[Numpy-discussion] Put type annotations in NumPy proper?

Stephan Hoyer shoyer at gmail.com
Tue Mar 24 13:00:19 EDT 2020

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

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

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


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