PEP 827: Type-Level Computation — Prototype Implementation

This is a prototype implementation of PEP 827 (Type Manipulation) in mypy.

The PEP introduces type-level computation operators for introspecting and constructing types.

Most of the main features are prototyped, and this should be suitable for experimentation, but it it not yet production quality or ready to be a PR yet. (This prototype has been AI assisted, and at least some slop has made it in that will need to be fixed; it might benefit from a full history rewrite, also.)

For the original mypy README, see REAL_README.md.

What's implemented

• Type operators: IsAssignable, IsEquivalent, Bool, GetArg, GetArgs, GetMember, GetMemberType, Members, Attrs, FromUnion, Length, Slice, Concat, Uppercase, Lowercase, Capitalize, Uncapitalize, RaiseError
• Conditional types: true_type if BoolType else false_type
• Type-level comprehensions: *[T for x in Iter[...]] with filtering
• Dot notation: member.name, member.type, param.type, etc.
• Boolean operators: and, or, not on type booleans
• Data types: Member[name, type, quals, init, definer], Param[name, type, quals]
• Extended callables: Callable[Params[Param[...], ...], RetType]
• Object construction: NewProtocol[*Members], NewTypedDict[*Members]
• Class modification: UpdateClass[*Members] as return type of decorators / __init_subclass__
• InitField: Keyword argument capture with literal type inference
• Callable introspection: GetArg[SomeCallable, Callable, Literal[0]] returns Param types
• Incremental mode

What's not yet implemented

• GetSpecialAttr[T, Attr] — extract __name__, __module__, __qualname__

• GenericCallable[Vs, lambda <vs>: Ty] — generic callable types with lambda binding

• Overloaded[*Callables] — overloaded function type construction

• any(comprehension) / all(comprehension) — quantification over type booleans

• classmethod/staticmethod representation in type-level computation

• Any attempt to make it perform well

• Fine-grained mode is currently broken

Key files

• mypy/typelevel.py — All type operator evaluation logic
• mypy/typeanal.py — Desugaring of conditional types, comprehensions, dot notation, extended callables
• mypy/typeshed/stdlib/_typeshed/typemap.pyi — Stub declarations for all operators and data types
• test-data/unit/check-typelevel-*.test — Test suite

Some implementation notes

• Evaluating NewProtocol creates a new anonymous TypeInfo that doesn't go into any symbol tables. That TypeInfo hangs on to the NewProtocol invocation that created it, and when we serialize Instances that refer to it, we serialize the NewProtocol invocation instead. This allows us to avoid needing to serialize the anonymous TypeInfos.