The inference of generic types for classes in most type-checkers assume that all the generic types can be inferred from the parameters to__init__. This is not always the case. For example, it is a common pattern to create an empty container class and add elements to it later, e.g.:

a= []a.append("hello world")

Currently for builtin classes, such aslist ordict, this idiom is understood by MyPy, but not by Pyright. Moreover, even MyPy is incapable of applying this type inference to custom types (seepython/mypy#13134). As mentioned in that issue, the problem is not only with empty containers, but also with some widely used APIs, such as theEstimator API from scikit-learn and related projects, in which the type of the data used is only known when thefit method is called.

My proposal would be to add functionality to the type system to define which methods can be used to infer generic types, apart from__init__. This could be made for example with a decorator (e.g.allow_generic_inference), so that one can define a method such as:

@allow_generic_inferencedefappend(value:T):    ...

In this case the value of the class generic parameterT could be inferred fromappend if it was undefined before.

I think this proposal may also be similar to the previously suggestedTypeAssert mentioned by@erictraut in#1013 (comment). However I am not completely sure thatTypeAssert could do what I explained here. I am also not sure that the proposedTypeAssert syntax is better than a decorator.

I hope you too consider this proposal useful, and I look forward to your feedback.