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A wrapper around NumPy and other array libraries to make them compatible with the Array API standard

Project description

Array API compatibility library

This is a small wrapper around NumPy and CuPy that is compatible with the Array API standard. See also NEP 47.

Unlike numpy.array_api, this is not a strict minimal implementation of the Array API, but rather just an extension of the main NumPy and CuPy namespaces with changes needed to be compliant with the Array API.

Library authors using the Array API may wish to test against numpy.array_api to ensure they are not using functionality outside of the standard, but prefer this implementation for the default when working with NumPy or CuPy arrays.

See https://numpy.org/doc/stable/reference/array_api.html for a full list of changes. In particular, unlike numpy.array_api, this package does not use a separate Array object, but rather just uses numpy.ndarray directly.

Note that some of the functionality in this library is backwards incompatible with NumPy.

This library also supports CuPy in addition to NumPy. If you want support for other array libraries, please open an issue.

Library authors using the Array API may wish to test against numpy.array_api to ensure they are not using functionality outside of the standard, but prefer this implementation for end users who use NumPy arrays.

Usage

To use this library replace

import numpy as np

with

import array_api_compat.numpy as np

and replace

import cupy as cp

with

import array_api_compat.cupy as cp

Each will include all the functions from the normal NumPy/CuPy namespace, except that functions that are part of the array API are wrapped so that they have the correct array API behavior. In each case, the array object used will be thew same array object from the wrapped library.

Helper Functions

In addition to the default NumPy/CuPy namespace and functions in the array API specification, there are several helper functions included that aren't part of the specification but which are useful for using the array API:

  • is_array_api_obj(x): Return True if x is an array API compatible array object.

  • get_namespace(*xs): Get the corresponding array API namespace for the arrays xs. If the arrays are NumPy or CuPy arrays, the returned namespace will be array_api_compat.numpy or array_api_compat.cupy so that it is array API compatible.

  • device(x): Equivalent to x.device in the array API specification. Included because numpy.ndarray does not include the device attribute and this library does not wrap or extend the array object. Note that for NumPy, device is always "cpu".

  • to_device(x, device, /, *, stream=None): Equivalent to x.to_device. Included because neither NumPy's nor CuPy's ndarray objects include this method. For NumPy, this function effectively does nothing since the only supported device is the CPU, but for CuPy, this method supports CuPy CUDA Device and Stream objects.

Known Differences from the Array API Specification

There are some known differences between this library and the array API specification:

  • The array methods __array_namespace__, device (for NumPy), to_device, and mT are not defined. This reuses np.ndarray and cp.ndarray and we don't want to monkeypatch or wrap it. The helper functions device() and to_device() are provided to work around these missing methods (see above). x.mT can be replaced with xp.linalg.matrix_transpose(x). get_namespace(x) should be used instead of x.__array_namespace__.

  • NumPy value-based casting for scalars will be in effect unless explicitly disabled with the environment variable NPY_PROMOTION_STATE=weak or np._set_promotion_state('weak') (requires NumPy 1.24 or newer, see NEP 50 and https://github.com/numpy/numpy/issues/22341)

  • Functions which are not wrapped may not have the same type annotations as the spec.

  • Functions which are not wrapped may not use positional-only arguments.

Vendoring

This library supports vendoring as an installation method. To vendor the library, simply copy array_api_compat into the appropriate place in the library, like

cp -R array_api_compat/ mylib/vendored/array_api_compat

You may also rename it to something else if you like (nowhere in the code references the name "array_api_compat").

Alternatively, the library may be installed as dependency on PyPI.

Implementation

As noted before, the goal of this library is to reuse the NumPy and CuPy array objects, rather than wrapping or extending them. This means that the functions need to accept and return np.ndarray for NumPy and cp.ndarray for CuPy.

Each namespace (array_api_compat.numpy and array_api_compat.cupy) is populated with the normal library namespace (like from numpy import *). Then specific functions are replaced with wrapped variants. Wrapped functions that have the same logic between NumPy and CuPy (which is most functions) are in array_api_compat/common/. These functions are defined like

# In array_api_compat/common/_aliases.py

def acos(x, /, xp):
    return xp.arccos(x)

The xp argument refers to the original array namespace (either numpy or cupy). Then in the specific array_api_compat/numpy and array_api_compat/cupy namespace, the get_xp decorator is applied to these functions, which automatically removes the xp argument from the function signature and replaces it with the corresponding array library, like

# In array_api_compat/numpy/_aliases.py

from ..common import _aliases

import numpy as np

acos = get_xp(np)(_aliases.acos)

This acos now has the signature acos(x, /) and calls numpy.arccos.

Similarly, for CuPy:

# In array_api_compat/cupy/_aliases.py

from ..common import _aliases

import cupy as cp

acos = get_xp(cp)(_aliases.acos)

Since NumPy and CuPy are nearly identical in their behaviors, this allows writing the wrapping logic for both libraries only once. If support is added for other libraries which differ significantly from NumPy, their wrapper code should go in their specific sub-namespace instead of common/.

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