What type is a sklearn model?

I think the most generic class that all models inherit from would be sklearn.base.BaseEstimator.

If you want to be more specific, maybe use sklearn.base.ClassifierMixin or sklearn.base.RegressorMixin.

So I would do:

from sklearn.base import RegressorMixin

def model_tester(model: RegressorMixin, parameter: int) -> np.ndarray:
     """An example function with type hints."""

     # do stuff to model 

     return values

I am no expert in type checking, so correct me if this is not right.

From Python 3.8 on (or earlier using typing-extensions), you can use typing.Protocol. Using protocols, you can use a concept called structural subtyping to define exactly the type's expected structure:

from typing import Protocol
# from typing_extensions import Protocol  # for Python <3.8

class ScikitModel(Protocol):
    def fit(self, X, y, sample_weight=None): ...
    def predict(self, X): ...
    def score(self, X, y, sample_weight=None): ...
    def set_params(self, **params): ...

which you can then use as a type hint:

def do_stuff(model: ScikitModel) -> Any:
    model.fit(train_data, train_labels)  # this type checks 
    score = model.score(test_data, test_labels)  # this type checks