In [1]: import numpy as np

In [2]: class MyArray:
   ...:     def __array_function__(*args, **kwargs):
   ...:         return NotImplemented
   ...:

In [3]: np.sum(MyArray())
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-3-c8a80bb1d37e> in <module>()
----> 1 np.sum(MyArray())

~/dev/numpy/numpy/core/overrides.py in public_api(*args, **kwargs)
    149             relevant_args = dispatcher(*args, **kwargs)
    150             return array_function_implementation_or_override(
--> 151                 implementation, public_api, relevant_args, args, kwargs)
    152
    153         # TODO: remove this when we drop Python 2 support (functools.wraps

~/dev/numpy/numpy/core/overrides.py in array_function_implementation_or_override(implementation, public_api, relevant_args, args, kwargs)
    108     raise TypeError('no implementation found for {} on types that implement '
    109                     '__array_function__: {}'
--> 110                     .format(public_api, list(map(type, overloaded_args))))
    111
    112

TypeError: no implementation found for <function sum at 0x10e070bf8> on types that implement __array_function__: [<class '__main__.MyArray'>]