copy() method for UTCDateTime

[claudiodsf]

Just found myself needing to copy a UTCDateTime() object.
I thought that it might be useful.

[trichter]

Why not just assign utc2=utc instead of copy utc2=utc.copy()?

EDIT: OK, for a real copy you need to do utc2=utc+0

[claudiodsf]

@trichter Yep, but that solution is pretty inelegant...
Another solution is utc2 = UTCDateTime(utc). This is what I do in my code, but it happened that, after few months, I forgot why I was doing that and I reverted my code to urc2 = utc which obviously didn't give the good results.
That's why I thought that utc.copy() would be a nice solution because it clearly says what you want to do.

[krischer]

At one point we discussed making the UTCDateTime object immutable which would still be nice. Assuming we would have that I still don't think one can make Python objects "copy at assignment" which is effectively how integers work in Python.

So I think we might want to have this method. It still needs a test.

[claudiodsf]

Sure. Will add a test later today.

[jkmacc-LANL]

@krischer Immutable? I've got a lot of dt += 1.5 usage; would it just .copy() as part of the __add__?

[megies]

UTCDateTime doesn't have __iadd__ implemented, that means it's falling back to __add__, creating a new object and returning it. (edit: since UTCDateTime is circling around a single float, it kind of behaves like one, immutability-wise)

I think currently the only way to really change a UTCDateTime object in place is using one of the private ._set_...() methods or manually setting the .timestamp.

If we really add the copy() method, we also might want to think about making properties like year settable (via _set_year()), I think. So that's basically saying, either we make it strictly immutable or also allow modifying via convenience properties (e.g. t.year += 1).

[claudiodsf]

@megies I'm probably missing something: isn't t.year += 1 already possible?

[megies]

Oh, indeed.. 😊 didn't see the property() call (usually use the decorator notation myself..)
👍 for the copy() method..

[krischer]

I think we should still make UTCDateTime immutable. += would return a new object and dt.year = 1 and consorts should raise an error. I reopened #568 to discuss this in a bit more detail.

[claudiodsf]

I'm holding on this one, until you decide whether UTCDateTime should become immutable or not.