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datetime gotcha

Hi all

It took me a while to track down a bug that stemmed from this -

 >>> from datetime import date as dt, time as tm, datetime as dtm
 >>> x = dt.today()
 >>> isinstance(x, dt)
 >>> isinstance(x, dtm)
 >>> y = dtm.now()
 >>> isinstance(y, dt)
True                   <--- ??
 >>> isinstance(y, dtm)
 >>> isinstance(y, tm)

Why is a dtm instance also an instance of dt?

 From the docs, "A datetime object is a single object containing all the 
information from a date object and a time object."

If it was using multiple inheritance, a dtm should also be an instance 
of tm, but it is not.

This is using Python 3.7.2.

Frank Millman