Pandas DatetimeIndex 奇怪的行为

Question

我处理一个DataFrame，索引是字符串，年月，例如：

index = ['2007-01', '2007-03', ...]

但是，索引未满。例如2007-02 丢失。我想要的是用完整索引重新索引 DataFrame。

我尝试过的：

In [60]: pd.DatetimeIndex(start='2007-01', end='2007-12', freq='M')
Out[60]: 
DatetimeIndex(['2007-01-31', '2007-02-28', '2007-03-31', '2007-04-30',
           '2007-05-31', '2007-06-30', '2007-07-31', '2007-08-31',
           '2007-09-30', '2007-10-31', '2007-11-30'],
          dtype='datetime64[ns]', freq='M')

索引是每个月的月末。

In [64]: pd.DatetimeIndex(['2007-01', '2007-03', '2007-04', '2007-05'])
Out[64]: DatetimeIndex(['2007-01-01', '2007-03-01', '2007-04-01', '2007-05-01'],    dtype='datetime64[ns]', freq=None)

索引是每个月的开始。

如何处理这个问题？

Answer 1

如果需要每月第一天的频率，我想你需要添加参数freq='MS'：

print (pd.DatetimeIndex(start='2007-01', end='2007-12', freq='MS'))
DatetimeIndex(['2007-01-01', '2007-02-01', '2007-03-01', '2007-04-01',
               '2007-05-01', '2007-06-01', '2007-07-01', '2007-08-01',
               '2007-09-01', '2007-10-01', '2007-11-01', '2007-12-01'],
              dtype='datetime64[ns]', freq='MS')

Link 到 Offset Aliases in pandas documentation, thank you .

另一种解决方案是使用 PeriodIndex 生成月份：

print (pd.PeriodIndex(start='2007-01', end='2007-12', freq='M'))
PeriodIndex(['2007-01', '2007-02', '2007-03', '2007-04', '2007-05', '2007-06',
             '2007-07', '2007-08', '2007-09', '2007-10', '2007-11', '2007-12'],
            dtype='int64', freq='M')

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