calendar.monthrange() - ValueError：系列的真值不明确。 使用 a.empty、a.bool()、a.item()、a.any() 或 a.all()

Question

I have a pandas dataframe that has a date field. I'm trying to use this field to calculate the number of days in the month of that date. However, when I use the code below, I get the following error:

ValueError: The truth value of a Series is ambiguous. Use a.empty, a.bool(), a.item(), a.any() or a.all()

from calendar import monthrange
df['NumDays'] = monthrange(df['Date_Field'].map(lambda x: x.year),df['Date_Field'].map(lambda x: x.month))[1]

I've tested to see if the .year and .month are working by creating two separate columns in the DF with that data.

df['year'] = df['Date_Field'].map(lambda x: x.year)
df['month'] = df['Date_Field'].map(lambda x: x.month)
df['NumDays'] = monthrange(df['year'], df['month'])[1]

The resulting 'year' column had the correct year, and the 'month' column had the correct month, but when I tried to use them in monthrange() I get the same ValueError.

Any guidance would be appreciated.

Thanks.

Answer 1

Use days_in_month attribute of DatetimeIndex:

df = pd.DataFrame({'date':pd.date_range('01/01/2019', periods=12, freq='MS')})
df['No of Days in this Month'] = df['date'].dt.days_in_month
df

Output:

         date  No of Days in this Month
0  2019-01-01                        31
1  2019-02-01                        28
2  2019-03-01                        31
3  2019-04-01                        30
4  2019-05-01                        31
5  2019-06-01                        30
6  2019-07-01                        31
7  2019-08-01                        31
8  2019-09-01                        30
9  2019-10-01                        31
10 2019-11-01                        30
11 2019-12-01                        31

Answer 2

Can be something like this:

# dt is referred to datetime
df['NumDays'] = df['Date_Field'].dt.days_in_month