If no prerelease value is explicitly set, `Specifier[Set].contains` should default to `prerelease=True`.

[notatallshaw]

The spec says:

accept remotely available pre-releases for version specifiers where there is no final or post release that satisfies the version specifier

Now, the spec never elucidates on what "remotely" means, but this is already implemented for filter:

list(Specifier("<=2.0").filter(["1.0b0"]))
['1.0b0']

However it is not implemented for contains which can only receive one version and therefore there is no other version that could satisfy it:

>>> Specifier("<=2.0").contains("1.0b0")
False

The contradiction being: how can 1.0b0 pass the <=2.0 filter when <=2.0 does not "contain" 1.0b0?

[notatallshaw]

This also needs to be fixed in SpecifierSet.contains. I'm starting to familiarize myself with the code, I will make a PR soon.

[notatallshaw]

There is a design issue:

• Specifier.contains depends on Specifier.prerelease
• Specifier.filter depends on Specifier.contains
• Specifier.filter depends on Specifier.prerelease in a seperate way
• The semantics of Version.prerelease, Specifier.contains, and Specifier.filter are defined by the spec but Specifier.prerelease is not

I'm going to try and implement the spec so only one of Specifier.contains or Specifier.filter directly depends on Specifier.prerelease so this logic is easily to follow.

[brettcannon]

I'm trying to think how this might break someone. I guess something that was failing may suddenly start succeeding in terms of dependencies? That seems fine to me since https://packaging.pypa.io/en/stable/specifiers.html#packaging.specifiers.Specifier.contains has a way to specify that pre-releases should be left out (although this does change __contains__()).

@pradyunsg @henryiii any opinions?

[notatallshaw]

@brettcannon apologies I was on vacation when you posted that and missed it sorry.

This change in default is definitely going to be surprising to some users, but as you say they do have a way to opt into the old behavior by setting pre-release to False. I'm not quite sure what to do other than to call out clearly in the change log.

For pip I'm going to call this out in the release notes, and I'm hoping to have a feature ready for pip soon where users can turn off pre-releaes for everything or for individual projects.