Put the not isinstance(...) condition first to not do the potentially expensive all(mask) when it's not needed?

I had wondered about it but felt quantity input was less likely, but thinking again, the isinstance is really negligible compared to everything else while going over the full mask may not be. Anyway, done now.

Is that really important though? The internal representation is always the same. So why not simply always create an empty time instance for empty input?

I guess the question then is what the format of the empty time instance should be.

Also, don't forget this line is also for all-masked values.

I guess the question then is what the format of the empty time instance should be.

That's always confusing me. Why are things like "format" and "precision" instance properties? The internal representation is always the same. It's affecting only affecting the default repr, right?

Oops, this change is actually introducing a new regression since Quantity is only allowed for TimeDelta not Time.

In [3]: Time([] * u.s)
Out[3]: <Time object: scale='utc' format='ymdhms' value=[]>

Instead for Time this should raise an exception like it currently does. The logic is getting ugly, but I think it could look like:

            if not (isinstance(self, TimeDelta) and isinstance(val, u.Quantity)) and (
                empty_array or np.all(mask)
            ):

I guess the question then is what the format of the empty time instance should be.

For TimeDelta the default format of jd is fine. As mentioned for Time we don't have to worry about it since an empty input with no format should always raise an exception.

@taldcroft - OK, I pushed your change

@maxnoe - I agree that it is a little strange to error for what indeed is in a sense a change in repr - though this affects things like .value as well. But it is a bit out of scope here - I just want to fix a regression! Also, as noted earlier, the same logic is applied for fully masked input.

@mhvk's point about .value is crucial. Without a defined .format then .value is undefined. The .value is a primary functional output of the Time/TimeDelta classes.

And format also tells the class initializer and the object how to interpret input values. E.g.

tm = Time(["2023-01-01"])

tm[0] = 100.0
# ValueError

tm.format = "unix"
tm[0] = 100.0
# OK