First release of the Array API Standard

This first release of the standards document and accompanying test suite marks the start of the community review period.

Array and tensor libraries - from NumPy, TensorFlow and PyTorch to Dask, JAX, MXNet and beyond - could benefit greatly from a uniform API for creating and working with multi-dimensional arrays (a.k.a tensors), as we discussed in our previous blog post. Today we’re pleased to announce a first version of our array API standard (document, repo) for review by the wider community. Getting to this point took slightly longer than we had initially announced because, well, it’s 2020 and hence nothing quite goes according to plan.

The current status of the standard is that it is a coherent story (or at least, we hope it is) that gives readers enough context about goals and scope to understand and review the design decisions already taken and APIs it contains. However, it is not yet complete and we can still change direction and make significant changes based on community feedback. This is important — no one likes a “take it or leave it” approach, and more eyes can make the final result better. There’s still a few TODOs in places, and a couple of key sections to be finished. The most important of those are the API for device support, and the Python API for the data interchange protocol (proposed to be based on DLPack).

It is worth repeating the main goal of this standard: make it easier to switch from one array library to another one, or to support multiple array libraries as compute backends in downstream packages. We’d also like to emphasize that if some functionality is not present in the API standard, that does not mean it’s unimportant, or that we’re asking existing array libraries to deprecate it. Instead it simply means that that functionality at present isn’t supported - likely due to it not being present in all or most current array libraries, or not being used widely enough to have been included so far. The use cases section of the standard may provide more insight into important goals.

Some key design topics

Two topics stood out so far in terms of complexity and choices that were hard to make in such a way that they’d work well for all existing libraries: mutability & copy/view behaviour, and dtype casting rules.

The standard will contain common mutable operations such as slice assignment, but will generally avoid in-place mutation in APIs like the out keyword

NumPy, PyTorch, CuPy and MXNet provide strided arrays, and rely heavily on mutating values in existing arrays and on the concept of a “view” for performance. TensorFlow, JAX and Dask on the other hand have no or limited support, given that they rely on an execution graph and/or JIT compiler which provides constraints on how much mutability can be supported. The design decisions described here will allow the most heavily used types of mutability - inplace operators, item assignment and slice assignment - to be retained, while avoiding the use of the out= keyword which is problematic to support for some libraries and arguably a suboptimal API to begin with.

For libraries like SciPy and scikit-learn, the supported features are essential. Code like this, from scikit-learn’s ForestClassifier:

for k in range(self.n_outputs_):
    predictions[k][unsampled_indices, :] += p_estimator[k]

or this, from SciPy’s optimize.linprog:

r = b - A@x
A[r < 0] = -A[r < 0]
b[r < 0] = -b[r < 0]
r[r < 0] *= -1

is quite common and we see it as fundamental to how users work with array libraries. out= is less essential though, and leaving it out is important for JAX, TensorFlow, Dask, and future libraries designed on immutable data structures.

Casting rules for mixed type families will not be specified and are implementation specific

Casting rules are relatively straightforward when all involved dtypes are of the same kind (e.g. all integer), but when mixing for example integers and floats it quickly becomes clear that array libraries don’t agree with each other. One may get exceptions, or dtypes with different precision. Therefore we had to make the choice to leave the rules for “mixed kind dtype casting” undefined - when users want to write portable code, they should avoid this situation or use explicit casts to obtain the same results from different array libraries. An example as simple as this one:

x = np.arange(5)  # will be integer
y = np.ones(5, dtype=float16)
(x * y).dtype

will show the issue. NumPy will produce float64 here, PyTorch will produce float16, and TensorFlow will raise InvalidArgumentError because it does not support mixing integer and float dtypes.

See this section of the standard for more details on casting rules.

A portable test suite

With the array API standard document we are also working on a test suite. This test suite will be implemented with Pytest and Hypothesis, and won’t rely on any particular array implementation, and is meant to test compliance with the API standard.

It is still very much a work-in-progress, but the aim is to complete it by the time the community review of the API standard wraps up. However, the community is encouraged to check out the current work on the test suite on GitHub and try it out and comment on it. The README in the test suite repo contains more information on how to run it and contribute to it.

The test suite will be runnable with any existing library. This can be done by specifying the array implementation namespace to be tested via an environment variable:

$ ARRAY_API_TESTS_MODULE=jax.numpy pytest

The test suite will also support vendoring so that array libraries can easily include it in their own test suites.

The result of running the test suite will be an overview of the level of compliance with the standard. We expect it will take time for libraries to get to 100%; anything less shouldn’t just mean “fail”, 98% would be a major step towards portable code compared to today.

People & projects

So who was involved in getting the API standard to this point, and which libraries do we hope will adopt this standard? The answer to the latter is “all existing and new array and tensor libraries with a Python API”. As for who was involved, we were lucky to get contributions from creators and senior maintainers of almost every project of interest - here’s a brief description:

  • NumPy: Stephan Hoyer and Ralf Gommers are both long-time NumPy maintainers. In addition we got to consult regularly with Travis Oliphant, creator of NumPy, on the history behind some decisions made early on in NumPy’s life.
  • TensorFlow: Alexandre Passos was a technical lead on the TensorFlow team, and has been heavily involved until a few weeks ago. Paige Bailey is the product manager for TensorFlow APIs at Google Research. Edward Loper and Ashish Agarwal, TensorFlow maintainers, replaced Alexandre recently as Consortium members.
  • PyTorch: Adam Paszke is one of the co-creators of PyTorch. Ralf Gommers leads a team of engineers contributing to PyTorch.
  • MXNet: Sheng Zha is a long-time MXNet maintainer. Markus Weimer is an Apache PMC member and mentor for the MXNet incubation process into the Apache Foundation.
  • JAX: Stephan Hoyer and Adam Paszke are two maintainers of JAX.
  • XArray: Stephan Hoyer is one of the co-creators, and still a maintainer, of Xarray.
  • Dask: Tom Augspurger is a senior Dask maintainer.
  • CuPy: we have no active participant from CuPy. However we have talked to the CuPy team at Preferred Networks, who are supportive of the goals and committed to following NumPy’s lead on APIs.
  • ONNX: Sheng Zha is an ONNX Steering Committee member.

Many other people have made contributions so far, including the Consortium members listed at

Next steps to a first complete standard

We are now looking for feedback from the wider community, and in particular maintainers of array libraries. For each of those libraries, a Consortium member involved in the library will be soliciting feedback from their own project. We’d like to get to the point where it’s clear for each library that there are no blockers to adoption and that the overall shape of the API standard is considered valuable enough to support.

In addition, given that this API standard is completely new and drafting something like it hasn’t been attempted before in this community, we’d love to get meta feedback - is anything missing or in need of shaping in the standard document, the goal and scope, ways to participate, or any other such topic?

To provide feedback on the array API standard, please open issues or pull requests on For larger discussions and meta-feedback, please open GitHub Discussion topics at

Published by in Consortium and Standardization and tagged APIs, arrays, community, consortium and standard using 1425 words.