[Numpy-discussion] Deprecating unitless timedelta64 and "safe" casting of integers to timedelta64

[Stephan Hoyer]

As part of the datetime64 cleanup I've been working on over the past few
days, I noticed that NumPy's casting rules for np.datetime64('NaT') were
not working properly:
https://github.com/numpy/numpy/pull/6465

This led to my discovery that NumPy currently supports unit-less timedeltas
(e.g., "np.timedelta64(5)"), which indicate some sort of generic time
difference. The current behavior is to take the time units from the other
argument when these are used in a binary operation.

Even worse, we currently support "safe" casting of integers to timedelta64,
which means that integer + datetime64 and integer + timedelta64 arithmetic
works:

In [4]: np.datetime64('2000-01-01T00') + 10
Out[4]: numpy.datetime64('2000-01-01T10:00-0800','h')

Based on the principle that NumPy's datetime support should mirror the
standard library as much as possible, both of these behaviors seem like a
bad idea. We have datetime types precisely to disambiguate these sort of
situations.

I'd like to propose deprecating such casting in NumPy 1.11, with the intent
of removing it entirely as soon as practical.

Stephan
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