Get the first day of the week for a Pandas series

Question

I have the following df :

import pandas as pd
from datetime import datetime, timedelta

df = pd.DataFrame([
        ["A", "2018-08-03"],
        ["B", "2018-08-20"]
])
df.columns = ["Item", "Date"]

I want to get the first day of the week for every line of my df. I tried to do this :

df['Date'] =  pd.to_datetime(df['Date'], format='%Y-%m-%d')
df["Day_of_Week"] = df.Date.dt.weekday

df["First_day_of_the_week"] = df.Date - timedelta(days=df.Day_of_Week)

But I got that error message :

TypeError: unsupported type for timedelta days component: Series

How can I get the first day of the week for a Series ? My expected result is that :

• "A", "2018-08-03", "2018-07-30"
• "B", "2018-08-20", "2018-08-20"

Answer 1

A vectorised solution is possible with NumPy:

df['First_day'] = df['Date'] - df['Date'].dt.weekday * np.timedelta64(1, 'D')

print(df)

  Item       Date  First_day
0    A 2018-08-03 2018-07-30
1    B 2018-08-20 2018-08-20