numpy.ndarray的python运算符就像类一样

Question

I have a class that has a numpy.ndarray as a member and behaves similar to ndarray by overloading __getitem__ and __getattr__ :

class Foo(object):
    def __init__(values):
        # numpy.ndarray
        self._values = values

    def __getitem__(self, key):
        return self._values[key]

    def __getattr__(self, name):
        return getattr(self._values, name)

Thus I can use the numpy method like shape, size, ... directly on an object of this class. I can also do things like obj.__add__(1) , which will add 1 to obj._values . However, if I try obj + 1 it raises "unsupported operand type(s)". I would like to get the same behaviour for obj + 1 as obj.__add__(1) . Is this possible without adding __add__ to Foo ?

Answer 1

I can see what you're trying to do here, but it's not going to work the way you think it should. This is a very non-obvious subtlety in Python.

What you're thinking is that what you do obj + 1 Python is actually calling obj.__add__(1) and that, failing to find an __add__ attribute on obj it will fall through to its __getattr__ .

But this is not exactly how it works for arithmetic operators, the implementation of which is actually significantly more complicated. In this case if obj does not have an __add__ method, it will attempt to call the right-hand operand's __radd__ (for "right add") method to see if 1 knows how to add with the left-hand operator. It does not so you get an exception.

There are other subtleties involving type slots that I won't get into.

If you want your class to act as a proxy for an ndarray you have a few options. It really depends on what you're actually trying to accomplish, which you might consider asking in a separate question. You might just be able to subclass ndarray directly and implement your additional functionality in the subclass.

If you don't want a subclass of ndarray you might also consider using a proxy, such as the ObjectProxy from wrapt . This may or may not be what you want. It will make an object that walks, talks, quacks like, and is even named ndarray , though you can still subclass ObjectProxy to override methods that you don't want proxied.

Otherwise there's the tedious manual method.