fix(markers): apply lazy evaluation to markers

[frostming]

Signed-off-by: Frost Ming me@frostming.com

Fix #774

Change the marker evaluation to lazy evaluation so it is possible for boolean shortcuts. I didn't test but there should also be a performance gain.

[wimglenn]

Is this different to #834 ?

[frostming]

@wimglenn Thank you, I just noticed this PR. In fact, #834 is still eager, but it allows exceptions to exist and considers them during evaluation. The implementation also doesn't look so clean

By making the evaluation lazy there would be a minor breaking change that illegal markers will be missed if being shortcutted.

[edwardpeek-crown-public]

FWIW PR #834 started off as a simple lazy evaluation like this but discussion on issue #774 indicated an aversion to the edge cases this approach introduces with order dependency and silently ignoring malformed expressions.

[notatallshaw]

Yes, the packaging maintainers need to give clear guidance here, I think we have narrowed down to there only being 4 options on solving the practical problem users are facing:

1. Implement the standard that there should be a fallback to Python string comparison
2. Implement lazy comparison and accept that markers could be invalid when not evaluated
3. Do a pass to check markers are valid and then do a lazy comparison
4. Refuse all changes until spec is changed or clarified

If the answer is 4 I am willing to start the discussion on clarifying the spec, specifically the two separate standards topics: packaging rejecting solution 1. And whether the spec allows for 2 or 3.

I do not have the capacity to propose a new solution, but I am willing to explain the practical issues and put forth the already outlined options to see if there is a consensus in the Python packaging community.

[frostming]

silently ignoring malformed expressions.

This is not true, malformed expressions will be rejected by the parser and are never caught by the evaluation.

[notatallshaw]

silently ignoring malformed expressions.

This is not true, malformed expressions will be rejected by the parser and are never caught by the evaluation.

Can you please add additional tests for this, assuming they don't exist already.

My experience working on pre-releaes is packaging is far from exhaustively tested on edge cases.

[edwardpeek-crown-public]

This is not true, malformed expressions will be rejected by the parser and are never caught by the evaluation.

I should have been more specific, but I meant this repo's non-standard error in cases of "bad" version comparisons.

[frostming]

I should have been more specific, but I meant this repo's non-standard error in cases of "bad" version comparisons.

IIUC you mean os_name ~= "2", then yes it isn't caught by the parser. But that is also an implementation detail and should be clarified by the spec IMO.

[henryiii]

Closing since #939 was accepted and will be in the RC (coming soon). Please test and reopen if consideration on this still is needed. Combined with other changes, hopefully this all works.