Writing custom array containers

Numpy’s dispatch mechanism, introduced in numpy version v1.16 is the recommended approach for writing custom N-dimensional array containers that are compatible with the numpy API and provide custom implementations of numpy functionality. Applications include dask arrays, an N-dimensional array distributed across multiple nodes, and cupy arrays, an N-dimensional array on a GPU.

To get a feel for writing custom array containers, we’ll begin with a simple example that has rather narrow utility but illustrates the concepts involved.

>>> import numpy as np
>>> class DiagonalArray:
...     def __init__(self, N, value):
...         self._N = N
...         self._i = value
...     def __repr__(self):
...         return f"{self.__class__.__name__}(N={self._N}, value={self._i})"
...     def __array__(self):
...         return self._i * np.eye(self._N)
...

Our custom array can be instantiated like:

>>> arr = DiagonalArray(5, 1)
>>> arr
DiagonalArray(N=5, value=1)

We can convert to a numpy array using numpy.array or numpy.asarray, which will call its __array__ method to obtain a standard numpy.ndarray.

>>> np.asarray(arr)
array([[1., 0., 0., 0., 0.],
       [0., 1., 0., 0., 0.],
       [0., 0., 1., 0., 0.],
       [0., 0., 0., 1., 0.],
       [0., 0., 0., 0., 1.]])

If we operate on arr with a numpy function, numpy will again use the __array__ interface to convert it to an array and then apply the function in the usual way.

>>> np.multiply(arr, 2)
array([[2., 0., 0., 0., 0.],
       [0., 2., 0., 0., 0.],
       [0., 0., 2., 0., 0.],
       [0., 0., 0., 2., 0.],
       [0., 0., 0., 0., 2.]])

Notice that the return type is a standard numpy.ndarray.

>>> type(arr)
numpy.ndarray

How can we pass our custom array type through this function? Numpy allows a class to indicate that it would like to handle computations in a custom-defined way through the interaces __array_ufunc__ and __array_function__. Let’s take one at a time, starting with _array_ufunc__. This method covers Universal functions (ufunc), a class of functions that includes, for example, numpy.multiply and numpy.sin.

The __array_ufunc__ receives:

• ufunc, a function like numpy.multiply
• method, a string, differentiating between numpy.multiply(...) and variants like numpy.multiply.outer, numpy.multiply.accumulate, and so on. For the common case, numpy.multiply(...), method == '__call__'.
• inputs, which could be a mixture of different types
• kwargs, keyword arguments passed to the function

For this example we will only handle the method '__call__.

>>> from numbers import Number
>>> class DiagonalArray:
...     def __init__(self, N, value):
...         self._N = N
...         self._i = value
...     def __repr__(self):
...         return f"{self.__class__.__name__}(N={self._N}, value={self._i})"
...     def __array__(self):
...         return self._i * np.eye(self._N)
...     def __array_ufunc__(self, ufunc, method, *inputs, **kwargs):
...         if method == '__call__':
...             N = None
...             scalars = []
...             for input in inputs:
...                 if isinstance(input, Number):
...                     scalars.append(input)
...                 elif isinstance(input, self.__class__):
...                     scalars.append(input._i)
...                     if N is not None:
...                         if N != self._N:
...                             raise TypeError("inconsistent sizes")
...                     else:
...                         N = self._N
...                 else:
...                     return NotImplemented
...             return self.__class__(N, ufunc(*scalars, **kwargs))
...         else:
...             return NotImplemented
...

Now our custom array type passes through numpy functions.

>>> arr = DiagonalArray(5, 1)
>>> np.multiply(arr, 3)
DiagonalArray(N=5, value=3)
>>> np.add(arr, 3)
DiagonalArray(N=5, value=4)
>>> np.sin(arr)
DiagonalArray(N=5, value=0.8414709848078965)

At this point arr + 3 does not work.

>>> arr + 3
TypeError: unsupported operand type(s) for *: 'DiagonalArray' and 'int'

To support it, we need to define the Python interfaces __add__, __lt__, and so on to dispatch to the corresponding ufunc. We can achieve this conveniently by inheriting from the mixin NDArrayOperatorsMixin.

