Thanks for re-starting this discussion, Stephan! I think there is definitely significant interest in this topic: https://github.com/numpy/numpy/issues/7370 is the issue with the largest number of user likes in the issue tracker (FWIW).

Having them in numpy, as opposed to a separate numpy-stubs repository would indeed be ideal from a user perspective. When looking into it in the past, I was never sure how well in sync numpy-stubs was. Putting aside ndarray, as more challenging, even annotations for numpy functions and method parameters with built-in types would help, as a start.

To add to the previously listed projects that would benefit from this, we are currently considering to start using some (minimal) type annotations in scikit-learn.

--
Roman Yurchak

On 24/03/2020 18:00, Stephan Hoyer wrote:
When we started numpy-stubs [1] a few years ago, putting type annotations in NumPy itself seemed premature. We still supported Python 2, which meant that we would need to use awkward comments for type annotations.

Over the past few years, using type annotations has become increasingly popular, even in the scientific Python stack. For example, off-hand I know that at least SciPy, pandas and xarray have at least part of their APIs type annotated. Even without annotations for shapes or dtypes, it would be valuable to have near complete annotations for NumPy, the project at the bottom of the scientific stack.

Unfortunately, numpy-stubs never really took off. I can think of a few reasons for that: 1. Missing high level guidance on how to write type annotations, particularly for how (or if) to annotate particularly dynamic parts of NumPy (e.g., consider __array_function__), and whether we should prioritize strictness or faithfulness [2]. 2. We didn't have a good experience for new contributors. Due to the relatively low level of interest in the project, when a contributor would occasionally drop in, I often didn't even notice their PR for a few weeks. 3. Developing type annotations separately from the main codebase makes them a little harder to keep in sync. This means that type annotations couldn't serve their typical purpose of self-documenting code. Part of this may be necessary for NumPy (due to our use of C extensions), but large parts of NumPy's user facing APIs are written in Python. We no longer support Python 2, so at least we no longer need to worry about putting annotations in comments.

We eventually could probably use a formal NEP (or several) on how we want to use type annotations in NumPy, but I think a good first step would be to think about how to start moving the annotations from numpy-stubs into numpy proper.

Any thoughts? Anyone interested in taking the lead on this?

Cheers,
Stephan

[1] https://github.com/numpy/numpy-stubs
[2] https://github.com/numpy/numpy-stubs/issues/12

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