Why is it possible to add pd.Timedelta and None?

Question

The code

None + pd.Timedelta("1 day")

evaluates to a strange result:

Timedelta('-106751 days +00:12:43:145224')

Why does this happen?

Answer 1

As mentioned in the comments, this behavior has been fixed in later versions of Pandas. From going over the code, it seems to have been fixed for version 0.23 in this commit.

In a bit more detail - the code for the function that calculates the delta, _binary_op_method_timedeltalike, contains the condition:

elif other is NaT:  # (N)ot-(A)-(T)ime, the time equivalent of NaN
    return NaT

But None is not NaT, and that's the reason for the bug:

>>> None is NaT
False

In the later version, a second condition has been added, first converting other to a Timedelta object, and then testing the condition again, so effectively testing:

>>> Timedelta(None) is NaT
True

So now NaT is returned in the case of None + Timedelta.