pandas.DataFrame.loc：尝试根据日期时间修改列的值时返回类型错误

Question

I have a data frame that contains a column of dates and another column that I'd like to modify according to the date. However when I try to do this using the.loc method, I get

TypeError: '<' not supported between instances of 'str' and 'datetime.datetime'

Could anyone please explain 1) why this error comes up - the dates are datetime objects, and 2) how I can modify the second column. I include a MWE below.

Many thanks

from datetime import datetime as DT

import numpy as np
import pandas as pd

def random_dates(start, end, n, unit='D', seed=None):
    if not seed:  # from piR's answer
        np.random.seed(0)

    ndays = (end - start).days + 1
    return pd.to_timedelta(np.random.rand(n) * ndays, unit=unit) + start


start_date = pd.to_datetime('2010-01-01')
end_date   = pd.to_datetime('2020-01-01')
df = pd.DataFrame(columns=['Date', 'Names'])

N = 10
df['Date'] = random_dates(start_date, end_date, N)
df = df.assign(Names = ['A'] * N)

df.loc['Date' < DT(2015, 1, 1), 'Names'] = 'B'

(random_dates function from this post )

Answer 1

Switch the line

df.loc['Date' < DT(2015, 1, 1), 'Names'] = 'B'

to

df.loc[df.Date < DT(2015, 1, 1), 'Names'] = 'B'

This would solve it.

Answer 2

You are not using the df.loc statement properly. For your particular logic, you want to change the values of column Names to "B" when a row's corresponding date is < 2015-1-1 . When you want to include any conditions in df.loc , the proper way to use it is like this:

df.loc[(df['Date'] < DT(2015, 1, 1)), 'Names'] = 'B'

For a detailed guide on how to use conditions with df.loc , you can refer this link