python - 为什么 astype timedelta64 不能在 ndarray 上工作？

Question

为什么在以下示例中.astype('timedelta64[D]')无法转换ndarray？

df = pd.DataFrame(pd.date_range('2017-01-01', periods=5, freq='W'), columns=['Val'])
df['Base'] = pd.datetime(2015, 1, 1)
df['Days'] = (df['Val'] - df['Base']).astype('timedelta64[D]') # Success
df['FailCast'] = (df['Val'].values - df['Base'].values).astype('timedelta64[D]') # Failure

print (df)

         Val       Base   Days   FailCast
0 2017-01-01 2015-01-01  731.0   731 days
1 2017-01-08 2015-01-01  738.0   738 days
2 2017-01-15 2015-01-01  745.0   745 days
3 2017-01-22 2015-01-01  752.0   752 days
4 2017-01-29 2015-01-01  759.0   759 days

特别考虑到在事后作为单独的操作应用时它将成功地转换值:例如，

df['FailCast'] = df['FailCast'].astype('timedelta64[D]') # Success!

Answer 1

请注意，这些是等效的:

df['FailCast'] = df['FailCast'].values.astype('timedelta64[D]')

还有

df['FailCast'] = np.array([731, 738, 745, 752, 759], dtype='timedelta64[D]')

在获得相同的输出后传递.astype(int)。