2.replace(month=col1), df['Col1'], df['Col2']))))

print(df)

Output -