PeriodIndex.is_monotonic only for monotonic increasing

[jbrockmendel]

dti = pd.date_range('2016-01-01', periods=3)
pi = dti.to_period('D')

>>> pi.is_monotonic
True
>>> pi[::-1].is_monotonic
False

[gfyoung]

Oops. Patch is most welcome!

[uds5501]

@jbrockmendel I guess you would like to check out pandas 0.23.0 documentation where it is stated that series.is_monotonic returns boolean if values in the object are monotonic_increasing.

If you want to check the monotnicity of the series, how about using this:
>>> pi[::-1].is_monotonic_increasing or pi[::-1].is_monotonic_increasing

[gfyoung]

@uds5501 : That's a good point. I forgot about that myself 😂

That being said, it does seem a little counter-intuitive, especially since we have both is_monotonic_increasing and is_monotonic_decreasing.