Let’s fix extras in Core Metadata 3.0

Slides for a discussion in the PyCon US 2022 Packaging Summit. This probably won’t make much sense unless you read the minute notes. You can also find there other topics discussed during the summit.

https://hackmd.io/Rv_FmGp6TniDWM4EXyZmMw

Transcript

1. Let’s fix extras in Core Metadata 3.0
   1
   

   View Slide

2. Extras
   Name normalisation and comparison
   PEP 685
   Make sense of extra comparison in markers
   == “works” but is wrong
   Default extras
   2
   

   View Slide

3. Name normalisation
   Covered by Brett’s PEP 685 but has consequences
   Extra name must be normalised in markers
   Incompatibility
   Easy for tools to get wrong
   3
   

   View Slide

4. Extra in marker is a mess
   extra == "foo" is wrong
   pip install package[docs,tests]
   any(marker.evaluate({"extra": x}) for x in extras)
   No other operators make sense
   “Negative extra” is impossible
   Exclude a dependency when an extra is specified
   4
   

   View Slide

5. extras instead of extra
   pytest>=6.0; 'tests' in extras
   Use not in for negation
   Special semantics for operators
   Normalise operands on comparison
   Should drop extra to avoid confusion
   Incompatibility
   5
   

   View Slide

6. Default extra group
   I think Pradyun showed interests
   Probably can use help from interested folks
   A “default extra” specification in metadata
   Requires-Dist: package asks for that extra group
   Semantic incompatibility
   What happens when pip install package[another] ?
   package[] requests to not install the defaul group?
   6
   

   View Slide

7. Proposals (1/2)
   Metadata 3.0 or not?
   PEP 685 already (slightly) broke compatibility
   Or we keep extra and stay on 2.x
   But if we bump to 3.0…
   Default extra group
   I should get in touch with Pradyun
   7
   

   View Slide

8. Proposal (2/2)
   Drop old fields made problematic by PEP 621
   Home-page
   The Author and Maintainer divide
   I think there are more
   There must be more we can do
   8
   

   View Slide