[Numpy-discussion] subclassing ndarray in python3

[Darren Dale]

Now that the trunk has some support for python3, I am working on
making Quantities work with python3 as well. I'm running into some
problems related to subclassing ndarray that can be illustrated with a
simple script, reproduced below. It looks like there is a problem with
the reflected operations, I see problems with __rmul__ and __radd__,
but not with __mul__ and __add__:

import numpy as np


class A(np.ndarray):
    def __new__(cls, *args, **kwargs):
        return np.ndarray.__new__(cls, *args, **kwargs)

class B(A):
    def __mul__(self, other):
        return self.view(A).__mul__(other)
    def __rmul__(self, other):
        return self.view(A).__rmul__(other)
    def __add__(self, other):
        return self.view(A).__add__(other)
    def __radd__(self, other):
        return self.view(A).__radd__(other)

a = A((10,))
b = B((10,))

print('A __mul__:')
print(a.__mul__(2))
# ok
print(a.view(np.ndarray).__mul__(2))
# ok
print(a*2)
# ok

print('A __rmul__:')
print(a.__rmul__(2))
# yields NotImplemented
print(a.view(np.ndarray).__rmul__(2))
# yields NotImplemented
print(2*a)
# ok !!??

print('B __mul__:')
print(b.__mul__(2))
# ok
print(b.view(A).__mul__(2))
# ok
print(b.view(np.ndarray).__mul__(2))
# ok
print(b*2)
# ok

print('B __add__:')
print(b.__add__(2))
# ok
print(b.view(A).__add__(2))
# ok
print(b.view(np.ndarray).__add__(2))
# ok
print(b+2)
# ok

print('B __rmul__:')
print(b.__rmul__(2))
# yields NotImplemented
print(b.view(A).__rmul__(2))
# yields NotImplemented
print(b.view(np.ndarray).__rmul__(2))
# yields NotImplemented
print(2*b)
# yields: TypeError: unsupported operand type(s) for *: 'int' and 'B'

print('B __radd__:')
print(b.__radd__(2))
# yields NotImplemented
print(b.view(A).__radd__(2))
# yields NotImplemented
print(b.view(np.ndarray).__radd__(2))
# yields NotImplemented
print(2+b)
# yields: TypeError: unsupported operand type(s) for +: 'int' and 'B'