Fix Python – Type hinting / annotation (PEP 484) for numpy.ndarray


Asked By – Inon

Has anyone implemented type hinting for the specific numpy.ndarray class?

Right now, I’m using typing.Any, but it would be nice to have something more specific.

For instance if the NumPy people added a type alias for their array_like object class. Better yet, implement support at the dtype level, so that other objects would be supported, as well as ufunc.

Now we will see solution for issue: Type hinting / annotation (PEP 484) for numpy.ndarray



Check recent numpy versions for a new typing module

dated answer

It looks like typing module was developed at:

The main numpy repository is at

Python bugs and commits can be tracked at

The usual way of adding a feature is to fork the main repository, develop the feature till it is bomb proof, and then submit a pull request. Obviously at various points in the process you want feedback from other developers. If you can’t do the development yourself, then you have to convince someone else that it is a worthwhile project.

cython has a form of annotations, which it uses to generate efficient C code.

You referenced the array-like paragraph in numpy documentation. Note its typing information:

A simple way to find out if the object can be converted to a numpy array using array() is simply to try it interactively and see if it works! (The Python Way).

In other words the numpy developers refuse to be pinned down. They don’t, or can’t, describe in words what kinds of objects can or cannot be converted to np.ndarray.

In [586]: np.array({'test':1})   # a dictionary
Out[586]: array({'test': 1}, dtype=object)

In [587]: np.array(['one','two'])  # a list
array(['one', 'two'], 

In [589]: np.array({'one','two'})  # a set
Out[589]: array({'one', 'two'}, dtype=object)

For your own functions, an annotation like

def foo(x: np.ndarray) -> np.ndarray:

works. Of course if your function ends up calling some numpy function that passes its argument through asanyarray (as many do), such an annotation would be incomplete, since your input could be a list, or np.matrix, etc.

When evaluating this question and answer, pay attention to the date. 484 was a relatively new PEP back then, and code to make use of it for standard Python still in development. But it looks like the links provided are still valid.

This question is answered By – hpaulj

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