Meta issue tracker, pt 2

This is the sequel to https://github.com/python/mypy/issues/11201. It's awesome to see most of the things in that issue fixed.

Type System Features

• PEP 695 https://github.com/python/mypy/issues/15238
• PEP 702 https://github.com/python/mypy/issues/16111
• PEP 675 https://github.com/python/mypy/issues/12554
• PEP 696 https://github.com/python/mypy/issues/14851

Behaviour changes

• Ternary unions https://github.com/python/mypy/issues/12056

  We should use unions in some places we currently join, see https://github.com/python/mypy/issues/12056#issuecomment-1694422734

  Update: We now use unions in more places, including ternary expressions. Feel free to investigate this more though!

• Always narrow types https://github.com/python/mypy/issues/2008#issuecomment-296213676 (proposal 3)

  Users find it surprising because it seems obvious that mypy should know the variable is of the narrower type. There was a big discussion in typing-sig and all the type checkers agreed this behaviour was probably the best. The change is very easy, but exposes bugs in binder

• Default value of generic parameters https://github.com/python/mypy/issues/3737#issuecomment-552840010

  It's totally reasonable to want to have generic parameters with defaults, we should error at call sites in case of badness

• Support narrowing Literal https://github.com/python/mypy/issues/12535

  It's hard to really use Literal types without a good way to narrow them. We should also narrow on ==. I think last time I tried this it exposed bugs in binder

• Support property setters with broader types https://github.com/python/mypy/issues/3004

  Should also allow covariant overrides of getter

• Better handling of __new__ https://github.com/python/mypy/pull/16020

  This is part of the Python language and comes up weirdly frequently.

• Love for match statement https://github.com/python/mypy/issues?q=is%3Aopen+label%3Atopic-match-statement+sort%3Areactions-%2B1-desc

  Update: I fixed the most popular of these issues, but there's still a long tail of them

• PEP 688 https://github.com/python/mypy/issues/15313

  Easy, is just a somewhat breaking change

UI

• Per module strict=true https://github.com/python/mypy/issues/11401

  There's maybe some other UI stuff we can think about for groups of flags and error codes, as discussed in meetup. I also don't like the new --extra-checks since flag name doesn't mean anything and can't be decomposed.

• Ability to override PEP 561 https://github.com/python/mypy/issues/8545

  It would be nice to be able to opt-in to treating packages missing py.typed as having py.typed. mypy can still provide a lot of value just based on function shape.

  Update: this is now --follow-untyped-imports (can be set per-module)

• Exclude should imply per-module follow_import=skip https://github.com/python/mypy/issues/10377

  Currently --exclude only affects file discovery on command line, not import following. This is a source of frustration for users because the relevant modules can still end up being type checked.

• Provide flag to generate stubs requirements https://github.com/python/mypy/issues/10600#issuecomment-950170169

  --install-types is slow, results in unstable type checking, doesn't let users manage their own envs, can fail unclearly if cache isn't written

• Rethink stub suggestion logic https://github.com/python/mypy/issues/10619

  Our current behaviour was needed to help the migration to modular typeshed, but is not behaviour we'd have designed from scratch.

Gradual typing

• Make check_untyped_defs = True the default https://github.com/python/mypy/issues/3948#issuecomment-890060233 / https://github.com/python/mypy/issues/6646

  We decided not to do this for mypy 1.0, but could still make sense in the future.
  It could make sense to investigate and introduce other changes that make this more palatable, e.g. enable --allow-redefinition(-new), swallow var-annotated in untyped defs, etc

• Infer obvious return types https://github.com/python/mypy/issues/4409 / https://github.com/python/mypy/issues/10149

  As a 1/3-way measure, we could also do https://github.com/python/mypy/issues/9413

• Infer method types from superclass https://github.com/python/mypy/issues/3903#issuecomment-408833797

Performance

• Parallel type checking https://github.com/python/mypy/issues/933
• Faster caching https://github.com/python/mypy/issues/3456

Misc

• Member access https://github.com/python/mypy/issues/7724

• Merge configuration docs https://github.com/python/mypy/issues/12347

See also a similar issue for mypyc here: https://github.com/mypyc/mypyc/issues/785

(other maintainers, feel free to edit)

Start by reviewing the still-open linked issues under Type System Features, Behaviour changes, UI, Gradual typing, and Misc, along with their referenced discussions. This tracker names no files or tests; completion requires resolving the selected outstanding items or moving them into separately scoped work.