__array_prepare__ doesn't get called.

gistfile1.txt

import numpy as np

class Quantity(object):

    def __init__(self, value, unit):
        self.unit = unit
        self.value = value
        self.exception = False

    def __getitem__(self, key):
        return Quantity(self.value[key], unit=self.unit)

    def __len__(self):
        if self.exception:
            raise RuntimeError()
        return len(self.value)

    def __array_prepare__(self):
        print "__array_prepare__"
        self.exception = True

    def __array_wrap__(self):
        print "__array_wrap__"
        self.exception = False

import numpy as np
q = Quantity(np.array([1,2,3]), 'degree')
print(repr(np.array(q)))