Set pandas.tseries.index.DatetimeIndex.freq with inferred_freq

Question

consider the DatetimeIndex tidx

tidx = pd.to_datetime(['2016-07-29', '2016-08-31', '2016-09-30'])
print(tidx.freq)
print(tidx.inferred_freq)
print(tidx)

None
BM
DatetimeIndex(['2016-07-29', '2016-08-31', '2016-09-30'], \
              dtype='datetime64[ns]', freq=None)

I want the freq attribute to assume the inferred_freq attribute... so I

tidx.freq = tidx.inferred_freq
print(tidx)

--------------------------------------------------------------------------- AttributeError                            Traceback (most recent call last) <ipython-input-24-b49b104fda04> in <module>()
      1 tidx.freq = tidx.inferred_freq
----> 2 print(tidx)

C:\Users\ssmith\AppData\Local\Continuum\Anaconda2\lib\site-packages\pandas\core\base.pyc in __str__(self)
     45         if compat.PY3:
     46             return self.__unicode__()
---> 47         return self.__bytes__()
     48 
     49     def __bytes__(self):

C:\Users\ssmith\AppData\Local\Continuum\Anaconda2\lib\site-packages\pandas\core\base.pyc in __bytes__(self)
     57 
     58         encoding = get_option("display.encoding")
---> 59         return self.__unicode__().encode(encoding, 'replace')
     60 
     61     def __repr__(self):

C:\Users\ssmith\AppData\Local\Continuum\Anaconda2\lib\site-packages\pandas\indexes\base.pyc in __unicode__(self)
    581         klass = self.__class__.__name__
    582         data = self._format_data()
--> 583         attrs = self._format_attrs()
    584         space = self._format_space()
    585 

C:\Users\ssmith\AppData\Local\Continuum\Anaconda2\lib\site-packages\pandas\tseries\base.pyc in _format_attrs(self)
    485         for attrib in self._attributes:
    486             if attrib == 'freq':
--> 487                 freq = self.freqstr
    488                 if freq is not None:
    489                     freq = "'%s'" % freq

C:\Users\ssmith\AppData\Local\Continuum\Anaconda2\lib\site-packages\pandas\tseries\base.pyc in freqstr(self)
    223         if self.freq is None:
    224             return None
--> 225         return self.freq.freqstr
    226 
    227     @cache_readonly

AttributeError: 'str' object has no attribute 'freqstr'

---

question
What is the appropriate way to have my index assume it's inferred_freq?

Answer 1

It's unclear why the docs state you can set the freq attribute but then it doesn't persist but if you reconstruct the datetimeindex again but pass a freq param then it works:

In [56]:
tidx = pd.DatetimeIndex(tidx.values, freq = tidx.inferred_freq)
tidx

Out[56]:
DatetimeIndex(['2016-07-29', '2016-08-31', '2016-09-30'], dtype='datetime64[ns]', freq='BM')

Answer 2

You can directly use the DatetimeIndex constructor with your list of strings and pass 'infer' as the freq:

In [2]: tidx = pd.DatetimeIndex(['2016-07-29', '2016-08-31', '2016-09-30'], freq='infer')

In [3]: tidx
Out[3]: DatetimeIndex(['2016-07-29', '2016-08-31', '2016-09-30'], dtype='datetime64[ns]', freq='BM')