How floor a date to the first date of that month?

Question

I have a pandas DataFrame with index column = date.

Input:

            value date     1986-01-31  22.93 1986-02-28  15.46 

I want to floor the date to the first day of that month

Output:

            value date     1986-01-01  22.93 1986-02-01  15.46 

What I tried:

df.index.floor('M') ValueError: <MonthEnd> is a non-fixed frequency 

This is potentially because the df is generated by df = df.resample("M").sum() (The output of this code is the input at the beginning of the question)

I also tried df = df.resample("M", convention='start').sum(). However, it does not work.

I know in R, it is easy to just call floor(date, 'M').

Answer 1

there is a pandas issue about the floor problem

the suggested way is

import pandas as pd pd.to_datetime(df.date).dt.to_period('M').dt.to_timestamp()