Pandas dataframe 在每月的第 n 天重新采样日期时间索引

Question

Imagine the ohlc data indexed in DateTime. I am going to resample this dataframe on every nth day of the month.

for example:

.
.
.
2020-09-24  1.0990  1.1000  1.0982  1.0991
2020-09-25  1.1018  1.1025  1.0964  1.0995
2020-09-26  1.1011  1.1020  1.1009  1.1018
.
.
.
2020-10-24  1.1045  1.1068  1.0995  1.1017
2020-10-25  1.1031  1.1074  1.1021  1.1045
2020-10-26  1.1071  1.1076  1.1012  1.1031
.
.
.
2020-11-23  1.1005  1.1075  1.0989  1.1071
2020-11-26  1.1079  1.1086  1.0992  1.1005
2020-11-27  1.1076  1.1087  1.1068  1.1079
.
.
.
2020-12-24  1.1058  1.1110  1.1054  1.1071
2020-12-25  1.1010  1.1087  1.0926  1.1058
2020-12-26  1.1049  1.1056  1.0983  1.1010
.
.
.
2021-01-24  1.1049  1.1059  1.1029  1.1048
2021-01-25  1.1025  1.1068  1.1014  1.1049
2021-01-26  1.1025  1.1028  1.1022  1.1025

what I need is:

2020-09-25  1.1018  1.1025  1.0964  1.0995
2020-10-25  1.1031  1.1074  1.1021  1.1045
2020-11-25  1.1005  1.1075  1.0989  1.1071
2020-12-25  1.1010  1.1087  1.0926  1.1058
2021-01-25  1.1025  1.1068  1.1014  1.1049

In fact, I need to resample on every 25th day of the month and if there were no data for it, it must be filled with the nearest previous data.

Answer 1

df[df.groupby(df['Date']+df['Date'].apply(lambda x: pd.DateOffset(days=25-x.day) if x.day<=25 else pd.DateOffset(days=25-x.day,months=1)))['Date'].transform(max)==df['Date']]

Answer 2

Simplest solution would be as follows,

#Sort the dataframe
df = df.sort_values('date')

#Use ffill to fill nearst previous value (by timestamp) for Null elements
df = df.ffill(axis=0)

#Simpelly query by day
df.loc[df['date'].dt.day == float('25')]

Output:

date    a   b   c   d
1   2020-09-25  1.1018  1.1025  1.0964  1.0995
4   2020-10-25  1.1031  1.1074  1.1021  1.1045
10  2020-12-25  1.1010  1.1087  1.0926  1.1058
13  2021-01-25  1.1025  1.1068  1.1014  1.1049