Interpretation of argument for use_legacy_defaults is not clear

[murrayrm]

My sense of the way that use_legacy_defaults should work is that if I had code that was working in some version and it stops working in a future version, I should be able to use use_legacy_defaults to return things to the version that was working.

So, for example, if I had things working in 0.8.2 and I upgrade to 0.8.4 or 0.9.0 and things stop working, I should be able to say

use_legacy_defaults('0.8.2')

and my code will work again.

This particular call definitely won't work (use_legacy_defaults only recognizes '0.8.3') and it is not clear to me how it could work. Basically, it seems like we need to track when a change in default behavior occurred and then "revert" any defaults that are different between the version I was using and the current version.

@sawyerbfuller Does my issue make sense? This is related to PR #431, I think.

[sawyerbfuller]

I don’t have much familiarity with backwards-incompatible changes before 0.8.3 but I assumed that we could use this function to add such functionality as needed from feedback from users.

My take on it is that the function should be available as a “good enough” solution that allows users to avoid the thorniest issues that come up whenever there are big changes in the code (a few of which I anticipated in that PR). But not something that will include every version number and provide compatibility with minor revision of how the code works.

In short, we should be a little bit “opinionated” about how things ought to work rather than to aim to maintain perfect backwards compatibility at all times.

So, if I update to the latest version of python-control and something breaks in my code, the first thing I should try is use-legacy-defaults on various previous version numbers, one of which might fix things, but maybe not everything.

[sawyerbfuller]

We could alternatively include every recent release in that function, most of which would set the same defaults as each other.

[murrayrm]

I agree with the philosophy, especially the summary sentence:

So, if I update to the latest version of python-control and something breaks in my code, the first thing I should try is use-legacy-defaults on various previous version numbers, one of which might fix things, but maybe not everything.

But this means that use_legacy_defaults needs to take an arbitrary version number and do the "right thing", which I don't think it currently does. Eg, if I were using 0.8.2 and things were working, then I got to 0.8.4 and things break, in the current implementation I would need to know to call use_legacy_defaults('0.8.3') even though I never personally used 0.8.3.

I think a bit of clever parsing of the version number and lexicographical comparison would allow an implementation where if I enter a version number I get the legacy defaults for anything that changed after that version number.