Slicing pandas dataframe via index datetime

Question

I tried to slice a pandas dataframe, that was read from the CSV file and the index was set from the first column of dates.

IN:

df = pd.read_csv(r'E:\...\^d.csv')
df["Date"] = pd.to_datetime(df["Date"])

OUT:

        Date  Open  High   Low  Close  Volume
0 1920-01-02  9.52  9.52  9.52   9.52     NaN
1 1920-01-03  9.62  9.62  9.62   9.62     NaN
2 1920-01-05  9.57  9.57  9.57   9.57     NaN
3 1920-01-06  9.46  9.46  9.46   9.46     NaN
4 1920-01-07  9.47  9.47  9.47   9.47     NaN
            Date     Open     High      Low    Close        Volume
26798 2020-10-26  3441.42  3441.42  3364.86  3400.97  2.435787e+09
26799 2020-10-27  3403.15  3409.51  3388.71  3390.68  2.395102e+09
26800 2020-10-28  3342.48  3342.48  3268.89  3271.03  3.147944e+09
26801 2020-10-29  3277.17  3341.05  3259.82  3310.11  2.752626e+09
26802 2020-10-30  3293.59  3304.93  3233.94  3269.96  3.002804e+09

IN:

df = df.set_index(['Date'])
print("my index type is ")
print(df.index.dtype)
print(type(df.index)) #type of index

OUT:

               Open     High      Low    Close        Volume
Date                                                        
2007-01-03  1418.03  1429.42  1407.86  1416.60  1.905089e+09
2007-01-04  1416.95  1421.84  1408.22  1418.34  1.669144e+09
2007-01-05  1418.34  1418.34  1405.75  1409.71  1.621889e+09
2007-01-08  1409.22  1414.98  1403.97  1412.84  1.535189e+09
2007-01-09  1412.85  1415.61  1405.42  1412.11  1.687989e+09
            ...      ...      ...      ...           ...
2009-12-24  1120.59  1126.48  1120.59  1126.48  7.042833e+08
2009-12-28  1126.48  1130.38  1123.51  1127.78  1.509111e+09
2009-12-29  1127.78  1130.38  1126.08  1126.19  1.383900e+09
2009-12-30  1126.19  1126.42  1121.94  1126.42  1.265167e+09
2009-12-31  1126.42  1127.64  1114.81  1115.10  1.153883e+09

my index type is 
datetime64[ns]
<class 'pandas.core.indexes.datetimes.DatetimeIndex'>

I try to slice for Mondays using

monday_dow = df["Date"].dt.dayofweek==0

OUT (Spyder returns):

KeyError: 'Date'

I've read a lot and similar answers on stackoverflow, but could fix this, although I understand I do something wrong with index, it should be called another way?

Answer 1

You need filter by DatetimeIndex by DatetimeIndex.dayofweek (removed .dt used only for columns):

monday_dow = df.index.dayofweek==0

So if need all rows:

df1 = df[monday_dow]

Also here is possible simplify code for set DatimeIndex in read_csv :

df = pd.read_csv(r'E:\...\^d.csv', index_col=['Date'], parse_dates=['Date'])

monday_dow = df.index.dayofweek==0

df1 = df[monday_dow]