Datetime comparison

[jswhit]

copy of Unidata/netcdf4-python#639

When a netcdftime datetime object is unable to be compared to another object, the richcmp method raises a TypeError. In previous versions of the module the method would return NotImplemented, causing the appropriate method on the other object to be called and allowing classes to be written which could be compared against netcdftime datetime objects. With the TypeError behaviour this is no longer the case.

Can the previous behaviour be restored?

[jswhit]

Seems like simply changing TypeError to NotImplementedError in richcmp would fix this. Are they any downsides to this?

[shoyer]

CC @djkirkham

Seems like simply changing TypeError to NotImplementedError in richcmp would fix this. Are they any downsides to this?

It needs to be return NotImplemented to properly defer to the implemention on the other object, not raise NotImplementedError

[pdearnshaw]

This problem has arisen in a project I am working on so I thought to try this and got unexpected results, basically cftime.datetime objects become comparable to anything! ie. breaks not_comparable_4 test in test_richcmp.

Is there anything else that can be done?