Trying to learn how to write statically typed Python, but got confused for quite some time. Here is a toy example:

fromtypingimportTypedDictfromcollections.abcimportMapping# represent the loose idea of a point as a mapping (i.e. dict or variants) from coordinates to floatstypePointLike=Mapping[str,float]# a specific type of pointclassPoint2D(TypedDict):x:floaty:float# other types of points may be defined here or elsewhere (thanks to duck typing)# examples: 3D space, RGB...# the point (pun intended) is to implement transformations between different point types# those should be abstracted with a Protocol later on# here is a trivial transformationdefidentity_transform[T:PointLike](p:T)->T:# return the point unchangedreturnp# let's test itmy_point=Point2D(x=0.7,y=-0.3)identity_transform(my_point)# runs fine, but type checker error!

Bothmypy andpyright will error on the last line, as can be seen in the playground links provided.

My main question is: why is this? After thinking about it for way too long, my guess would be that, since Python dictionaries are mutable, there is nothing stopping you from adding another key of a different type tomy_point. Therefore, type checkers cannot infer its type asMapping[str, float], and are inferring theTypedDict instance to be likeMapping[str, Any]. Please correct me if I am wrong.

A second question is: what would, then, be the appropriate way to implement the idea in the problem while keeping static type checkers happy?