Set frequency 'MS' to pandas datatime object - python

Question

I have this dataframe in pandas:

df = pd.read_csv('data_stack.csv',index_col='month',parse_dates=True)

If I look at the parameter freq it's automatically infered as None

DatetimeIndex(['2018-09-01', '2018-08-01', '2018-07-01', '2018-06-01',
               '2018-05-01', '2018-04-01', '2018-03-01', '2018-02-01',
               '2018-01-01', '2017-12-01',
               ...
               '2018-11-01', '2019-01-01', '2018-12-01', '2018-11-01',
               '2019-01-01', '2018-12-01', '2018-11-01', '2019-01-01',
               '2018-12-01', '2018-11-01'],
              dtype='datetime64[ns]', name='month', length=4325, freq=None)

I want to put it as Monthly started 'MS':

df.index.freq = 'MS'

but I get this error:

ValueError                                Traceback (most recent call last)
<ipython-input-99-0dc1e7b74d6b> in <module>
----> 1 df.index.freq = 'MS'

~/opt/anaconda3/lib/python3.7/site-packages/pandas/core/indexes/extension.py in fset(self, value)
     64 
     65             def fset(self, value):
---> 66                 setattr(self._data, name, value)
     67 
     68             fget.__name__ = name

~/opt/anaconda3/lib/python3.7/site-packages/pandas/core/arrays/datetimelike.py in freq(self, value)
    925         if value is not None:
    926             value = frequencies.to_offset(value)
--> 927             self._validate_frequency(self, value)
    928 
    929         self._freq = value

~/opt/anaconda3/lib/python3.7/site-packages/pandas/core/arrays/datetimelike.py in _validate_frequency(cls, index, freq, **kwargs)
   1001             #  message.
   1002             raise ValueError(
-> 1003                 f"Inferred frequency {inferred} from passed values "
   1004                 f"does not conform to passed frequency {freq.freqstr}"
   1005             )

ValueError: Inferred frequency None from passed values does not conform to passed frequency MS

I have looked for similar cases and I found this one: pandas.DatetimeIndex frequency is None and can't be set

I have tried it but I obtain the same error, could anyone tell me why?

The data is in this repository: https://github.com/jordi-crespo/stack-questions

Answer 1

There's no frequency as you have duplicate values in your index. So I guess the only thing you can do with such an index in order to set a frequency is to aggregate the data somehow, e.g.

>>> df.resample('MS').mean().index
DatetimeIndex(['2017-01-01', '2017-02-01', '2017-03-01', '2017-04-01',
               '2017-05-01', '2017-06-01', '2017-07-01', '2017-08-01',
               '2017-09-01', '2017-10-01', '2017-11-01', '2017-12-01',
               '2018-01-01', '2018-02-01', '2018-03-01', '2018-04-01',
               '2018-05-01', '2018-06-01', '2018-07-01', '2018-08-01',
               '2018-09-01', '2018-10-01', '2018-11-01', '2018-12-01',
               '2019-01-01'],
              dtype='datetime64[ns]', name='month', freq='MS')

which gives you an index of the desired frequency. But I'm not sure if this is what you really want.