pythonpandasdataframedatedatetime

Select DataFrame rows between two dates

I am creating a DataFrame from a csv as follows:

stock = pd.read_csv('data_in/' + filename + '.csv', skipinitialspace=True)

The DataFrame has a date column. Is there a way to create a new DataFrame (or just overwrite the existing one) which only contains rows with date values that fall within a specified date range or between two specified date values?

Solution

  • There are two possible solutions:

    Using a boolean mask:

    Ensure df['date'] is a Series with dtype datetime64[ns]:

    df['date'] = pd.to_datetime(df['date'])

    Make a boolean mask. start_date and end_date can be datetime.datetimes, np.datetime64s, pd.Timestamps, or even datetime strings:

    #greater than the start date and smaller than the end date
mask = (df['date'] > start_date) & (df['date'] <= end_date)

    Select the sub-DataFrame:

    df.loc[mask]

    or re-assign to df

    df = df.loc[mask]

    For example,

    import numpy as np
import pandas as pd

df = pd.DataFrame(np.random.random((200,3)))
df['date'] = pd.date_range('2000-1-1', periods=200, freq='D')
mask = (df['date'] > '2000-6-1') & (df['date'] <= '2000-6-10')
print(df.loc[mask])

    yields

                0         1         2       date
153  0.208875  0.727656  0.037787 2000-06-02
154  0.750800  0.776498  0.237716 2000-06-03
155  0.812008  0.127338  0.397240 2000-06-04
156  0.639937  0.207359  0.533527 2000-06-05
157  0.416998  0.845658  0.872826 2000-06-06
158  0.440069  0.338690  0.847545 2000-06-07
159  0.202354  0.624833  0.740254 2000-06-08
160  0.465746  0.080888  0.155452 2000-06-09
161  0.858232  0.190321  0.432574 2000-06-10

    Using a DatetimeIndex:

    If you are going to do a lot of selections by date, it may be quicker to set the date column as the index first. Then you can select rows by date using df.loc[start_date:end_date].

    import numpy as np
import pandas as pd

df = pd.DataFrame(np.random.random((200,3)))
df['date'] = pd.date_range('2000-1-1', periods=200, freq='D')
df = df.set_index(['date'])
print(df.loc['2000-6-1':'2000-6-10'])

    yields

                       0         1         2
date                                    
2000-06-01  0.040457  0.326594  0.492136    # <- includes start_date
2000-06-02  0.279323  0.877446  0.464523
2000-06-03  0.328068  0.837669  0.608559
2000-06-04  0.107959  0.678297  0.517435
2000-06-05  0.131555  0.418380  0.025725
2000-06-06  0.999961  0.619517  0.206108
2000-06-07  0.129270  0.024533  0.154769
2000-06-08  0.441010  0.741781  0.470402
2000-06-09  0.682101  0.375660  0.009916
2000-06-10  0.754488  0.352293  0.339337

    While Python list indexing, e.g. seq[start:end] includes start but not end, in contrast, Pandas df.loc[start_date : end_date] includes both end-points in the result if they are in the index. Neither start_date nor end_date has to be in the index however.

    Also note that pd.read_csv has a parse_dates parameter which you could use to parse the date column as datetime64s. Thus, if you use parse_dates, you would not need to use df['date'] = pd.to_datetime(df['date']).