(An earlier version of this post usedTypeAnnotation rather thanTypeForm as the initially proposed spelling for the concept described here)

Feature

A new special formTypeForm[T] which is conceptually similar toType[T] but is inhabited by not only regular types likeint andstr, but also by anything "typelike" that can be used in the position of a type annotation at runtime, including special forms likeUnion[int, str],Literal['foo'],List[int],MyTypedDict, etc.

Pitch

Being able to represent something likeTypeForm[T] enables writing type signatures for new kinds of functions that can operate on arbitrary type annotation objects at runtime. For example:

# Returns `value` if it conforms to the specified type annotation using typechecker subtyping rules.def trycast(typelike: TypeForm[T], value: object) -> Optional[T]: ...# Returns whether the specified value can be assigned to a variable with the specified type annotation using typechecker subtyping rules.def isassignable(value: object, typelike: TypeForm[T]) -> bool: ...

Several people have indicated interest in a way to spell this concept:

• @ltworf:Type[T] -> T has a strange behaviour #9003
• @glyph:Type[T] -> T has a strange behaviour #9003 (comment)
• @davidfstr (yours truly):Type[T] -> T has a strange behaviour #9003 (comment)
• @Nico-Kialo:Version 0.780 does not recognize python internal types astyping.Type anymore #8992
• @hmvp:Version 0.780 does not recognize python internal types astyping.Type anymore #8992 (comment)

For a more in-depth motivational example showing how I can use something like TypeForm[T] to greatly simplify parsing JSON objects received by Python web applications, see my recent thread on typing-sig:

• Spelling the type of a runtime type annotation object (typing-sig thread)

If there is interest from the core mypy developers, I'm willing to do the related specification and implementation work in mypy.