>>> import numpy.lib.mixins
>>> class DiagonalArray(numpy.lib.mixins.NDArrayOperatorsMixin):
...     def __init__(self, N, value):
...         self._N = N
...         self._i = value
...     def __repr__(self):
...         return f"{self.__class__.__name__}(N={self._N}, value={self._i})"
...     def __array__(self):
...         return self._i * np.eye(self._N)
...     def __array_ufunc__(self, ufunc, method, *inputs, **kwargs):
...         if method == '__call__':
...             N = None
...             scalars = []
...             for input in inputs:
...                 if isinstance(input, Number):
...                     scalars.append(input)
...                 elif isinstance(input, self.__class__):
...                     scalars.append(input._i)
...                     if N is not None:
...                         if N != self._N:
...                             raise TypeError("inconsistent sizes")
...                     else:
...                         N = self._N
...                 else:
...                     return NotImplemented
...             return self.__class__(N, ufunc(*scalars, **kwargs))
...         else:
...             return NotImplemented
...

>>> arr = DiagonalArray(5, 1)
>>> arr + 3
DiagonalArray(N=5, value=4)
>>> arr > 0
DiagonalArray(N=5, value=True)

Now let’s tackle __array_function__. We’ll create dict that maps numpy functions to our custom variants.

>>> HANDLED_FUNCTIONS = {}
>>> class DiagonalArray(numpy.lib.mixins.NDArrayOperatorsMixin):
...     def __init__(self, N, value):
...         self._N = N
...         self._i = value
...     def __repr__(self):
...         return f"{self.__class__.__name__}(N={self._N}, value={self._i})"
...     def __array__(self):
...         return self._i * np.eye(self._N)
...     def __array_ufunc__(self, ufunc, method, *inputs, **kwargs):
...         if method == '__call__':
...             N = None
...             scalars = []
...             for input in inputs:
...                 # In this case we accept only scalar numbers or DiagonalArrays.
...                 if isinstance(input, Number):
...                     scalars.append(input)
...                 elif isinstance(input, self.__class__):
...                     scalars.append(input._i)
...                     if N is not None:
...                         if N != self._N:
...                             raise TypeError("inconsistent sizes")
...                     else:
...                         N = self._N
...                 else:
...                     return NotImplemented
...             return self.__class__(N, ufunc(*scalars, **kwargs))
...         else:
...             return NotImplemented
...    def __array_function__(self, func, types, args, kwargs):
...        if func not in HANDLED_FUNCTIONS:
...            return NotImplemented
...        # Note: this allows subclasses that don't override
...        # __array_function__ to handle DiagonalArray objects.
...        if not all(issubclass(t, self.__class__) for t in types):
...            return NotImplemented
...        return HANDLED_FUNCTIONS[func](*args, **kwargs)
...

A convenient pattern is to define a decorator implements that can be used to add functions to HANDLED_FUNCTIONS.

>>> def implements(np_function):
...    "Register an __array_function__ implementation for DiagonalArray objects."
...    def decorator(func):
...        HANDLED_FUNCTIONS[np_function] = func
...        return func
...    return decorator
...

Now we write implementations of numpy functions for DiagonalArray. For completeness, to support the usage arr.sum() add a method sum that calls numpy.sum(self), and the same for mean.

>>> @implements(np.sum)
... def sum(a):
...     "Implementation of np.sum for DiagonalArray objects"
...     return arr._i * arr._N
...
>>> @implements(np.mean)
... def mean(arr):
...     "Implementation of np.mean for DiagonalArray objects"
...     return arr._i / arr._N
...
>>> arr = DiagonalArray(5, 1)
>>> np.sum(arr)
5
>>> np.mean(arr)
0.2

If the user tries to use any numpy functions not included in HANDLED_FUNCTIONS, a TypeError will be raised by numpy, indicating that this operation is not supported. For example, concatenating two DiagonalArrays does not produce another diagonal array, so it is not supported.

>>> np.concatenate([arr, arr])
TypeError: no implementation found for 'numpy.concatenate' on types that implement __array_function__: [<class '__main__.DiagonalArray'>]

Additionally, our implementations of sum and mean do not accept the optional arguments that numpy’s implementation does.

>>> np.sum(arr, axis=0)
TypeError: sum() got an unexpected keyword argument 'axis'

The user always has the option of converting to a normal numpy.ndarray with numpy.asarray and using standard numpy from there.

>>> np.concatenate([np.asarray(arr), np.asarray(arr)])
array([[1., 0., 0., 0., 0.],
       [0., 1., 0., 0., 0.],
       [0., 0., 1., 0., 0.],
       [0., 0., 0., 1., 0.],
       [0., 0., 0., 0., 1.],
       [1., 0., 0., 0., 0.],
       [0., 1., 0., 0., 0.],
       [0., 0., 1., 0., 0.],
       [0., 0., 0., 1., 0.],
       [0., 0., 0., 0., 1.]])

Refer to the dask source code and cupy source code for more fully-worked examples of custom array containers.

See also NEP 18.