如何正确使用fillna（）作为熊猫数据框中的日期时间列（不起作用）？

Question

I have a dataframe as shown below:

       Age  CreatedDate   TestDate               Performance
544    51   2015-11-23    2015-12-11                   1
325    51   2015-11-23    2016-01-04                   0
1043   51   2016-01-26    2016-01-25                   1
303    51   2016-01-26    2016-02-15                   1
1076   50   2015-04-29           NaT                   0

I want to update my TestDate which has value of NaT with the data from Created date, as such:

       Age  CreatedDate   TestDate               Performance
544    51   2015-11-23    2015-12-11                   1
325    51   2015-11-23    2016-01-04                   0
1043   51   2016-01-26    2016-01-25                   1
303    51   2016-01-26    2016-02-15                   1
1076   50   2015-04-29    2015-04-29                   0

I've tried to use the method fillna(), however, my data is not being updated though there is no error emitted.

df['TestDate'].fillna(pd.to_datetime(df['CreatedDate']))

Any advice? Thanks Before.

Answer 1

You should convert your datetime columns TestDate and CreatedDate into datetime format before filling NaT :

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

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

then remember to add inplace=True to your statement:

In [20]: df['TestDate'].fillna(df['CreatedDate'], inplace=True)

In [21]: df
Out[21]: 
      Age CreatedDate   TestDate  Performance
544    51  2015-11-23 2015-12-11            1
325    51  2015-11-23 2016-01-04            0
1043   51  2016-01-26 2016-01-25            1
303    51  2016-01-26 2016-02-15            1
1076   50  2015-04-29 2015-04-29            0

Checking dtypes:

In [22]: df.dtypes
Out[22]: 
Age                     int64
CreatedDate    datetime64[ns]
TestDate       datetime64[ns]
Performance             int64
dtype: object