API/DEPR: re-instate timedelta[non-unit] conversions as a no-op

[jreback]

when I originally added first class timedelta support I allowed .astype('m8[unit]') to return a float, except if unit==ns. IOW it was de-facto the same a dividing by Timedelta(1, unit=unit)

In [1]: s = Series(pd.to_timedelta(['1 day', '1 minutes', '1 second']))

In [2]: s
Out[2]: 
0   1 days 00:00:00
1   0 days 00:01:00
2   0 days 00:00:01
dtype: timedelta64[ns]

In [3]: s.astype('timedelta64[s]')
Out[3]: 
0    86400.0
1       60.0
2        1.0
dtype: float64

In [4]: s / pd.Timedelta('1 s')
Out[4]: 
0    86400.0
1       60.0
2        1.0
dtype: float64

In [5]: s.astype('timedelta64[ns]')
Out[5]: 
0   1 days 00:00:00
1   0 days 00:01:00
2   0 days 00:00:01
dtype: timedelta64[ns]

note that for [5], however we just return a timedelta64[ns] (which happens to be the underlying data representation).

#19224 fixes construction of non-ns (and preserves the astype freq conversions. I think that this is confusing to the user and we should revert this and have a m8[unit] just work the same as it does for M8[unit] namely that you get back a timedelta dtype (and not a float), that is the same

So [3] would be the same as [5]

We do this for datetime types now

In [13]: Series(pd.date_range('20170101', periods=3)).astype('M8[s]')
Out[13]: 
0   2017-01-01
1   2017-01-02
2   2017-01-03
dtype: datetime64[ns]

sure this is slightly convenient but a bit confusing.

I would propose that we provide a deprecation warning and change it in the next version.

[jreback]

@jorisvandenbossche @TomAugspurger

cc @jbrockmendel

[jorisvandenbossche]

My first reaction was: and why not having the actual timedelta resolution? As we support this when doing astype("m8[s]) starting from numerical data.

But, this is actually maybe a bug / also not consistent, as in theory we only support 'ns' resolution:

In [4]: pd.Series([1,2,3]).astype('m8[s]')
Out[4]: 
0   00:00:01
1   00:00:02
2   00:00:03
dtype: timedelta64[s]    <---- kept the 's' resolution!

and for datetime it is interpreted correctly as well but then converted to ns:

In [7]: pd.Series([1,2,3]).astype('M8[s]')
Out[7]: 
0   1970-01-01 00:00:01
1   1970-01-01 00:00:02
2   1970-01-01 00:00:03
dtype: datetime64[ns]    <---- it are seconds, but dtype resolution is 'ns'

On the actual issue, I have to think a bit further about it, but agree that the float result is very strange

[jreback]

[4] is fixed by PR #19224 (I don't know if we had an issue about that before)

[7] is also fixed by that (its not a regression per-se because I broke it in a prior PR in master/0.23).

but note that the examples are not relevant to THIS issue. We are taking something that is already a dateimelike type and astyping it.

[jorisvandenbossche]

I wanted to open a separate issue for that (as the timedelta[s] series should not be created, as once you do operations with it you get bugs by it sometimes being interpreted as ns and sometimes as s), but we already have #12425 were this is also discussed.

[jorisvandenbossche]

[7] is also fixed by that (its not a regression per-se because I broke it in a prior issue).

[7] that I showed was already correct no? So what is exactly fixed/changed there?

[jreback]

#12425 is covered by #19224

[7] is the issue in #19223 (not broken in 0.22, rather in master)