ENH/API: Decide what to return Period - Period subtraction

[sinhrks]

Code Sample, a copy-pastable example if possible

Split from #13071. Currently it returns int and loses freq info.

pd.Period('2011-03', freq='M') - pd.Period('2011-01', freq='M') 
#2

Expected Output

Current ideas are:

1. Return Timedelta adding support of Monthly freq.
2. Return Perioddelta newly created.
3. Return DateOffset.

output of pd.show_versions()

on current master.

CC: @jreback @MaximilianR

[shoyer]

I don't think we want to add other units like months to Timedelta without a full overhauling of time units.

I also don't think we want to add a new PeriodDelta type.

So I guess that leaves returns DateOffsets. Are there any differences of Periods that could not be represented with DateOffsets? The other reasonable choice is to simply raise a TypeError.

[jreback]

I don't think we want to add other units like months to Timedelta without a full overhauling of time units.

I am not sure this will work, but what is the issue here

[jorisvandenbossche]

I don't think we want to add other units like months to Timedelta without a full overhauling of time units.

I am not sure this will work, but what is the issue here

A month has not a well defined length, so does not fit in the current concept of Timedelta's.

@sinhrks Do you want to do this for 0.19? (changing to DateOffset, if possible) Or push to next release?