Resample Daily Data to Monthly with Pandas (date formatting)

Question

I am trying to resample some data from daily to monthly in a Pandas DataFrame. I am new to pandas and maybe I need to format the date and time first before I can do this, but I am not finding a good tutorial out there on the correct way to work with imported time series data. Everything I find is automatically importing data from Yahoo or Quandl.

Here is what I have in my DataFrame: dataframe segment screenshot

Here is the code I used to create my DataFrame:

#Import excel file into a Pandas DataFrame
df = pd.read_excel(open('2016_forex_daily_returns.xlsx','rb'), sheetname='Sheet 1')

#Calculate the daily returns
df['daily_ret'] = df['Equity'].pct_change()

# Assume an average annual risk-free rate over the period of 5%
df['excess_daily_ret'] = df['daily_ret'] - 0.05/252

Can someone help me understand what I need to do with the "Date" and "Time" columns in my DataFrame so I can resample?

Answer 1

For create DataFrame is possible use:

df = pd.read_excel('2016_forex_daily_returns.xlsx', sheetname='Sheet 1')
print (df)
        Date      Time  Equity
0 2016-01-03  22:16:22  300.38
1 2016-01-04  22:16:00  300.65
2 2016-01-05  14:26:02  301.65
3 2016-01-06  19:08:13  302.10
4 2016-01-07  18:39:00  302.55
5 2016-01-08  22:16:04  308.24
6 2016-01-11  02:49:39  306.69
7 2016-01-14  15:46:39  307.93
8 2016-01-19  15:56:31  308.18

I think you can first cast to_datetime column date and then use resample with some aggregating functions like sum or mean:

df.Date = pd.to_datetime(df.Date)
df1 = df.resample('M', on='Date').sum()
print (df1)
             Equity  excess_daily_ret
Date                                 
2016-01-31  2738.37          0.024252

df2 = df.resample('M', on='Date').mean()
print (df2)
                Equity  excess_daily_ret
Date                                    
2016-01-31  304.263333          0.003032

df3 = df.set_index('Date').resample('M').mean()
print (df3)
                Equity  excess_daily_ret
Date                                    
2016-01-31  304.263333          0.003032

Answer 2

To resample from daily data to monthly, you can use the resample method. Specifically for daily returns, the example below demonstrates a possible solution.

The following data is taken from an analysis performed by AQR. It represents the market daily returns for May, 2019. The following code may be used to construct the data as a pd.DataFrame.

import pandas as pd

dates = pd.DatetimeIndex(['2019-05-01', '2019-05-02', '2019-05-03', '2019-05-06',
                         '2019-05-07', '2019-05-08', '2019-05-09', '2019-05-10',
                         '2019-05-13', '2019-05-14', '2019-05-15', '2019-05-16',
                         '2019-05-17', '2019-05-20', '2019-05-21', '2019-05-22',
                         '2019-05-23', '2019-05-24', '2019-05-27', '2019-05-28',
                         '2019-05-29', '2019-05-30', '2019-05-31'],
                         dtype='datetime64[ns]', name='DATE', freq=None)

daily_returns = array([-7.73787813e-03, -1.73277604e-03,  1.09124031e-02, -3.80437796e-03,
                       -1.66513456e-02, -1.67262934e-03, -2.77427734e-03,  4.01713274e-03,
                       -2.50407102e-02,  9.23270367e-03,  5.41897568e-03,  8.65419524e-03,
                       -6.83456209e-03, -6.54787106e-03,  9.04322511e-03, -4.05811322e-03,
                       -1.33152640e-02,  2.73398876e-03, -9.52000000e-05, -7.91438809e-03,
                       -7.16881982e-03,  1.19255102e-03, -1.24209547e-02])

daily_returns = pd.DataFrame(index = index, data= may.values, columns = ["returns"])

Assuming you don't have daily price data, you can resample from daily returns to monthly returns using the following code.

>>> daily_returns.resample("M").apply(lambda x: ((x + 1).cumprod() - 1).last("D"))
-0.06532

If you refer to their monthly dataset, this confirms that the market return for May 2019 was approximated to be -6.52% or -0.06532.