Determine the root cause of the is_equivalent_frame change and document it

astropy/astroplan#382 is showing a single test failure that appeared in astropy v3.1rc1. This is odd because it seems to be in a place that wasn't obviously modified in 3.1. But it could be some subtle thing related to some of the preformance improvements.

At any rate, whether it's a bug that needs fixing or a subtle behavior change, in either case it should be either fixed or documented in the changelog/what's new.

@adrn

Turns out this may be a "real" failure: looking more deeply at astropy/astroplan#382, it turns out the real problem lies in is_equivalent_frame returning False in a case where it looks like it should return True. Trying to work up a simple test case of this...

cc @adrn

Here's a test case demonstrating the core of the problem. This fails in 3.1rc2 and passes in 3.0.x:

>>> from astropy.coordinates import *
>>> from astropy import units as u
>>> sc = SkyCoord(1*u.deg, 2*u.deg)
>>> newframe = GCRS()
>>> sc2 = sc.transform_to(newframe)
>>> assert newframe.obsgeoloc == sc2.obsgeoloc

To summarize: something changed in the the frame initializer that makes it so that frame attributes are copied in transform_to instead of referenced. Arguable this is a feature... (although we should document it as an API change - I'll make a quick PR doing just that right now).

However, the problem that astropy/astroplan#382 reveals (and shown in the above) is that this now causes some counter-intuitive behavior, because is_equivalent_frame uses equality, which isn't implemented on representations (or more accurately, is using the object.__eq__, which is the same as "is the same object"). I think the fix is probably to implement a reasonable equality for representations.

create regression test for astropy#8138

@eteq

@eteq - I suppose this has been fixed now, can you confirm and close or remilestone?

Yes, fixed by #8218 (apparently my wording in the commit message was too ambiguous for github...)

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Determine the root cause of the is_equivalent_frame change and document it #8138

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Determine the root cause of the is_equivalent_frame change and document it #8138

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eteq commented on Dec 3, 2018

eteq commented on Dec 3, 2018

bsipocz commented on Dec 6, 2018

eteq commented on Dec 6, 2018

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