Add NaTType to tslib public api

[dhirschfeld]

When testing the new 0.20.0 release candidate I found that the removal of NaTType from the tslib public api breaks odo.

If this api breakage is actually intentional/desired I'm ok with this being closed however it should probably get a mention in the backwards incompatible API changes section of the whatsnew.

Whilst a deprecation warning is raised in 648ae4f#diff-d6b2c5dbe1e7fbda4ed4b87e0c49399f the removal of NaTType breaks the previous public api rendering the warning ineffective and not giving any 3rd party libraries a chance to make the necessary changes and cut a release

[codecov]

Codecov Report

Merging #16137 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #16137   +/-   ##
=======================================
  Coverage   90.84%   90.84%           
=======================================
  Files         159      159           
  Lines       50779    50779           
=======================================
  Hits        46128    46128           
  Misses       4651     4651

Flag	Coverage Δ
#multiple	88.62% <ø> (ø)	⬆️
#single	40.33% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
pandas/tslib.py	100% <ø> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update d50b162...06eafff. Read the comment docs.

[codecov]

Codecov Report

Merging #16137 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #16137   +/-   ##
=======================================
  Coverage   90.84%   90.84%           
=======================================
  Files         159      159           
  Lines       50779    50779           
=======================================
  Hits        46128    46128           
  Misses       4651     4651

Flag	Coverage Δ
#multiple	88.62% <ø> (ø)	⬆️
#single	40.33% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
pandas/tslib.py	100% <ø> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update d50b162...06eafff. Read the comment docs.

[jreback]

NaTType was never a public api
who/what is using this?

[dhirschfeld]

odo was using that

[jorisvandenbossche]

We could maybe expose NaTType in pandas.api.types ?

[jorisvandenbossche]

Depending on what we decide (whether we find it worth to expose NaTType publicly or not), you also have to adapt __init__.py here: https://github.com/pandas-dev/pandas/blob/master/pandas/__init__.py#L76 to provide the correct deprecation message.

[dhirschfeld]

It would be ok for odo to use a private attribute if they're aware that it is meant to be private which the new import from pd._libs.tslib would indicate.

It would be nice I guess if they didn't have to though so I'd be in favour of exposing it in pd.api.types.

[jreback]

superseded by #16146

[shoyer]

The right way to use this going forward is probably type(pd.NaT). I'm not sure what the point of exposing NaTType is, given that it's a singleton. Python 2 had types.NoneType but they removed it in Python 3.