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

Answer 1

dt_1 = "2016-02-01"
def first_day(dt):
    lt_split = dt.split("-")
    return "-".join([lt_split[0], lt_split[1], "01"])

print first_day(dt_1)

For Panda's DataFrame, you can use dt["col_name_date"].apply(first_day).

Answer 2

Here's another 'pandonic' way to do it:

df.date - pd.Timedelta('1 day') * (df.date.dt.day - 1)